shairport-sync/pair_ap/pair_homekit.c

/*
 * Adaption of ap2-sender by ViktoriiaKh:
 *    <https://github.com/ViktoriiaKh/ap2-sender>
 *
 * To test, it is useful to try with this receiver:
 *    <https://github.com/ckdo/airplay2-receiver>
 *
 * The Secure Remote Password 6a implementation is adapted from:
 *  - Tom Cocagne
 *    <https://github.com/cocagne/csrp>
 *
 * TLV helpers are adapted from ESP homekit:
 *    <https://github.com/maximkulkin/esp-homekit>
 *
 * The MIT License (MIT)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
 * of the Software, and to permit persons to whom the Software is furnished to do
 * so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>

#include <assert.h>

#include <sodium.h>

#include "pair-internal.h"
#include "pair-tlv.h"


/* ----------------------------- DEFINES ETC ------------------------------- */

#define USERNAME "Pair-Setup"
#define AUTHTAG_LENGTH 16
#define NONCE_LENGTH 12 // 96 bits according to chacha poly1305
#define REQUEST_BUFSIZE 4096
#define ENCRYPTED_LEN_MAX 0x400

// #define DEBUG_SHORT_A 1

enum pair_keys
{
  PAIR_SETUP_MSG01 = 0,
  PAIR_SETUP_MSG02,
  PAIR_SETUP_MSG03,
  PAIR_SETUP_MSG04,
  PAIR_SETUP_MSG05,
  PAIR_SETUP_MSG06,
  PAIR_SETUP_CONTROLLER_SIGN,
  PAIR_SETUP_ACCESSORY_SIGN,
  PAIR_VERIFY_MSG01,
  PAIR_VERIFY_MSG02,
  PAIR_VERIFY_MSG03,
  PAIR_VERIFY_MSG04,
  PAIR_CONTROL_WRITE,
  PAIR_CONTROL_READ,
  PAIR_EVENTS_WRITE,
  PAIR_EVENTS_READ,
};

struct pair_keys_map
{
  uint8_t state;
  const char *salt;
  const char *info;
  const char nonce[8];
};

static struct pair_keys_map pair_keys_map[] =
{
  // Used for /pair-setup
  { 0x01, NULL, NULL, "" },
  { 0x02, NULL, NULL, "" },
  { 0x03, NULL, NULL, "" },
  { 0x04, NULL, NULL, "" },
  { 0x05, "Pair-Setup-Encrypt-Salt", "Pair-Setup-Encrypt-Info", "PS-Msg05" },
  { 0x06, "Pair-Setup-Encrypt-Salt", "Pair-Setup-Encrypt-Info", "PS-Msg06" },
  { 0, "Pair-Setup-Controller-Sign-Salt", "Pair-Setup-Controller-Sign-Info", "" },
  { 0, "Pair-Setup-Accessory-Sign-Salt", "Pair-Setup-Accessory-Sign-Info", "" },

  // Used for /pair-verify
  { 0x01, NULL, NULL, "" },
  { 0x02, "Pair-Verify-Encrypt-Salt", "Pair-Verify-Encrypt-Info", "PV-Msg02" },
  { 0x03, "Pair-Verify-Encrypt-Salt", "Pair-Verify-Encrypt-Info", "PV-Msg03" },
  { 0x04, NULL, NULL, "" },

  // Encryption/decryption of control channel
  { 0, "Control-Salt", "Control-Write-Encryption-Key", "" },
  { 0, "Control-Salt", "Control-Read-Encryption-Key", "" },

  // Encryption/decryption of event channel
  { 0, "Events-Salt", "Events-Write-Encryption-Key", "" },
  { 0, "Events-Salt", "Events-Read-Encryption-Key", "" },
};

enum pair_method {
  PairingMethodPairSetup          = 0x00,
  PairingMethodPairSetupWithAuth  = 0x01,
  PairingMethodPairVerify         = 0x02,
  PairingMethodAddPairing         = 0x03,
  PairingMethodRemovePairing      = 0x04,
  PairingMethodListPairings       = 0x05
};

enum pair_flags {
  PairingFlagsTransient           = 0x10,
};

// Forwards
const struct pair_definition pair_client_homekit_normal;
const struct pair_definition pair_client_homekit_transient;
const struct pair_definition pair_server_homekit;


/* ---------------------------------- SRP ----------------------------------- */

typedef enum
{
  SRP_NG_2048,
  SRP_NG_3072,
  SRP_NG_CUSTOM
} SRP_NGType;

typedef struct
{
  int N_len;
  bnum N;
  bnum g;
} NGConstant;

struct SRPUser
{
  enum hash_alg     alg;
  NGConstant        *ng;

  bnum a;
  bnum A;
  bnum S;

  const unsigned char *bytes_A;
  int           authenticated;

  char          *username;
  unsigned char *password;
  int           password_len;

  unsigned char M           [SHA512_DIGEST_LENGTH];
  unsigned char H_AMK       [SHA512_DIGEST_LENGTH];
  unsigned char session_key [SHA512_DIGEST_LENGTH];
  int           session_key_len;
};

struct SRPVerifier
{
  enum hash_alg     alg;
  NGConstant        *ng;

  const unsigned char *bytes_B;
  int           authenticated;

  char          *username;

  unsigned char M           [SHA512_DIGEST_LENGTH];
  unsigned char H_AMK       [SHA512_DIGEST_LENGTH];
  unsigned char session_key [SHA512_DIGEST_LENGTH];
  int           session_key_len;
};

struct NGHex
{
  int N_len;
  const char *n_hex;
  const char *g_hex;
};

// These constants here were pulled from Appendix A of RFC 5054
static struct NGHex global_Ng_constants[] =
{
  { /* 2048 */
    256,
    "AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050A37329CBB4"
    "A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50E8083969EDB767B0CF60"
    "95179A163AB3661A05FBD5FAAAE82918A9962F0B93B855F97993EC975EEAA80D740ADBF4FF"
    "747359D041D5C33EA71D281E446B14773BCA97B43A23FB801676BD207A436C6481F1D2B907"
    "8717461A5B9D32E688F87748544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB37861"
    "60279004E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DB"
    "FBB694B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73",
    "2"
  },
  { /* 3072 */
    384,
    "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B"
    "139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485"
    "B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1F"
    "E649286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F83655D23"
    "DCA3AD961C62F356208552BB9ED529077096966D670C354E4ABC9804F1746C08CA18217C32"
    "905E462E36CE3BE39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF69558"
    "17183995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D04507A33A85521"
    "ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7ABF5AE8CDB0933D7"
    "1E8C94E04A25619DCEE3D2261AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B1817"
    "7B200CBBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFCE0FD108E4B82"
    "D120A93AD2CAFFFFFFFFFFFFFFFF",
    "5"
  },
  {0} /* null sentinel */
};


static NGConstant *
new_ng(SRP_NGType ng_type, const char *n_hex, const char *g_hex)
{
  NGConstant *ng = calloc(1, sizeof(NGConstant));

  if ( ng_type != SRP_NG_CUSTOM )
    {
      n_hex = global_Ng_constants[ng_type].n_hex;
      g_hex = global_Ng_constants[ng_type].g_hex;
    }

  bnum_hex2bn(ng->N, n_hex);
  bnum_hex2bn(ng->g, g_hex);

  ng->N_len = bnum_num_bytes(ng->N);

  assert(ng_type == SRP_NG_CUSTOM || ng->N_len == global_Ng_constants[ng_type].N_len);

  return ng;
}

static void
free_ng(NGConstant * ng)
{
  if (!ng)
    return;

  bnum_free(ng->N);
  bnum_free(ng->g);
  free(ng);
}

static int
N_len(SRP_NGType ng_type)
{
  if (ng_type == SRP_NG_CUSTOM)
    return -1;

  return global_Ng_constants[ng_type].N_len;
}

static bnum
calculate_x(enum hash_alg alg, const bnum salt, const char *username, const unsigned char *password, int password_len)
{
  unsigned char ucp_hash[SHA512_DIGEST_LENGTH];
  HashCTX       ctx;

  hash_init( alg, &ctx );
  hash_update( alg, &ctx, username, strlen(username) );
  hash_update( alg, &ctx, ":", 1 );
  hash_update( alg, &ctx, password, password_len );
  hash_final( alg, &ctx, ucp_hash );

  return H_ns( alg, salt, ucp_hash, hash_length(alg) );
}

static void
calculate_M(enum hash_alg alg, NGConstant *ng, unsigned char *dest, const char *I, const bnum s,
            const bnum A, const bnum B, const unsigned char *K, int K_len)
{
  unsigned char H_N[ SHA512_DIGEST_LENGTH ];
  unsigned char H_g[ SHA512_DIGEST_LENGTH ];
  unsigned char H_I[ SHA512_DIGEST_LENGTH ];
  unsigned char H_xor[ SHA512_DIGEST_LENGTH ];
  HashCTX       ctx;
  int           i = 0;
  int           hash_len = hash_length(alg);

  hash_num( alg, ng->N, H_N );
  hash_num( alg, ng->g, H_g );

  hash(alg, (const unsigned char *)I, strlen(I), H_I);

  for (i=0; i < hash_len; i++ )
    H_xor[i] = H_N[i] ^ H_g[i];
    
  hash_init( alg, &ctx );

  hash_update( alg, &ctx, H_xor, hash_len );
  hash_update( alg, &ctx, H_I,   hash_len );
  update_hash_n( alg, &ctx, s );
  update_hash_n( alg, &ctx, A );
  update_hash_n( alg, &ctx, B );
  hash_update( alg, &ctx, K, K_len );

  hash_final( alg, &ctx, dest );
}

static void
calculate_H_AMK(enum hash_alg alg, unsigned char *dest, const bnum A, const unsigned char * M, const unsigned char * K, int K_len)
{
  HashCTX ctx;

  hash_init( alg, &ctx );

  update_hash_n( alg, &ctx, A );
  hash_update( alg, &ctx, M, hash_length(alg) );
  hash_update( alg, &ctx, K, K_len );

  hash_final( alg, &ctx, dest );
}

/* ----------------------- SRP for the client side -------------------------- */

static struct SRPUser *
srp_user_new(enum hash_alg alg, SRP_NGType ng_type, const char *username,
             const unsigned char *bytes_password, int len_password,
             const char *n_hex, const char *g_hex)
{
  struct SRPUser  *usr  = calloc(1, sizeof(struct SRPUser));
  int              ulen = strlen(username) + 1;

  if (!usr)
    goto err_exit;

  usr->alg = alg;
  usr->ng  = new_ng( ng_type, n_hex, g_hex );

  bnum_new(usr->a);
  bnum_new(usr->A);
  bnum_new(usr->S);

  if (!usr->ng || !usr->a || !usr->A || !usr->S)
    goto err_exit;

  usr->username     = malloc(ulen);
  usr->password     = malloc(len_password);
  usr->password_len = len_password;

  if (!usr->username || !usr->password)
    goto err_exit;

  memcpy(usr->username, username,       ulen);
  memcpy(usr->password, bytes_password, len_password);

  usr->authenticated = 0;
  usr->bytes_A = 0;

  return usr;

 err_exit:
  if (!usr)
    return NULL;

  bnum_free(usr->a);
  bnum_free(usr->A);
  bnum_free(usr->S);

  free(usr->username);
  if (usr->password)
    {
      memset(usr->password, 0, usr->password_len);
      free(usr->password);
    }
  free(usr);

  return NULL;
}

static void
srp_user_free(struct SRPUser *usr)
{
  if(!usr)
    return;

  bnum_free(usr->a);
  bnum_free(usr->A);
  bnum_free(usr->S);

  free_ng(usr->ng);

  memset(usr->password, 0, usr->password_len);

  free(usr->username);
  free(usr->password);
  free((char *)usr->bytes_A);

  memset(usr, 0, sizeof(*usr));
  free(usr);
}

static int
srp_user_is_authenticated(struct SRPUser *usr)
{
  return usr->authenticated;
}

static const unsigned char *
srp_user_get_session_key(struct SRPUser *usr, int *key_length)
{
  if (key_length)
    *key_length = usr->session_key_len;
  return usr->session_key;
}

#ifdef DEBUG_SHORT_A
// This value of "a" will yield a 383 byte A
static uint8_t short_a[] = {
 0xef, 0xb5, 0x93, 0xf5, 0x03, 0x97, 0x69, 0x8e, 0x15, 0xed, 0xee, 0x5b, 0xf2, 0xf9, 0x23, 0x6c,
 0xf0, 0x59, 0x6c, 0xe2, 0x77, 0xf2, 0x14, 0x16, 0xac, 0x99, 0xfa, 0x31, 0xae, 0x2b, 0xd3, 0x41,
};
#endif

/* Output: username, bytes_A, len_A */
static void
srp_user_start_authentication(struct SRPUser *usr, const char **username,
                              const unsigned char **bytes_A, int *len_A)
{
//  BN_hex2bn(&(usr->a), "D929DFB605687233C9E9030C2280156D03BDB9FDCF3CCE3BC27D9CCFCB5FF6A1");
  bnum_random(usr->a, 256);
#ifdef DEBUG_SHORT_A
  bnum_bin2bn(usr->a, short_a, sizeof(short_a));
#endif
#ifdef DEBUG_PAIR
  bnum_dump("Random value of usr->a:\n", usr->a);
#endif

  bnum_modexp(usr->A, usr->ng->g, usr->a, usr->ng->N);
    
  *len_A   = bnum_num_bytes(usr->A);
  *bytes_A = malloc(*len_A);

  if (!*bytes_A)
    {
      *len_A = 0;
      *bytes_A = 0;
      *username = 0;
      return;
    }

  bnum_bn2bin(usr->A, (unsigned char *) *bytes_A, *len_A);

  usr->bytes_A = *bytes_A;
  *username = usr->username;
}

/* Output: bytes_M. Buffer length is SHA512_DIGEST_LENGTH */
static void
srp_user_process_challenge(struct SRPUser *usr, const unsigned char *bytes_s, int len_s,
                           const unsigned char *bytes_B, int len_B,
                           const unsigned char **bytes_M, int *len_M )
{
  bnum s, B, k, v;
  bnum tmp1, tmp2, tmp3;
  bnum u, x;

  *len_M = 0;
  *bytes_M = NULL;

  bnum_bin2bn(s, bytes_s, len_s);
  bnum_bin2bn(B, bytes_B, len_B);

  k = H_nn_pad(usr->alg, usr->ng->N, usr->ng->g, usr->ng->N_len);

  bnum_new(v);
  bnum_new(tmp1);
  bnum_new(tmp2);
  bnum_new(tmp3);

  if (!s || !B || !k || !v || !tmp1 || !tmp2 || !tmp3)
    goto cleanup1;

  u = H_nn_pad(usr->alg, usr->A, B, usr->ng->N_len);
  x = calculate_x(usr->alg, s, usr->username, usr->password, usr->password_len);
  if (!u || !x)
    goto cleanup2;

  // SRP-6a safety check
  if (!bnum_is_zero(B) && !bnum_is_zero(u))
    {
      bnum_modexp(v, usr->ng->g, x, usr->ng->N);

      // S = (B - k*(g^x)) ^ (a + ux)
      bnum_mul(tmp1, u, x);
      bnum_add(tmp2, usr->a, tmp1);        // tmp2 = (a + ux)
      bnum_modexp(tmp1, usr->ng->g, x, usr->ng->N);
      bnum_mul(tmp3, k, tmp1);             // tmp3 = k*(g^x)
      bnum_sub(tmp1, B, tmp3);             // tmp1 = (B - K*(g^x))
      bnum_modexp(usr->S, tmp1, tmp2, usr->ng->N);

      hash_num(usr->alg, usr->S, usr->session_key);
      usr->session_key_len = hash_length(usr->alg);

      calculate_M(usr->alg, usr->ng, usr->M, usr->username, s, usr->A, B, usr->session_key, usr->session_key_len);
      calculate_H_AMK(usr->alg, usr->H_AMK, usr->A, usr->M, usr->session_key, usr->session_key_len);

      *bytes_M = usr->M;
      *len_M = hash_length(usr->alg);
    }

 cleanup2:
  bnum_free(x);
  bnum_free(u);
 cleanup1:
  bnum_free(tmp3);
  bnum_free(tmp2);
  bnum_free(tmp1);
  bnum_free(v);
  bnum_free(k);
  bnum_free(B);
  bnum_free(s);
}

static void
srp_user_verify_session(struct SRPUser *usr, const unsigned char *bytes_HAMK)
{
  if (memcmp(usr->H_AMK, bytes_HAMK, hash_length(usr->alg)) == 0)
    usr->authenticated = 1;
}


/* ----------------------- SRP for the server side -------------------------- */

static int
srp_create_salted_verification_key(enum hash_alg alg,
                                   SRP_NGType ng_type, const char *username,
                                   const unsigned char *password, int len_password,
                                   unsigned char **bytes_s, int *len_s, 
                                   unsigned char **bytes_v, int *len_v,
                                   const char *n_hex, const char *g_hex )
{
  bnum s, v, x;
  NGConstant *ng;

  bnum_new(s);
  bnum_new(v);
  x = NULL;

  ng = new_ng(ng_type, n_hex, g_hex);

  *bytes_s = NULL;
  *bytes_v = NULL;

  if (!s || !v || !ng)
    goto error;

  bnum_random(s, 128); // MODIFIED from csrp's BN_rand(s, 32, -1, 0)
#ifdef DEBUG_PAIR
  bnum_dump("Random value of s:\n", s);
#endif

  x = calculate_x(alg, s, username, password, len_password);
  if (!x)
    goto error;

  bnum_modexp(v, ng->g, x, ng->N);

  *len_s = bnum_num_bytes(s);
  *len_v = bnum_num_bytes(v);

  *bytes_s = malloc(*len_s);
  *bytes_v = malloc(*len_v);
  if (!(*bytes_s) || !(*bytes_v))
    goto error;

  bnum_bn2bin(s, (unsigned char *) *bytes_s, *len_s);
  bnum_bn2bin(v, (unsigned char *) *bytes_v, *len_v);

  free_ng(ng);
  bnum_free(s);
  bnum_free(v);
  bnum_free(x);
  return 0;

 error:
  free(*bytes_s);
  free(*bytes_v);
  free_ng(ng);
  bnum_free(s);
  bnum_free(v);
  bnum_free(x);
  return -1;
}

static int
srp_verifier_start_authentication(enum hash_alg alg, SRP_NGType ng_type,
                                  const unsigned char *bytes_v, int len_v,
                                  unsigned char **bytes_b, int *len_b,
                                  unsigned char **bytes_B, int *len_B,
                                  const char *n_hex, const char *g_hex)
{
  bnum v, k, b, B, tmp1, tmp2;
  NGConstant *ng;

  v = NULL;
  k = NULL;
  bnum_new(b);
  bnum_new(B);
  bnum_new(tmp1);
  bnum_new(tmp2);

  *len_b   = 0;
  *bytes_b = NULL;
  *len_B   = 0;
  *bytes_B = NULL;

  ng = new_ng(ng_type, n_hex, g_hex);

  if (!b || !B || !tmp1 || !tmp2 || !ng)
    goto error;

  bnum_bin2bn(v, bytes_v, len_v);

  bnum_random(b, 256); // MODIFIED from BN_rand(b, 256, -1, 0)
#ifdef DEBUG_PAIR
  bnum_dump("Random value of b:\n", b);
#endif

  k = H_nn_pad(alg, ng->N, ng->g, ng->N_len); // MODIFIED from H_nn(alg, ng->N, ng->g)
  if (!k)
    goto error;

  // B = kv + g^b
  bnum_mul(tmp1, k, v);
  bnum_modexp(tmp2, ng->g, b, ng->N);
  bnum_modadd(B, tmp1, tmp2, ng->N);

  *len_B = bnum_num_bytes(B);
  *len_b = bnum_num_bytes(b);

  *bytes_B = malloc(*len_B);
  *bytes_b = malloc(*len_b);
  if (!(*bytes_B) || !(*bytes_b))
    goto error;

  bnum_bn2bin(B, (unsigned char *) *bytes_B, *len_B);
  bnum_bn2bin(b, (unsigned char *) *bytes_b, *len_b);

  bnum_free(b);
  bnum_free(B);
  bnum_free(v);
  bnum_free(k);
  bnum_free(tmp1);
  bnum_free(tmp2);
  free_ng(ng);
  return 0;

 error:
  free(*bytes_B);
  free(*bytes_b);
  bnum_free(b);
  bnum_free(B);
  bnum_free(v);
  bnum_free(k);
  bnum_free(tmp1);
  bnum_free(tmp2);
  free_ng(ng);
  return -1;
}

static void
srp_verifier_free(struct SRPVerifier *ver)
{
  if (!ver)
    return;

  free_ng(ver->ng);

  free(ver->username);

  memset(ver, 0, sizeof(struct SRPVerifier));
  free(ver);
}

static struct SRPVerifier *
srp_verifier_new(enum hash_alg alg, SRP_NGType ng_type, const char *username,
                 const unsigned char *bytes_s, int len_s,
                 const unsigned char *bytes_v, int len_v,
                 const unsigned char *bytes_A, int len_A,
                 const unsigned char *bytes_b, int len_b,
                 const unsigned char *bytes_B, int len_B,
                 const char *n_hex, const char *g_hex )
{
  struct SRPVerifier *ver = NULL;
  bnum s, v, A, b, B, S, tmp1, tmp2, u, k;
  NGConstant *ng;
  size_t ulen;

  bnum_bin2bn(s, bytes_s, len_s);
  bnum_bin2bn(v, bytes_v, len_v);
  bnum_bin2bn(A, bytes_A, len_A);
  bnum_bin2bn(b, bytes_b, len_b);
  bnum_bin2bn(B, bytes_B, len_B);
  bnum_new(S);
  bnum_new(tmp1);
  bnum_new(tmp2);
  u = NULL;
  k = NULL;

  ng = new_ng(ng_type, n_hex, g_hex);

  if (!s || !v || !A || !B || !S || !b || !tmp1 || !tmp2 || !ng)
    goto error;

  ver = calloc(1, sizeof(struct SRPVerifier));
  if (!ver)
    goto error;

  ulen = strlen(username) + 1;

  ver->alg = alg;
  ver->ng  = ng;

  ver->username = malloc(ulen);
  if (!ver->username)
    goto error;

  memcpy(ver->username, username, ulen);

  ver->authenticated = 0;

  // SRP-6a safety check
  bnum_mod(tmp1, A, ng->N);
  if (bnum_is_zero(tmp1))
    goto error;

  k = H_nn_pad(alg, ng->N, ng->g, ng->N_len); // MODIFIED from H_nn(alg, ng->N, ng->g)
  u = H_nn_pad(alg, A, B, ng->N_len); // MODIFIED from H_nn(alg, A, B)

  // S = (A *(v^u)) ^ b
  bnum_modexp(tmp1, v, u, ng->N);
  bnum_mul(tmp2, A, tmp1);
  bnum_modexp(S, tmp2, b, ng->N);

  hash_num(alg, S, ver->session_key);
  ver->session_key_len = hash_length(ver->alg);

  calculate_M(alg, ng, ver->M, username, s, A, B, ver->session_key, ver->session_key_len);
  calculate_H_AMK(alg, ver->H_AMK, A, ver->M, ver->session_key, ver->session_key_len);

  ver->bytes_B = bytes_B;

  bnum_free(s);
  bnum_free(v);
  bnum_free(A);
  bnum_free(u);
  bnum_free(k);
  bnum_free(B);
  bnum_free(S);
  bnum_free(b);
  bnum_free(tmp1);
  bnum_free(tmp2);
  return ver;

 error:
  srp_verifier_free(ver);
  bnum_free(s);
  bnum_free(v);
  bnum_free(A);
  bnum_free(u);
  bnum_free(k);
  bnum_free(B);
  bnum_free(S);
  bnum_free(b);
  bnum_free(tmp1);
  bnum_free(tmp2);
  return NULL;
}

// user_M must be exactly SHA512_DIGEST_LENGTH bytes in size
static void
srp_verifier_verify_session(struct SRPVerifier *ver, const unsigned char *user_M, const unsigned char **bytes_HAMK)
{
  if (memcmp(ver->M, user_M, hash_length(ver->alg)) == 0)
    {
      ver->authenticated = 1;
      *bytes_HAMK = ver->H_AMK;
    }
  else
    *bytes_HAMK = NULL;
}

static const unsigned char *
srp_verifier_get_session_key(struct SRPVerifier *ver, int *key_length)
{
  if (key_length)
    *key_length = hash_length(ver->alg);

  return ver->session_key;
}

/* -------------------------------- HELPERS --------------------------------- */

static void
hexread(uint8_t *out, size_t out_len, const char *in)
{
  char hex[] = { 0, 0, 0 };
  int i;

  for (i = 0; i < out_len; i++, in+=2)
    {
      hex[0] = in[0];
      hex[1] = in[1];
      out[i] = strtol(hex, NULL, 16);
    }
}

static pair_tlv_values_t *
message_process(const uint8_t *data, size_t data_len, const char **errmsg)
{
  pair_tlv_values_t *response;
  pair_tlv_t *error;
  int ret;

  response = pair_tlv_new();
  if (!response)
    {
      *errmsg = "Out of memory\n";
      return NULL;
    }

  ret = pair_tlv_parse(data, data_len, response);
  if (ret < 0)
    {
      *errmsg = "Could not parse TLV\n";
      goto error;
    }

#ifdef DEBUG_PAIR
  pair_tlv_debug(response);
#endif

  error = pair_tlv_get_value(response, TLVType_Error);
  if (error)
    {
      if (error->value[0] == TLVError_Authentication)
	*errmsg = "Device returned an authentication failure";
      else if (error->value[0] == TLVError_Backoff)
	*errmsg = "Device told us to back off pairing attempts\n";
      else if (error->value[0] == TLVError_MaxPeers)
	*errmsg = "Max peers trying to connect to device\n";
      else if (error->value[0] == TLVError_MaxTries)
	*errmsg = "Max pairing attemps reached\n";
      else if (error->value[0] == TLVError_Unavailable)
	*errmsg = "Device is unuavailble at this time\n";
      else
	*errmsg = "Device is busy/returned unknown error\n";

      goto error;
    }

  return response;

 error:
  pair_tlv_free(response);
  return NULL;
}

/* Executes SHA512 RFC 5869 extract + expand, writing a derived key to okm

   hkdfExtract(SHA512, salt, salt_len, ikm, ikm_len, prk);
   hkdfExpand(SHA512, prk, SHA512_LEN, info, info_len, okm, okm_len);
*/
static int
hkdf_extract_expand(uint8_t *okm, size_t okm_len, const uint8_t *ikm, size_t ikm_len, enum pair_keys pair_key)
{
#ifdef CONFIG_OPENSSL
#include <openssl/kdf.h>
  EVP_PKEY_CTX *pctx;

  if (okm_len > SHA512_DIGEST_LENGTH)
    return -1;
  if (! (pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL)))
    return -1;
  if (EVP_PKEY_derive_init(pctx) <= 0)
    goto error;
  if (EVP_PKEY_CTX_set_hkdf_md(pctx, EVP_sha512()) <= 0)
    goto error;
  if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, (const unsigned char *)pair_keys_map[pair_key].salt, strlen(pair_keys_map[pair_key].salt)) <= 0)
    goto error;
  if (EVP_PKEY_CTX_set1_hkdf_key(pctx, ikm, ikm_len) <= 0)
    goto error;
  if (EVP_PKEY_CTX_add1_hkdf_info(pctx, (const unsigned char *)pair_keys_map[pair_key].info, strlen(pair_keys_map[pair_key].info)) <= 0)
    goto error;
  if (EVP_PKEY_derive(pctx, okm, &okm_len) <= 0)
    goto error;

  EVP_PKEY_CTX_free(pctx);
  return 0;

 error:
  EVP_PKEY_CTX_free(pctx);
  return -1;
#elif CONFIG_GCRYPT
  uint8_t prk[SHA512_DIGEST_LENGTH];
  gcry_md_hd_t hmac_handle;

  if (okm_len > SHA512_DIGEST_LENGTH)
    return -1; // Below calculation not valid if output is larger than hash size
  if (gcry_md_open(&hmac_handle, GCRY_MD_SHA512, GCRY_MD_FLAG_HMAC) != GPG_ERR_NO_ERROR)
    return -1;
  if (gcry_md_setkey(hmac_handle, (const unsigned char *)pair_keys_map[pair_key].salt, strlen(pair_keys_map[pair_key].salt)) != GPG_ERR_NO_ERROR)
    goto error;
  gcry_md_write(hmac_handle, ikm, ikm_len);
  memcpy(prk, gcry_md_read(hmac_handle, 0), sizeof(prk));

  gcry_md_reset(hmac_handle);

  if (gcry_md_setkey(hmac_handle, prk, sizeof(prk)) != GPG_ERR_NO_ERROR)
    goto error;
  gcry_md_write(hmac_handle, (const unsigned char *)pair_keys_map[pair_key].info, strlen(pair_keys_map[pair_key].info));
  gcry_md_putc(hmac_handle, 1);

  memcpy(okm, gcry_md_read(hmac_handle, 0), okm_len);

  gcry_md_close(hmac_handle);
  return 0;

 error:
  gcry_md_close(hmac_handle);
  return -1;
#else
  return -1;
#endif
}

static int
encrypt_chacha(uint8_t *cipher, const uint8_t *plain, size_t plain_len, const uint8_t *key, size_t key_len, const void *ad, size_t ad_len, uint8_t *tag, size_t tag_len, const uint8_t nonce[NONCE_LENGTH])
{
#ifdef CONFIG_OPENSSL
  EVP_CIPHER_CTX *ctx;
  int len;

  if (! (ctx = EVP_CIPHER_CTX_new()))
    return -1;

  if (EVP_EncryptInit_ex(ctx, EVP_chacha20_poly1305(), NULL, key, nonce) != 1)
    goto error;

  if (EVP_CIPHER_CTX_set_padding(ctx, 0) != 1) // Maybe not necessary
    goto error;

  if (ad_len > 0 && EVP_EncryptUpdate(ctx, NULL, &len, ad, ad_len) != 1)
    goto error;

  if (EVP_EncryptUpdate(ctx, cipher, &len, plain, plain_len) != 1)
    goto error;

  assert(len == plain_len);

  if (EVP_EncryptFinal_ex(ctx, NULL, &len) != 1)
    goto error;

  if (EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_GET_TAG, tag_len, tag) != 1)
    goto error;

  EVP_CIPHER_CTX_free(ctx);
  return 0;

 error:
  EVP_CIPHER_CTX_free(ctx);
  return -1;
#elif CONFIG_GCRYPT
  gcry_cipher_hd_t hd;

  if (gcry_cipher_open(&hd, GCRY_CIPHER_CHACHA20, GCRY_CIPHER_MODE_POLY1305, 0) != GPG_ERR_NO_ERROR)
    return -1;

  if (gcry_cipher_setkey(hd, key, key_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_setiv(hd, nonce, NONCE_LENGTH) != GPG_ERR_NO_ERROR)
    goto error;

  if (ad_len > 0 && gcry_cipher_authenticate(hd, ad, ad_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_encrypt(hd, cipher, plain_len, plain, plain_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_gettag(hd, tag, tag_len) != GPG_ERR_NO_ERROR)
    goto error;

  gcry_cipher_close(hd);
  return 0;

 error:
  gcry_cipher_close(hd);
  return -1;
#else
  return -1;
#endif
}

static int
decrypt_chacha(uint8_t *plain, const uint8_t *cipher, size_t cipher_len, const uint8_t *key, size_t key_len, const void *ad, size_t ad_len, uint8_t *tag, size_t tag_len, const uint8_t nonce[NONCE_LENGTH])
{
#ifdef CONFIG_OPENSSL
  EVP_CIPHER_CTX *ctx;
  int len;

  if (! (ctx = EVP_CIPHER_CTX_new()))
    return -1;

  if (EVP_DecryptInit_ex(ctx, EVP_chacha20_poly1305(), NULL, key, nonce) != 1)
    goto error;

  if (EVP_CIPHER_CTX_set_padding(ctx, 0) != 1) // Maybe not necessary
    goto error;

  if (EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_AEAD_SET_TAG, tag_len, tag) != 1)
    goto error;

  if (ad_len > 0 && EVP_DecryptUpdate(ctx, NULL, &len, ad, ad_len) != 1)
    goto error;

  if (EVP_DecryptUpdate(ctx, plain, &len, cipher, cipher_len) != 1)
    goto error;

  if (EVP_DecryptFinal_ex(ctx, NULL, &len) != 1)
    goto error;

  EVP_CIPHER_CTX_free(ctx);
  return 0;

 error:
  EVP_CIPHER_CTX_free(ctx);
  return -1;
#elif CONFIG_GCRYPT
  gcry_cipher_hd_t hd;

  if (gcry_cipher_open(&hd, GCRY_CIPHER_CHACHA20, GCRY_CIPHER_MODE_POLY1305, 0) != GPG_ERR_NO_ERROR)
    return -1;

  if (gcry_cipher_setkey(hd, key, key_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_setiv(hd, nonce, NONCE_LENGTH) != GPG_ERR_NO_ERROR)
    goto error;

  if (ad_len > 0 && gcry_cipher_authenticate(hd, ad, ad_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_decrypt(hd, plain, cipher_len, cipher, cipher_len) != GPG_ERR_NO_ERROR)
    goto error;

  if (gcry_cipher_checktag(hd, tag, tag_len) != GPG_ERR_NO_ERROR)
    goto error;

  gcry_cipher_close(hd);
  return 0;

 error:
  gcry_cipher_close(hd);
  return -1;
#else
  return -1;
#endif
}

static int
create_info(uint8_t *info, size_t *info_len, uint8_t *a, size_t a_len, uint8_t *b, size_t b_len, uint8_t *c, size_t c_len)
{
  if (a_len + b_len + c_len > *info_len)
    return -1;

  *info_len = a_len + b_len + c_len;
  memcpy(info, a, a_len);
  memcpy(info + a_len, b, b_len);
  memcpy(info + a_len + b_len, c, c_len);

  return 0;
}

static int
create_and_sign_device_info(uint8_t *msg, size_t *msg_len, const char *device_id, uint8_t *device_x, size_t device_x_len, uint8_t *pk, size_t pk_len, uint8_t *sk)
{
  pair_tlv_values_t *tlv;
  uint8_t device_info[256];
  size_t device_info_len;
  size_t device_id_len;
  uint8_t signature[crypto_sign_BYTES];
  int ret;

  device_id_len = strlen(device_id);
  device_info_len = sizeof(device_info);

  ret = create_info(device_info, &device_info_len, device_x, device_x_len, (uint8_t *)device_id, device_id_len, pk, pk_len);
  if (ret < 0)
    return -1;

  crypto_sign_detached(signature, NULL, device_info, device_info_len, sk);

  tlv = pair_tlv_new();
  pair_tlv_add_value(tlv, TLVType_Identifier, (unsigned char *)device_id, device_id_len);
  pair_tlv_add_value(tlv, TLVType_Signature, signature, sizeof(signature));

  ret = pair_tlv_format(tlv, msg, msg_len);

  pair_tlv_free(tlv);
  return ret;
}

static int
create_and_sign_accessory_info(uint8_t *msg, size_t *msg_len, uint8_t *server_pk, size_t server_pk_len, const char *accessory_id, uint8_t *client_pk, size_t client_pk_len, uint8_t *sk)
{
  pair_tlv_values_t *tlv;
  uint8_t accessory_info[256];
  size_t accessory_info_len;
  size_t accessory_id_len;
  uint8_t signature[crypto_sign_BYTES];
  int ret;

  accessory_id_len = strlen(accessory_id);
  accessory_info_len = sizeof(accessory_info);

  ret = create_info(accessory_info, &accessory_info_len, server_pk, server_pk_len, (uint8_t *)accessory_id, accessory_id_len, client_pk, client_pk_len);
  if (ret < 0)
    return -1;

  crypto_sign_detached(signature, NULL, accessory_info, accessory_info_len, sk);

  tlv = pair_tlv_new();
  pair_tlv_add_value(tlv, TLVType_Identifier, (unsigned char *)accessory_id, accessory_id_len);
  pair_tlv_add_value(tlv, TLVType_Signature, signature, sizeof(signature));

  ret = pair_tlv_format(tlv, msg, msg_len);

  pair_tlv_free(tlv);
  return ret;
}

static int
verify_info(uint8_t *signature, uint8_t *pk, uint8_t *a, size_t a_len, uint8_t *b, size_t b_len, uint8_t *c, size_t c_len)
{
  uint8_t info[256];
  size_t info_len;
  int ret;

  info_len = sizeof(info);
  ret = create_info(info, &info_len, a, a_len, b, b_len, c, c_len);
  if (ret < 0)
    return -1;

  return crypto_sign_verify_detached(signature, info, info_len, pk);
}


/* ------------------------- CLIENT IMPLEMENTATION -------------------------- */

static int
client_setup_new(struct pair_setup_context *handle, const char *pin, pair_cb add_cb, void *cb_arg, const char *device_id)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;

  if (!is_initialized())
    return -1;

  if (!pin && handle->type == &pair_client_homekit_transient)
    pin = "3939";
  else if (!pin)
    return -1;

  if (device_id && strlen(device_id) >= PAIR_AP_DEVICE_ID_LEN_MAX)
    return -1;

  sctx->pin = strdup(pin);
  if (!sctx->pin)
    return -1;

  sctx->add_cb = add_cb;
  sctx->add_cb_arg = cb_arg;

  if (device_id)
    memcpy(sctx->device_id, device_id, strlen(device_id));

  crypto_sign_keypair(sctx->public_key, sctx->private_key);

#ifdef DEBUG_PAIR
  hexdump("Client public key:\n", sctx->public_key, sizeof(sctx->public_key));
  hexdump("Client private key:\n", sctx->private_key, sizeof(sctx->private_key));
#endif

  return 0;
}

static void
client_setup_free(struct pair_setup_context *handle)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;

  srp_user_free(sctx->user);

  free(sctx->pkB);
  free(sctx->M2);
  free(sctx->salt);
  free(sctx->epk);
  free(sctx->authtag);
  free(sctx->pin);
}

static uint8_t *
client_setup_request1(size_t *len, struct pair_setup_context *handle)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *request;
  uint8_t *data;
  size_t data_len;
  uint8_t method;
  uint8_t flags;
  int endian_test = 1;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  request = pair_tlv_new();

  // Test here instead of setup_new() so we can give an error message
  if(*(char *)&endian_test != 1)
    {
      handle->errmsg = "Setup request 1: No support for big endian architechture";
      goto error;
    }

  sctx->user = srp_user_new(HASH_SHA512, SRP_NG_3072, USERNAME, (unsigned char *)sctx->pin, strlen(sctx->pin), 0, 0);
  if (!sctx->user)
    {
      handle->errmsg = "Setup request 1: Create SRP user failed";
      goto error;
    }

  method = PairingMethodPairSetup;
  pair_tlv_add_value(request, TLVType_State, &pair_keys_map[PAIR_SETUP_MSG01].state, sizeof(pair_keys_map[PAIR_SETUP_MSG01].state));
  pair_tlv_add_value(request, TLVType_Method, &method, sizeof(method));

  if (handle->type == &pair_client_homekit_transient)
    {
      flags = PairingFlagsTransient;
      pair_tlv_add_value(request, TLVType_Flags, &flags, sizeof(flags));
    }

  ret = pair_tlv_format(request, data, &data_len);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 1: pair_tlv_format returned an error";
      goto error;
    }

  *len = data_len;

  pair_tlv_free(request);
  return data;

 error:
  pair_tlv_free(request);
  free(data);
  return NULL;
}

static uint8_t *
client_setup_request2(size_t *len, struct pair_setup_context *handle)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *request;
  uint8_t *data;
  size_t data_len;
  const char *auth_username = NULL;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  request = pair_tlv_new();

  // Calculate A
  srp_user_start_authentication(sctx->user, &auth_username, &sctx->pkA, &sctx->pkA_len);

  // Calculate M1 (client proof)
  srp_user_process_challenge(sctx->user, (const unsigned char *)sctx->salt, sctx->salt_len, (const unsigned char *)sctx->pkB, sctx->pkB_len, &sctx->M1, &sctx->M1_len);

  pair_tlv_add_value(request, TLVType_State, &pair_keys_map[PAIR_SETUP_MSG03].state, sizeof(pair_keys_map[PAIR_SETUP_MSG03].state));
  pair_tlv_add_value(request, TLVType_PublicKey, sctx->pkA, sctx->pkA_len);
  pair_tlv_add_value(request, TLVType_Proof, sctx->M1, sctx->M1_len);

  ret = pair_tlv_format(request, data, &data_len);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 2: pair_tlv_format returned an error";
      goto error;
    }

  *len = data_len;

  pair_tlv_free(request);
  return data;

 error:
  pair_tlv_free(request);
  free(data);
  return NULL;
}

static uint8_t *
client_setup_request3(size_t *len, struct pair_setup_context *handle)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *request;
  uint8_t *data;
  size_t data_len;
  const unsigned char *session_key;
  int session_key_len;
  uint8_t device_x[32];
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  pair_tlv_values_t *append;
  size_t append_len;
  uint8_t *encrypted_data = NULL;
  size_t encrypted_data_len;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  request = pair_tlv_new();

  session_key = srp_user_get_session_key(sctx->user, &session_key_len);
  if (!session_key)
    {
      handle->errmsg = "Setup request 3: No valid session key";
      goto error;
    }

  ret = hkdf_extract_expand(device_x, sizeof(device_x), session_key, session_key_len, PAIR_SETUP_CONTROLLER_SIGN);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: hkdf error getting device_x";
      goto error;
    }

  ret = create_and_sign_device_info(data, &data_len, sctx->device_id, device_x, sizeof(device_x), sctx->public_key, sizeof(sctx->public_key), sctx->private_key);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: error creating signed device info";
      goto error;
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), session_key, session_key_len, PAIR_SETUP_MSG05);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: hkdf error getting derived_key";
      goto error;
    }

  // Append TLV-encoded public key to *data, which already has identifier and signature
  append = pair_tlv_new();
  append_len = REQUEST_BUFSIZE - data_len;
  pair_tlv_add_value(append, TLVType_PublicKey, sctx->public_key, sizeof(sctx->public_key));
  ret = pair_tlv_format(append, data + data_len, &append_len);
  pair_tlv_free(append);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: error appending public key to TLV";
      goto error;
    }
  data_len += append_len;

  memcpy(nonce + 4, pair_keys_map[PAIR_SETUP_MSG05].nonce, NONCE_LENGTH - 4);

  encrypted_data_len = data_len + sizeof(tag); // Space for ciphered payload and authtag
  encrypted_data = malloc(encrypted_data_len);

  ret = encrypt_chacha(encrypted_data, data, data_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: Could not encrypt";
      goto error;
    }

  memcpy(encrypted_data + data_len, tag, sizeof(tag));

  pair_tlv_add_value(request, TLVType_State, &pair_keys_map[PAIR_SETUP_MSG05].state, sizeof(pair_keys_map[PAIR_SETUP_MSG05].state));
  pair_tlv_add_value(request, TLVType_EncryptedData, encrypted_data, encrypted_data_len);

  data_len = REQUEST_BUFSIZE; // Re-using *data, so pass original length to pair_tlv_format
  ret = pair_tlv_format(request, data, &data_len);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: error appending public key to TLV";
      goto error;
    }

  *len = data_len;

  free(encrypted_data);
  pair_tlv_free(request);
  return data;

 error:
  free(encrypted_data);
  pair_tlv_free(request);
  free(data);
  return NULL;
}

static int
client_setup_response1(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *response;
  pair_tlv_t *pk;
  pair_tlv_t *salt;

  response = message_process(data, data_len, &handle->errmsg);
  if (!response)
    {
      return -1;
    }

  pk = pair_tlv_get_value(response, TLVType_PublicKey);
  if (!pk || pk->size > N_len(SRP_NG_3072)) // max 384 bytes
    {
      handle->errmsg = "Setup response 1: Missing or invalid public key";
      goto error;
    }

  salt = pair_tlv_get_value(response, TLVType_Salt);
  if (!salt || salt->size != 16)
    {
      handle->errmsg = "Setup response 1: Missing or invalid salt";
      goto error;
    }

  sctx->pkB_len = pk->size;
  sctx->pkB = malloc(sctx->pkB_len);
  memcpy(sctx->pkB, pk->value, sctx->pkB_len);

  sctx->salt_len = salt->size;
  sctx->salt = malloc(sctx->salt_len);
  memcpy(sctx->salt, salt->value, sctx->salt_len);

  pair_tlv_free(response);
  return 0;

 error:
  pair_tlv_free(response);
  return -1;
}

static int
client_setup_response2(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *response;
  pair_tlv_t *proof;
  const uint8_t *session_key;
  int session_key_len;

  response = message_process(data, data_len, &handle->errmsg);
  if (!response)
    {
      return -1;
    }

  proof = pair_tlv_get_value(response, TLVType_Proof);
  if (!proof || proof->size != SHA512_DIGEST_LENGTH)
    {
      handle->errmsg = "Setup response 2: Missing or invalid proof";
      goto error;
    }

  sctx->M2_len = proof->size;
  sctx->M2 = malloc(sctx->M2_len);
  memcpy(sctx->M2, proof->value, sctx->M2_len);

  // Check M2
  srp_user_verify_session(sctx->user, (const unsigned char *)sctx->M2);
  if (!srp_user_is_authenticated(sctx->user))
    {
      handle->errmsg = "Setup response 2: Server authentication failed";
      goto error;
    }

  if (handle->type == &pair_client_homekit_transient)
    {
      session_key = srp_user_get_session_key(sctx->user, &session_key_len);
      if (!session_key)
        {
	  handle->errmsg = "Setup response 2: Could not compute session key";
	  goto error;
	}

      assert(sizeof(handle->result.shared_secret) >= session_key_len);

      memcpy(handle->result.shared_secret, session_key, session_key_len);
      handle->result.shared_secret_len = session_key_len;

      handle->status = PAIR_STATUS_COMPLETED;
    }

  pair_tlv_free(response);
  return 0;

 error:
  pair_tlv_free(response);
  return -1;
}

static int
client_setup_response3(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_client_setup_context *sctx = &handle->sctx.client;
  pair_tlv_values_t *response;
  pair_tlv_t *encrypted_data;
  pair_tlv_t *device_id;
  pair_tlv_t *pk;
  pair_tlv_t *signature;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  size_t encrypted_len;
  uint8_t *decrypted_data = NULL;
  const uint8_t *session_key;
  int session_key_len;
  uint8_t device_x[32];
  int ret;

  response = message_process(data, data_len, &handle->errmsg);
  if (!response)
    {
      return -1;
    }

  encrypted_data = pair_tlv_get_value(response, TLVType_EncryptedData);
  if (!encrypted_data)
    {
      handle->errmsg = "Setup response 3: Missing encrypted_data";
      goto error;
    }

  session_key = srp_user_get_session_key(sctx->user, &session_key_len);
  if (!session_key)
    {
      handle->errmsg = "Setup response 3: No valid session key";
      goto error;
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), session_key, session_key_len, PAIR_SETUP_MSG06);
  if (ret < 0)
    {
      handle->errmsg = "Setup response 3: hkdf error getting derived_key";
      goto error;
    }

  // encrypted_data->value consists of the encrypted payload + the auth tag
  if (encrypted_data->size < AUTHTAG_LENGTH)
    {
      handle->errmsg = "Setup response 3: Invalid encrypted data";
      goto error;
    }

  encrypted_len = encrypted_data->size - AUTHTAG_LENGTH;
  memcpy(tag, encrypted_data->value + encrypted_len, AUTHTAG_LENGTH);
  memcpy(nonce + 4, pair_keys_map[PAIR_SETUP_MSG06].nonce, NONCE_LENGTH - 4);

  decrypted_data = malloc(encrypted_len);

  ret = decrypt_chacha(decrypted_data, encrypted_data->value, encrypted_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      handle->errmsg = "Setup response 3: Decryption error";
      goto error;
    }

  pair_tlv_free(response);
  response = message_process(decrypted_data, encrypted_len, &handle->errmsg);
  if (!response)
    {
      goto error;
    }

  ret = hkdf_extract_expand(device_x, sizeof(device_x), session_key, session_key_len, PAIR_SETUP_ACCESSORY_SIGN);
  if (ret < 0)
    {
      handle->errmsg = "Setup response 3: hkdf error getting device_x";
      goto error;
    }

  device_id = pair_tlv_get_value(response, TLVType_Identifier);
  pk = pair_tlv_get_value(response, TLVType_PublicKey);
  signature = pair_tlv_get_value(response, TLVType_Signature);
  if (!device_id || device_id->size >= sizeof(handle->result.device_id) || !pk || pk->size != crypto_sign_PUBLICKEYBYTES || !signature || signature->size != crypto_sign_BYTES)
    {
      handle->errmsg = "Setup response 3: Missing/invalid device ID, public key or signature";
      goto error;
    }

  ret = verify_info(signature->value, pk->value, device_x, sizeof(device_x), device_id->value, device_id->size, pk->value, pk->size);
  if (ret < 0)
    {
      handle->errmsg = "Setup request 3: Invalid signature";
      goto error;
    }

  assert(sizeof(handle->result.client_private_key) == sizeof(sctx->private_key));
  assert(sizeof(handle->result.client_public_key) == sizeof(sctx->public_key));
  assert(sizeof(handle->result.server_public_key) == pk->size);

  memcpy(handle->result.client_private_key, sctx->private_key, sizeof(sctx->private_key));
  memcpy(handle->result.client_public_key, sctx->public_key, sizeof(sctx->public_key));
  memcpy(handle->result.server_public_key, pk->value, pk->size);
  memcpy(handle->result.device_id, device_id->value, device_id->size);

  if (sctx->add_cb)
    sctx->add_cb(handle->result.server_public_key, handle->result.device_id, sctx->add_cb_arg);

  handle->status = PAIR_STATUS_COMPLETED;

  free(decrypted_data);
  pair_tlv_free(response);
  return 0;

 error:
  free(decrypted_data);
  pair_tlv_free(response);
  return -1;
}

static int
client_setup_result(struct pair_setup_context *handle)
{
  char *ptr;
  int i;

  assert(sizeof(handle->result_str) >= 2 * sizeof(handle->result.client_private_key) + 2 * sizeof(handle->result.server_public_key) + 1);

  // It is enough to export the private key, since the public key can be
  // extracted from that with crypto_sign_ed25519_sk_to_pk (it is the last 32
  // bytes)
  ptr = handle->result_str;
  for (i = 0; i < sizeof(handle->result.client_private_key); i++)
    ptr += sprintf(ptr, "%02x", handle->result.client_private_key[i]); // 2 x 64 bytes
  for (i = 0; i < sizeof(handle->result.server_public_key); i++)
    ptr += sprintf(ptr, "%02x", handle->result.server_public_key[i]); // 2 x 32 bytes
  *ptr = '\0';

  return 0;
}

static int
client_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, pair_cb cb, void *cb_arg, const char *device_id)
{
  struct pair_client_verify_context *vctx = &handle->vctx.client;
  size_t hexkey_len;

  if (!is_initialized())
    return -1;

  if (!device_id || strlen(device_id) >= PAIR_AP_DEVICE_ID_LEN_MAX)
    return -1;

  if (!client_setup_keys)
    return -1;

  hexkey_len = strlen(client_setup_keys);
  if (hexkey_len == 2 * sizeof(vctx->client_private_key) + 2 * sizeof(vctx->server_public_key))
    {
      hexread(vctx->client_private_key, sizeof(vctx->client_private_key), client_setup_keys);
      hexread(vctx->server_public_key, sizeof(vctx->server_public_key), client_setup_keys + 2 * sizeof(vctx->client_private_key));
      vctx->verify_server_signature = true;
    }
  else if (hexkey_len == 2 * sizeof(vctx->client_private_key)) // No server public key known, so signature validation will be skipped
    {
      hexread(vctx->client_private_key, sizeof(vctx->client_private_key), client_setup_keys);
    }
  else
    return -1;

  crypto_sign_ed25519_sk_to_pk(vctx->client_public_key, vctx->client_private_key);

  snprintf(vctx->device_id, sizeof(vctx->device_id), "%s", device_id);

  return 0;
}

static uint8_t *
client_verify_request1(size_t *len, struct pair_verify_context *handle)
{
  struct pair_client_verify_context *vctx = &handle->vctx.client;
//  const uint8_t basepoint[crypto_scalarmult_BYTES] = {9}; // 32 bytes
  pair_tlv_values_t *request;
  uint8_t *data;
  size_t data_len;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  request = pair_tlv_new();

  crypto_box_keypair(vctx->client_eph_public_key, vctx->client_eph_private_key);

/*
  // TODO keep around in case box_keypair doesn't work
  ret = crypto_scalarmult(vctx->client_eph_public_key, vctx->client_eph_private_key, basepoint);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 1: Curve 25519 returned an error";
      goto error;
    }
*/

  pair_tlv_add_value(request, TLVType_State, &pair_keys_map[PAIR_VERIFY_MSG01].state, sizeof(pair_keys_map[PAIR_VERIFY_MSG01].state));
  pair_tlv_add_value(request, TLVType_PublicKey, vctx->client_eph_public_key, sizeof(vctx->client_eph_public_key));

  ret = pair_tlv_format(request, data, &data_len);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 1: pair_tlv_format returned an error";
      goto error;
    }

  *len = data_len;

  pair_tlv_free(request);
  return data;

 error:
  pair_tlv_free(request);
  free(data);
  return NULL;
}

static uint8_t *
client_verify_request2(size_t *len, struct pair_verify_context *handle)
{
  struct pair_client_verify_context *vctx = &handle->vctx.client;
  pair_tlv_values_t *request;
  uint8_t *data;
  size_t data_len;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  uint8_t *encrypted_data = NULL;
  size_t encrypted_data_len;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  request = pair_tlv_new();

  ret = create_and_sign_device_info(data, &data_len, vctx->device_id, vctx->client_eph_public_key, sizeof(vctx->client_eph_public_key),
                                    vctx->server_eph_public_key, sizeof(vctx->server_eph_public_key), vctx->client_private_key);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 2: error creating signed device info";
      goto error;
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), vctx->shared_secret, sizeof(vctx->shared_secret), PAIR_VERIFY_MSG03);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 2: hkdf error getting derived_key";
      goto error;
    }

  memcpy(nonce + 4, pair_keys_map[PAIR_VERIFY_MSG03].nonce, NONCE_LENGTH - 4);

  encrypted_data_len = data_len + sizeof(tag); // Space for ciphered payload and authtag
  encrypted_data = malloc(encrypted_data_len);

  ret = encrypt_chacha(encrypted_data, data, data_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 2: Could not encrypt";
      goto error;
    }

  memcpy(encrypted_data + data_len, tag, sizeof(tag));

  pair_tlv_add_value(request, TLVType_State, &pair_keys_map[PAIR_VERIFY_MSG03].state, sizeof(pair_keys_map[PAIR_VERIFY_MSG03].state));
  pair_tlv_add_value(request, TLVType_EncryptedData, encrypted_data, encrypted_data_len);

  data_len = REQUEST_BUFSIZE; // Re-using *data, so pass original length to pair_tlv_format
  ret = pair_tlv_format(request, data, &data_len);
  if (ret < 0)
    {
      handle->errmsg = "Verify request 2: pair_tlv_format returned an error";
      goto error;
    }

  *len = data_len;

  free(encrypted_data);
  pair_tlv_free(request);
  return data;

 error:
  free(encrypted_data);
  pair_tlv_free(request);
  free(data);
  return NULL;
}

static int
client_verify_response1(struct pair_verify_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_client_verify_context *vctx = &handle->vctx.client;
  pair_tlv_values_t *response;
  pair_tlv_t *encrypted_data;
  pair_tlv_t *public_key;
  pair_tlv_t *device_id;
  pair_tlv_t *signature;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  size_t encrypted_len;
  uint8_t *decrypted_data = NULL;
  int ret;

  response = message_process(data, data_len, &handle->errmsg);
  if (!response)
    {
      return -1;
    }

  encrypted_data = pair_tlv_get_value(response, TLVType_EncryptedData);
  if (!encrypted_data)
    {
      handle->errmsg = "Verify response 1: Missing encrypted_data";
      goto error;
    }

  public_key = pair_tlv_get_value(response, TLVType_PublicKey);
  if (!public_key || public_key->size != sizeof(vctx->server_eph_public_key))
    {
      handle->errmsg = "Verify response 1: Missing or invalid public_key";
      goto error;
    }

  memcpy(vctx->server_eph_public_key, public_key->value, sizeof(vctx->server_eph_public_key));
  ret = crypto_scalarmult(vctx->shared_secret, vctx->client_eph_private_key, vctx->server_eph_public_key);
  if (ret < 0)
    {
      handle->errmsg = "Verify response 1: Curve 25519 returned an error";
      goto error;
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), vctx->shared_secret, sizeof(vctx->shared_secret), PAIR_VERIFY_MSG02);
  if (ret < 0)
    {
      handle->errmsg = "Verify response 1: hkdf error getting derived_key";
      goto error;
    }

  // encrypted_data->value consists of the encrypted payload + the auth tag
  if (encrypted_data->size < AUTHTAG_LENGTH)
    {
      handle->errmsg = "Verify response 1: Invalid encrypted data";
      goto error;
    }

  encrypted_len = encrypted_data->size - AUTHTAG_LENGTH;
  memcpy(tag, encrypted_data->value + encrypted_len, AUTHTAG_LENGTH);
  memcpy(nonce + 4, pair_keys_map[PAIR_VERIFY_MSG02].nonce, NONCE_LENGTH - 4);

  decrypted_data = malloc(encrypted_len);

  ret = decrypt_chacha(decrypted_data, encrypted_data->value, encrypted_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      handle->errmsg = "Verify response 1: Decryption error";
      goto error;
    }

  pair_tlv_free(response);
  response = message_process(decrypted_data, encrypted_len, &handle->errmsg);
  if (!response)
    {
      goto error;
    }

  device_id = pair_tlv_get_value(response, TLVType_Identifier);
  signature = pair_tlv_get_value(response, TLVType_Signature);
  if (!device_id || !signature || signature->size != crypto_sign_BYTES)
    {
      handle->errmsg = "Verify response 1: Missing device ID or signature";
      goto error;
    }

  if (vctx->verify_server_signature)
    {
      ret = verify_info(signature->value, vctx->server_public_key, vctx->server_eph_public_key, sizeof(vctx->server_eph_public_key),
                        device_id->value, device_id->size, vctx->client_eph_public_key, sizeof(vctx->client_eph_public_key));
      if (ret < 0)
        {
          handle->errmsg = "Verify response 1: Invalid signature";
          goto error;
        }
    }

  free(decrypted_data);
  pair_tlv_free(response);
  return 0;

 error:
  free(decrypted_data);
  pair_tlv_free(response);
  return -1;
}

static int
client_verify_response2(struct pair_verify_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_client_verify_context *vctx = &handle->vctx.client;
  pair_tlv_values_t *response;

  response = message_process(data, data_len, &handle->errmsg);
  if (!response)
    {
      return -1;
    }

  memcpy(handle->result.shared_secret, vctx->shared_secret, sizeof(vctx->shared_secret));
  handle->result.shared_secret_len = sizeof(vctx->shared_secret);

  handle->status = PAIR_STATUS_COMPLETED;

  pair_tlv_free(response);
  return 0;
}


/* ------------------------- SERVER IMPLEMENTATION -------------------------- */

// Use (unsecure) keys seeded from device_id. We need the keys to always be the
// same during pair setup and pair verify, since the client saves them after
// pair-setup 3, so that the signature in pair-verify 1 can be checked.
static void
server_keypair(uint8_t *public_key, uint8_t *private_key, const char *device_id)
{
  uint8_t seed[crypto_sign_SEEDBYTES] = { 0 };
  size_t len;

  len = strlen(device_id);
  if (len > sizeof(seed))
    len = sizeof(seed);

  memcpy(seed, device_id, len);

  crypto_sign_seed_keypair(public_key, private_key, seed);
}

static uint8_t *
server_auth_failed_response(size_t *len, enum pair_keys msg_state)
{
  pair_tlv_values_t *response;
  uint8_t *data;
  size_t data_len;
  uint8_t error = TLVError_Authentication;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));
  pair_tlv_add_value(response, TLVType_Error, &error, sizeof(error));

  pair_tlv_format(response, data, &data_len);
  pair_tlv_free(response);

  *len = data_len;

  return data;
}

static int
server_setup_new(struct pair_setup_context *handle, const char *pin, pair_cb add_cb, void *cb_arg, const char *device_id)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;

  if (!is_initialized())
    return -1;

  if (!pin)
    pin = "3939";

  if (!device_id || strlen(device_id) >= PAIR_AP_DEVICE_ID_LEN_MAX)
    return -1;

  sctx->pin = strdup(pin);
  if (!sctx->pin)
    return -1;

  sctx->add_cb = add_cb;
  sctx->add_cb_arg = cb_arg;

  snprintf(sctx->device_id, sizeof(sctx->device_id), "%s", device_id);

  server_keypair(sctx->public_key, sctx->private_key, sctx->device_id);

  return 0;
}

static void
server_setup_free(struct pair_setup_context *handle)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;

  srp_verifier_free(sctx->verifier);

  free(sctx->pkA);
  free(sctx->pkB);
  free(sctx->b);
  free(sctx->M1);
  free(sctx->v);
  free(sctx->salt);
  free(sctx->pin);
}

static int
server_setup_request1(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
//  enum pair_keys msg_state = PAIR_SETUP_MSG01;
  pair_tlv_values_t *request;
  pair_tlv_t *method;
  pair_tlv_t *type;
  int ret;

  request = message_process(data, data_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  method = pair_tlv_get_value(request, TLVType_Method);
  if (!method || method->size != 1 || method->value[0] != 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 1: Missing or unexpected pairing method in TLV");
    }

  type = pair_tlv_get_value(request, TLVType_Flags);
  sctx->is_transient = (type && type->size == 1 && type->value[0] == PairingFlagsTransient);

  // Note this is modified to return a 16 byte salt
  ret = srp_create_salted_verification_key(HASH_SHA512, SRP_NG_3072, USERNAME, (unsigned char *)sctx->pin, strlen(sctx->pin),
    &sctx->salt, &sctx->salt_len, &sctx->v, &sctx->v_len, NULL, NULL);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 1: Could not create verification key");
    }

  ret = srp_verifier_start_authentication(HASH_SHA512, SRP_NG_3072, sctx->v, sctx->v_len,
    &sctx->b, &sctx->b_len, &sctx->pkB, &sctx->pkB_len, NULL, NULL);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 1: Could not compute B");
    }

  pair_tlv_free(request);
  return 0;

 error:
  pair_tlv_free(request);
  return -1;
}

static int
server_setup_request2(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
//  enum pair_keys msg_state = PAIR_SETUP_MSG03;
  pair_tlv_values_t *request;
  pair_tlv_t *pk;
  pair_tlv_t *proof;

  request = message_process(data, data_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  pk = pair_tlv_get_value(request, TLVType_PublicKey);
  if (!pk || pk->size > N_len(SRP_NG_3072)) // 384 bytes or less
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 2: Missing og invalid public key");
    }

  proof = pair_tlv_get_value(request, TLVType_Proof);
  if (!proof || proof->size != SHA512_DIGEST_LENGTH)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 2: Missing or invalid proof");
    }

  sctx->pkA_len = pk->size;
  sctx->pkA = malloc(sctx->pkA_len);
  memcpy(sctx->pkA, pk->value, sctx->pkA_len);

  sctx->M1_len = proof->size;
  sctx->M1 = malloc(sctx->M1_len);
  memcpy(sctx->M1, proof->value, sctx->M1_len);

  sctx->verifier = srp_verifier_new(HASH_SHA512, SRP_NG_3072, USERNAME, sctx->salt, sctx->salt_len, sctx->v, sctx->v_len,
    sctx->pkA, sctx->pkA_len, sctx->b, sctx->b_len, sctx->pkB, sctx->pkB_len, NULL, NULL);
  if (!sctx->verifier)
    {
      handle->status = PAIR_STATUS_AUTH_FAILED;
      goto out;
    }

  sctx->M2_len = SHA512_DIGEST_LENGTH;
  srp_verifier_verify_session(sctx->verifier, sctx->M1, &sctx->M2);
  if (!sctx->M2)
    {
      handle->status = PAIR_STATUS_AUTH_FAILED;
      goto out; // Not an error, server should give proper TLV-formatet reply
    }

 out:
  pair_tlv_free(request);
  return 0;

 error:
  pair_tlv_free(request);
  return -1;
}

static int
server_setup_request3(struct pair_setup_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
  enum pair_keys msg_state = PAIR_SETUP_MSG05;
  pair_tlv_values_t *request;
  pair_tlv_t *encrypted_data;
  pair_tlv_t *device_id;
  pair_tlv_t *pk;
  pair_tlv_t *signature;
  const uint8_t *session_key;
  int session_key_len;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  size_t encrypted_len;
  uint8_t *decrypted_data = NULL;
  uint8_t device_x[32];
  int ret;

  request = message_process(data, data_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  session_key = srp_verifier_get_session_key(sctx->verifier, &session_key_len);
  if (!session_key)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: No valid session key");
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), session_key, session_key_len, msg_state);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: hkdf error getting derived_key");
    }

  encrypted_data = pair_tlv_get_value(request, TLVType_EncryptedData);
  if (!encrypted_data)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: Missing encrypted_data");
    }

  // encrypted_data->value consists of the encrypted payload + the auth tag
  if (encrypted_data->size < AUTHTAG_LENGTH)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: Invalid encrypted data");
    }

  encrypted_len = encrypted_data->size - AUTHTAG_LENGTH;
  memcpy(tag, encrypted_data->value + encrypted_len, AUTHTAG_LENGTH);
  memcpy(nonce + 4, pair_keys_map[msg_state].nonce, NONCE_LENGTH - 4);

  decrypted_data = malloc(encrypted_len);

  ret = decrypt_chacha(decrypted_data, encrypted_data->value, encrypted_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: Decryption error");
    }

  pair_tlv_free(request);
  request = message_process(decrypted_data, encrypted_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  ret = hkdf_extract_expand(device_x, sizeof(device_x), session_key, session_key_len, PAIR_SETUP_CONTROLLER_SIGN);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: hkdf error getting device_x");
    }

  device_id = pair_tlv_get_value(request, TLVType_Identifier);
  pk = pair_tlv_get_value(request, TLVType_PublicKey);
  signature = pair_tlv_get_value(request, TLVType_Signature);
  if (!device_id || device_id->size >= sizeof(handle->result.device_id) || !pk || pk->size != crypto_sign_PUBLICKEYBYTES || !signature || signature->size != crypto_sign_BYTES)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 3: Missing/invalid device ID, public key or signature");
    }

  ret = verify_info(signature->value, pk->value, device_x, sizeof(device_x), device_id->value, device_id->size, pk->value, pk->size);
  if (ret < 0)
    {
      handle->status = PAIR_STATUS_AUTH_FAILED;
      goto out;
    }

  memcpy(handle->result.device_id, device_id->value, device_id->size);
  memcpy(handle->result.client_public_key, pk->value, pk->size);

 out:
  free(decrypted_data);
  pair_tlv_free(request);
  return 0;

 error:
  free(decrypted_data);
  pair_tlv_free(request);
  return -1;
}

static uint8_t *
server_setup_response1(size_t *len, struct pair_setup_context *handle)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
  enum pair_keys msg_state = PAIR_SETUP_MSG02;
  pair_tlv_values_t *response;
  uint8_t *data;
  size_t data_len;
  int ret;

  if (handle->status == PAIR_STATUS_AUTH_FAILED)
    return server_auth_failed_response(len, msg_state);

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));
  pair_tlv_add_value(response, TLVType_Salt, sctx->salt, sctx->salt_len); // 16
  pair_tlv_add_value(response, TLVType_PublicKey, sctx->pkB, sctx->pkB_len); // 384

  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 1: pair_tlv_format returned an error");
    }

  *len = data_len;

  pair_tlv_free(response);
  return data;

 error:
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static uint8_t *
server_setup_response2(size_t *len, struct pair_setup_context *handle)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
  enum pair_keys msg_state = PAIR_SETUP_MSG04;
  pair_tlv_values_t *response;
  uint8_t *data;
  size_t data_len;
  const uint8_t *session_key;
  int session_key_len;
  int ret;

  if (handle->status == PAIR_STATUS_AUTH_FAILED)
    return server_auth_failed_response(len, msg_state);

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));
  pair_tlv_add_value(response, TLVType_Proof, sctx->M2, sctx->M2_len); // 384

  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 2: pair_tlv_format returned an error");
    }

  if (sctx->is_transient)
    {
      session_key = srp_verifier_get_session_key(sctx->verifier, &session_key_len);
      if (!session_key)
        {
	  RETURN_ERROR(PAIR_STATUS_INVALID, "Setup request 2: Could not compute session key");
	}

      assert(sizeof(handle->result.shared_secret) >= session_key_len);

      memcpy(handle->result.shared_secret, session_key, session_key_len);
      handle->result.shared_secret_len = session_key_len;

      handle->status = PAIR_STATUS_COMPLETED;
    }

  *len = data_len;

  pair_tlv_free(response);
  return data;

 error:
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static uint8_t *
server_setup_response3(size_t *len, struct pair_setup_context *handle)
{
  struct pair_server_setup_context *sctx = &handle->sctx.server;
  enum pair_keys msg_state = PAIR_SETUP_MSG06;
  const uint8_t *session_key;
  int session_key_len;
  pair_tlv_values_t *response;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  pair_tlv_values_t *append;
  size_t append_len;
  uint8_t *encrypted_data = NULL;
  size_t encrypted_data_len;
  uint8_t *data;
  size_t data_len;
  uint8_t device_x[32];
  int ret;

  if (handle->status == PAIR_STATUS_AUTH_FAILED)
    return server_auth_failed_response(len, msg_state);

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  session_key = srp_verifier_get_session_key(sctx->verifier, &session_key_len);
  if (!session_key)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: No valid session key");
    }

  ret = hkdf_extract_expand(device_x, sizeof(device_x), session_key, session_key_len, PAIR_SETUP_ACCESSORY_SIGN);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: hkdf error getting device_x");
    }

  ret = create_and_sign_device_info(data, &data_len, sctx->device_id, device_x, sizeof(device_x), sctx->public_key, sizeof(sctx->public_key), sctx->private_key);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: create device info returned an error");
    }

  // Append TLV-encoded public key to *data, which already has identifier and signature
  append = pair_tlv_new();
  append_len = REQUEST_BUFSIZE - data_len;
  pair_tlv_add_value(append, TLVType_PublicKey, sctx->public_key, sizeof(sctx->public_key));
  ret = pair_tlv_format(append, data + data_len, &append_len);
  pair_tlv_free(append);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: error appending public key to TLV");
    }
  data_len += append_len;

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), session_key, session_key_len, msg_state);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: hkdf error getting derived_key");
    }

  memcpy(nonce + 4, pair_keys_map[msg_state].nonce, NONCE_LENGTH - 4);

  encrypted_data_len = data_len + sizeof(tag); // Space for ciphered payload and authtag
  encrypted_data = malloc(encrypted_data_len);

  ret = encrypt_chacha(encrypted_data, data, data_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: Could not encrypt");
    }

  memcpy(encrypted_data + data_len, tag, sizeof(tag));

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));
  pair_tlv_add_value(response, TLVType_EncryptedData, encrypted_data, encrypted_data_len);

  data_len = REQUEST_BUFSIZE; // Re-using *data, so pass original length to pair_tlv_format
  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Setup response 3: error appending public key to TLV");
    }

  if (sctx->add_cb)
    sctx->add_cb(handle->result.client_public_key, handle->result.device_id, sctx->add_cb_arg);

  handle->status = PAIR_STATUS_COMPLETED;

  *len = data_len;

  free(encrypted_data);
  pair_tlv_free(response);
  return data;

 error:
  free(encrypted_data);
  free(data);
  pair_tlv_free(response);
  return NULL;
}


static int
server_verify_new(struct pair_verify_context *handle, const char *client_setup_keys, pair_cb cb, void *cb_arg, const char *device_id)
{
  struct pair_server_verify_context *vctx = &handle->vctx.server;

  if (!is_initialized())
    return -1;

  if (client_setup_keys)
    return -1;

  if (!device_id || strlen(device_id) >= PAIR_AP_DEVICE_ID_LEN_MAX)
    return -1;

  snprintf(vctx->device_id, sizeof(vctx->device_id), "%s", device_id);

  vctx->get_cb = cb;
  vctx->get_cb_arg = cb_arg;
  vctx->verify_client_signature = cb;

  server_keypair(vctx->server_public_key, vctx->server_private_key, vctx->device_id);

  return 0;
}

static int
server_verify_request1(struct pair_verify_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_server_verify_context *vctx = &handle->vctx.server;
//  enum pair_keys msg_state = PAIR_VERIFY_MSG01;
  pair_tlv_values_t *request;
  pair_tlv_t *pk;

  request = message_process(data, data_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  pk = pair_tlv_get_value(request, TLVType_PublicKey);
  if (!pk || pk->size != sizeof(vctx->client_eph_public_key))
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 1: Missing or invalid public_key");
    }

  memcpy(vctx->client_eph_public_key, pk->value, sizeof(vctx->client_eph_public_key));

  pair_tlv_free(request);
  return 0;

 error:
  pair_tlv_free(request);
  return -1;
}

static int
server_verify_request2(struct pair_verify_context *handle, const uint8_t *data, size_t data_len)
{
  struct pair_server_verify_context *vctx = &handle->vctx.server;
  enum pair_keys msg_state = PAIR_VERIFY_MSG03;
  pair_tlv_values_t *request;
  pair_tlv_t *encrypted_data;
  pair_tlv_t *device_id;
  pair_tlv_t *signature;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  size_t encrypted_len;
  uint8_t *decrypted_data = NULL;
  char id_str[PAIR_AP_DEVICE_ID_LEN_MAX] = { 0 };
  uint8_t client_public_key[crypto_sign_PUBLICKEYBYTES];
  int ret;

  request = message_process(data, data_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), vctx->shared_secret, sizeof(vctx->shared_secret), msg_state);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: hkdf error getting derived_key");
    }

  encrypted_data = pair_tlv_get_value(request, TLVType_EncryptedData);
  if (!encrypted_data)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: Missing encrypted_data");
    }

  // encrypted_data->value consists of the encrypted payload + the auth tag
  if (encrypted_data->size < AUTHTAG_LENGTH)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: Invalid encrypted data");
    }

  encrypted_len = encrypted_data->size - AUTHTAG_LENGTH;
  memcpy(tag, encrypted_data->value + encrypted_len, AUTHTAG_LENGTH);
  memcpy(nonce + 4, pair_keys_map[msg_state].nonce, NONCE_LENGTH - 4);

  decrypted_data = malloc(encrypted_len);

  ret = decrypt_chacha(decrypted_data, encrypted_data->value, encrypted_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: Decryption error");
    }

  pair_tlv_free(request);
  request = message_process(decrypted_data, encrypted_len, &handle->errmsg);
  if (!request)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, handle->errmsg);
    }

  device_id = pair_tlv_get_value(request, TLVType_Identifier);
  signature = pair_tlv_get_value(request, TLVType_Signature);
  if (!device_id || !signature || signature->size != crypto_sign_BYTES)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: Missing identifier or signature");
    }

  if (vctx->verify_client_signature)
    {
      if (device_id->size >= sizeof(id_str))
        {
          RETURN_ERROR(PAIR_STATUS_INVALID, "Verify request 2: Device ID from peer is too long");
        }

      memcpy(id_str, device_id->value, device_id->size);

      ret = vctx->get_cb(client_public_key, id_str, vctx->get_cb_arg);
      if (ret < 0)
        {
          handle->status = PAIR_STATUS_AUTH_FAILED;
          goto out;
        }

      ret = verify_info(signature->value, client_public_key, vctx->client_eph_public_key, sizeof(vctx->client_eph_public_key),
                        device_id->value, device_id->size, vctx->server_eph_public_key, sizeof(vctx->server_eph_public_key));
      if (ret < 0)
        {
          handle->status = PAIR_STATUS_AUTH_FAILED;
          goto out;
        }
    }

 out:
  free(decrypted_data);
  pair_tlv_free(request);
  return 0;

 error:
  free(decrypted_data);
  pair_tlv_free(request);
  return -1;
}

static uint8_t *
server_verify_response1(size_t *len, struct pair_verify_context *handle)
{
  struct pair_server_verify_context *vctx = &handle->vctx.server;
  enum pair_keys msg_state = PAIR_VERIFY_MSG02;
  pair_tlv_values_t *response;
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t derived_key[32];
  uint8_t *encrypted_data = NULL;
  size_t encrypted_data_len;
  uint8_t *data;
  size_t data_len;
  int ret;

  if (handle->status == PAIR_STATUS_AUTH_FAILED)
    return server_auth_failed_response(len, msg_state);

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  crypto_box_keypair(vctx->server_eph_public_key, vctx->server_eph_private_key);

  ret = crypto_scalarmult(vctx->shared_secret, vctx->server_eph_private_key, vctx->client_eph_public_key);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 1: Error generating shared secret");
    }

  ret = create_and_sign_accessory_info(data, &data_len, vctx->server_eph_public_key, sizeof(vctx->server_eph_public_key), vctx->device_id,
                                       vctx->client_eph_public_key, sizeof(vctx->client_eph_public_key), vctx->server_private_key);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 1: Error creating device info");
    }

  ret = hkdf_extract_expand(derived_key, sizeof(derived_key), vctx->shared_secret, sizeof(vctx->shared_secret), msg_state);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 1: hkdf error getting derived_key");
    }

  memcpy(nonce + 4, pair_keys_map[msg_state].nonce, NONCE_LENGTH - 4);

  encrypted_data_len = data_len + sizeof(tag); // Space for ciphered payload and authtag
  encrypted_data = malloc(encrypted_data_len);

  ret = encrypt_chacha(encrypted_data, data, data_len, derived_key, sizeof(derived_key), NULL, 0, tag, sizeof(tag), nonce);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 1: Could not encrypt");
    }

  memcpy(encrypted_data + data_len, tag, sizeof(tag));

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));
  pair_tlv_add_value(response, TLVType_PublicKey, vctx->server_eph_public_key, sizeof(vctx->server_eph_public_key));
  pair_tlv_add_value(response, TLVType_EncryptedData, encrypted_data, encrypted_data_len);

  data_len = REQUEST_BUFSIZE; // Re-using *data, so pass original length to pair_tlv_format
  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 1: pair_tlv_format returned an error");
    }

  *len = data_len;

  free(encrypted_data);
  pair_tlv_free(response);
  return data;

 error:
  free(encrypted_data);
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static uint8_t *
server_verify_response2(size_t *len, struct pair_verify_context *handle)
{
  struct pair_server_verify_context *vctx = &handle->vctx.server;
  enum pair_keys msg_state = PAIR_VERIFY_MSG04;
  pair_tlv_values_t *response;
  uint8_t *data;
  size_t data_len;
  int ret;

  if (handle->status == PAIR_STATUS_AUTH_FAILED)
    return server_auth_failed_response(len, msg_state);

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &pair_keys_map[msg_state].state, sizeof(pair_keys_map[msg_state].state));

  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      RETURN_ERROR(PAIR_STATUS_INVALID, "Verify response 2: pair_tlv_format returned an error");
    }

  *len = data_len;

  memcpy(handle->result.shared_secret, vctx->shared_secret, sizeof(vctx->shared_secret));
  handle->result.shared_secret_len = sizeof(vctx->shared_secret);

  handle->status = PAIR_STATUS_COMPLETED;

  pair_tlv_free(response);
  return data;

 error:
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static int
server_add_remove_request(pair_cb cb, void *cb_arg, const uint8_t *in, size_t in_len)
{
  const char *errmsg;
  pair_tlv_values_t *request;
  pair_tlv_t *device_id;
  pair_tlv_t *pk;
  char id_str[PAIR_AP_DEVICE_ID_LEN_MAX] = { 0 };
  uint8_t *public_key = NULL;

  request = message_process(in, in_len, &errmsg);
  if (!request)
    {
      goto error;
    }

  device_id = pair_tlv_get_value(request, TLVType_Identifier);
  if (!device_id || device_id->size >= sizeof(id_str))
    {
      goto error;
    }

  // Only present when adding
  pk = pair_tlv_get_value(request, TLVType_PublicKey);
  if (pk && pk->size == crypto_sign_PUBLICKEYBYTES)
    {
      public_key = pk->value;
    }

  memcpy(id_str, device_id->value, device_id->size);

  cb(public_key, id_str, cb_arg);

  pair_tlv_free(request);
  return 0;

 error:
  pair_tlv_free(request);
  return -1;
}

static uint8_t *
server_add_remove_response(size_t *len)
{
  pair_tlv_values_t *response;
  uint8_t *data;
  uint8_t state = 2;
  size_t data_len;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &state, sizeof(state));

  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      goto error;
    }

  *len = data_len;

  pair_tlv_free(response);
  return data;

 error:
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static int
server_add_remove(uint8_t **out, size_t *out_len, pair_cb cb, void *cb_arg, const uint8_t *in, size_t in_len)
{
  int ret;

  ret = server_add_remove_request(cb, cb_arg, in, in_len);
  if (ret < 0)
    return -1;

  *out = server_add_remove_response(out_len);
  if (!*out)
    return -1;

  return 0;
}

static int
server_list_cb(uint8_t public_key[crypto_sign_PUBLICKEYBYTES], const char *device_id, void *cb_arg)
{
  pair_tlv_values_t *response = cb_arg;
  pair_tlv_t *previous_id;
  uint8_t permissions = 1; // Means admin (TODO don't hardcode - let caller set)

  // If this isn't the first iteration (item) then we must add a separator
  previous_id = pair_tlv_get_value(response, TLVType_Identifier);
  if (previous_id)
    pair_tlv_add_value(response, TLVType_Separator, NULL, 0);

  pair_tlv_add_value(response, TLVType_Identifier, (unsigned char *)device_id, strlen(device_id));
  pair_tlv_add_value(response, TLVType_PublicKey, public_key, crypto_sign_PUBLICKEYBYTES);
  pair_tlv_add_value(response, TLVType_Permissions, &permissions, sizeof(permissions));

  return 0;
}

static uint8_t *
server_list_response(size_t *len, pair_list_cb cb, void *cb_arg)
{
  pair_tlv_values_t *response;
  uint8_t *data;
  uint8_t state = 2;
  size_t data_len;
  int ret;

  data_len = REQUEST_BUFSIZE;
  data = malloc(data_len);
  response = pair_tlv_new();

  pair_tlv_add_value(response, TLVType_State, &state, sizeof(state));

  cb(server_list_cb, response, cb_arg);

  ret = pair_tlv_format(response, data, &data_len);
  if (ret < 0)
    {
      goto error;
    }

  *len = data_len;

  pair_tlv_free(response);
  return data;

 error:
  free(data);
  pair_tlv_free(response);
  return NULL;
}

static int
server_list(uint8_t **out, size_t *out_len, pair_list_cb cb, void *cb_arg, const uint8_t *in, size_t in_len)
{
  // Skip reading the request, it just has state = 1 and pair method =
  // PairingMethodListPairings

  *out = server_list_response(out_len, cb, cb_arg);
  if (!*out)
    return -1;

  return 0;
}


/* ----------------------- CIPHERING IMPLEMENTATION ------------------------- */

static void
cipher_free(struct pair_cipher_context *cctx)
{
  if (!cctx)
    return;

  free(cctx);
}

static struct pair_cipher_context *
cipher_new(struct pair_definition *type, int channel, const uint8_t *shared_secret, size_t shared_secret_len)
{
  struct pair_cipher_context *cctx;
  enum pair_keys write_key;
  enum pair_keys read_key;
  int ret;

  // Note that events is opposite, probably because it is a reverse connection
  switch (channel)
    {
      case 0:
	write_key = PAIR_CONTROL_WRITE;
	read_key = PAIR_CONTROL_READ;
	break;
      case 1:
	write_key = PAIR_EVENTS_READ;
	read_key = PAIR_EVENTS_WRITE;
	break;
      case 2:
	write_key = PAIR_CONTROL_READ;
	read_key = PAIR_CONTROL_WRITE;
	break;
      case 3:
	write_key = PAIR_EVENTS_WRITE;
	read_key = PAIR_EVENTS_READ;
	break;
      default:
	return NULL;
    }

  cctx = calloc(1, sizeof(struct pair_cipher_context));
  if (!cctx)
    goto error;

  cctx->type = type;

  ret = hkdf_extract_expand(cctx->encryption_key, sizeof(cctx->encryption_key), shared_secret, shared_secret_len, write_key);
  if (ret < 0)
    goto error;

  ret = hkdf_extract_expand(cctx->decryption_key, sizeof(cctx->decryption_key), shared_secret, shared_secret_len, read_key);
  if (ret < 0)
    goto error;

  return cctx;

 error:
  pair_cipher_free(cctx);
  return NULL;
}

static ssize_t
encrypt(uint8_t **ciphertext, size_t *ciphertext_len, const uint8_t *plaintext, size_t plaintext_len, struct pair_cipher_context *cctx)
{
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  const uint8_t *plain_block;
  uint8_t *cipher_block;
  uint16_t block_len;
  int nblocks;
  int ret;
  int i;

  if (plaintext_len == 0 || !plaintext)
    return -1;

  // Encryption is done in blocks, where each block consists of a short, the
  // encrypted data and an auth tag. The short is the size of the encrypted
  // data. The encrypted data in the block cannot exceed ENCRYPTED_LEN_MAX.
  nblocks = 1 + ((plaintext_len - 1) / ENCRYPTED_LEN_MAX); // Ceiling of division

  *ciphertext_len = nblocks * (sizeof(block_len) + AUTHTAG_LENGTH) + plaintext_len;
  *ciphertext = malloc(*ciphertext_len);

  cctx->encryption_counter_prev = cctx->encryption_counter;

  for (i = 0, plain_block = plaintext, cipher_block = *ciphertext; i < nblocks; i++)
    {
      // If it is the last block we will encrypt only the remaining data
      block_len = (i + 1 == nblocks) ? (plaintext + plaintext_len - plain_block) : ENCRYPTED_LEN_MAX;

      memcpy(nonce + 4, &(cctx->encryption_counter), sizeof(cctx->encryption_counter));// TODO BE or LE?

      // Write the ciphered block
      memcpy(cipher_block, &block_len, sizeof(block_len)); // TODO BE or LE?
      ret = encrypt_chacha(cipher_block + sizeof(block_len), plain_block, block_len, cctx->encryption_key, sizeof(cctx->encryption_key), &block_len, sizeof(block_len), tag, sizeof(tag), nonce);
      if (ret < 0)
	{
	  cctx->errmsg = "Encryption with chacha poly1305 failed";
	  cctx->encryption_counter = cctx->encryption_counter_prev;
	  free(*ciphertext);
	  return -1;
	}
      memcpy(cipher_block + sizeof(block_len) + block_len, tag, AUTHTAG_LENGTH);

      plain_block += block_len;
      cipher_block += block_len + sizeof(block_len) + AUTHTAG_LENGTH;
      cctx->encryption_counter++;
    }

#ifdef DEBUG_PAIR
  hexdump("Encrypted:\n", *ciphertext, *ciphertext_len);
#endif

  return plain_block - plaintext;
}

static ssize_t
decrypt(uint8_t **plaintext, size_t *plaintext_len, const uint8_t *ciphertext, size_t ciphertext_len, struct pair_cipher_context *cctx)
{
  uint8_t nonce[NONCE_LENGTH] = { 0 };
  uint8_t tag[AUTHTAG_LENGTH];
  uint8_t *plain_block;
  const uint8_t *cipher_block;
  uint16_t block_len;
  int ret;

  if (ciphertext_len < sizeof(block_len) || !ciphertext)
    return -1;

  // This will allocate more than we need. Since we don't know the number of
  // blocks in the ciphertext yet we can't calculate the exact required length.
  *plaintext = malloc(ciphertext_len);

  cctx->decryption_counter_prev = cctx->decryption_counter;

  for (plain_block = *plaintext, cipher_block = ciphertext; cipher_block < ciphertext + ciphertext_len; )
    {
      memcpy(&block_len, cipher_block, sizeof(block_len)); // TODO BE or LE?
      if (cipher_block + block_len + sizeof(block_len) + AUTHTAG_LENGTH > ciphertext + ciphertext_len)
	{
	  // The remaining ciphertext doesn't contain an entire block, so stop
	  break;
	}

      memcpy(tag, cipher_block + sizeof(block_len) + block_len, sizeof(tag));
      memcpy(nonce + 4, &(cctx->decryption_counter), sizeof(cctx->decryption_counter));// TODO BE or LE?

      ret = decrypt_chacha(plain_block, cipher_block + sizeof(block_len), block_len, cctx->decryption_key, sizeof(cctx->decryption_key), &block_len, sizeof(block_len), tag, sizeof(tag), nonce);
      if (ret < 0)
	{
	  cctx->errmsg = "Decryption with chacha poly1305 failed";
	  cctx->decryption_counter = cctx->decryption_counter_prev;
	  free(*plaintext);
	  return -1;
	}

      plain_block += block_len;
      cipher_block += block_len + sizeof(block_len) + AUTHTAG_LENGTH;
      cctx->decryption_counter++;
    }

  *plaintext_len = plain_block - *plaintext;

#ifdef DEBUG_PAIR
  hexdump("Decrypted:\n", *plaintext, *plaintext_len);
#endif

  return cipher_block - ciphertext;
}

static int
state_get(const char **errmsg, const uint8_t *data, size_t data_len)
{
  pair_tlv_values_t *message;
  pair_tlv_t *state;
  int ret;

  if (!data || data_len == 0)
    {
      return 0; // state 0 = no incoming data yet -> first request
    }

  message = message_process(data, data_len, errmsg);
  if (!message)
    {
      goto error;
    }

  state = pair_tlv_get_value(message, TLVType_State);
  if (!state || state->size != 1)
    {
      *errmsg = "Could not get message state";
      goto error;
    }

  ret = state->value[0];

  pair_tlv_free(message);
  return ret;

 error:
  pair_tlv_free(message);
  return -1;
}

static void
public_key_get(uint8_t server_public_key[crypto_sign_PUBLICKEYBYTES], const char *device_id)
{
  uint8_t private_key[crypto_sign_SECRETKEYBYTES];
  server_keypair(server_public_key, private_key, device_id);
}

const struct pair_definition pair_client_homekit_normal =
{
  .pair_setup_new = client_setup_new,
  .pair_setup_free = client_setup_free,
  .pair_setup_result = client_setup_result,

  .pair_setup_request1 = client_setup_request1,
  .pair_setup_request2 = client_setup_request2,
  .pair_setup_request3 = client_setup_request3,

  .pair_setup_response1 = client_setup_response1,
  .pair_setup_response2 = client_setup_response2,
  .pair_setup_response3 = client_setup_response3,

  .pair_verify_new = client_verify_new,

  .pair_verify_request1 = client_verify_request1,
  .pair_verify_request2 = client_verify_request2,

  .pair_verify_response1 = client_verify_response1,
  .pair_verify_response2 = client_verify_response2,

  .pair_cipher_new = cipher_new,
  .pair_cipher_free = cipher_free,

  .pair_encrypt = encrypt,
  .pair_decrypt = decrypt,

  .pair_state_get = state_get,
};

const struct pair_definition pair_client_homekit_transient =
{
  .pair_setup_new = client_setup_new,
  .pair_setup_free = client_setup_free,

  .pair_setup_request1 = client_setup_request1,
  .pair_setup_request2 = client_setup_request2,
  .pair_setup_request3 = client_setup_request3,

  .pair_setup_response1 = client_setup_response1,
  .pair_setup_response2 = client_setup_response2,
  .pair_setup_response3 = client_setup_response3,

  .pair_verify_new = client_verify_new,

  .pair_verify_request1 = client_verify_request1,
  .pair_verify_request2 = client_verify_request2,

  .pair_verify_response1 = client_verify_response1,
  .pair_verify_response2 = client_verify_response2,

  .pair_cipher_new = cipher_new,
  .pair_cipher_free = cipher_free,

  .pair_encrypt = encrypt,
  .pair_decrypt = decrypt,

  .pair_state_get = state_get,
};

const struct pair_definition pair_server_homekit =
{
  .pair_setup_new = server_setup_new,
  .pair_setup_free = server_setup_free,

  .pair_setup_request1 = server_setup_response1,
  .pair_setup_request2 = server_setup_response2,
  .pair_setup_request3 = server_setup_response3,

  .pair_setup_response1 = server_setup_request1,
  .pair_setup_response2 = server_setup_request2,
  .pair_setup_response3 = server_setup_request3,

  .pair_verify_new = server_verify_new,

  .pair_verify_request1 = server_verify_response1,
  .pair_verify_request2 = server_verify_response2,

  .pair_verify_response1 = server_verify_request1,
  .pair_verify_response2 = server_verify_request2,

  .pair_add = server_add_remove,
  .pair_remove = server_add_remove,
  .pair_list = server_list,

  .pair_cipher_new = cipher_new,
  .pair_cipher_free = cipher_free,

  .pair_encrypt = encrypt,
  .pair_decrypt = decrypt,

  .pair_state_get = state_get,
  .pair_public_key_get = public_key_get,
};