kex: fix libgcrypt memory leaks of bignum

Fixes #168.

src/kex.c

@@ -96,7 +96,7 @@ static int diffie_hellman_sha1(LIBSSH2_SESSION *session,
         exchange_state->ctx = _libssh2_bn_ctx_new();
         exchange_state->x = _libssh2_bn_init(); /* Random from client */
         exchange_state->e = _libssh2_bn_init(); /* g^x mod p */
-        exchange_state->f = _libssh2_bn_init(); /* g^(Random from server) mod p */
+        exchange_state->f = _libssh2_bn_init_from_bin(); /* g^(Random from server) mod p */
         exchange_state->k = _libssh2_bn_init(); /* The shared secret: f^x mod p */
 
         /* Zero the whole thing out */
@@ -715,7 +715,7 @@ kex_method_diffie_hellman_group1_sha1_key_exchange(LIBSSH2_SESSION *session,
 
     if (key_state->state == libssh2_NB_state_idle) {
         /* g == 2 */
-        key_state->p = _libssh2_bn_init();      /* SSH2 defined value (p_value) */
+        key_state->p = _libssh2_bn_init_from_bin();      /* SSH2 defined value (p_value) */
         key_state->g = _libssh2_bn_init();      /* SSH2 defined value (2) */
 
         /* Initialize P and G */
@@ -790,7 +790,7 @@ kex_method_diffie_hellman_group14_sha1_key_exchange(LIBSSH2_SESSION *session,
     int ret;
 
     if (key_state->state == libssh2_NB_state_idle) {
-        key_state->p = _libssh2_bn_init();      /* SSH2 defined value (p_value) */
+        key_state->p = _libssh2_bn_init_from_bin();      /* SSH2 defined value (p_value) */
         key_state->g = _libssh2_bn_init();      /* SSH2 defined value (2) */
 
         /* g == 2 */
@@ -834,8 +834,8 @@ kex_method_diffie_hellman_group_exchange_sha1_key_exchange
     int rc;
 
     if (key_state->state == libssh2_NB_state_idle) {
-        key_state->p = _libssh2_bn_init();
-        key_state->g = _libssh2_bn_init();
+        key_state->p = _libssh2_bn_init_from_bin();
+        key_state->g = _libssh2_bn_init_from_bin();
         /* Ask for a P and G pair */
 #ifdef LIBSSH2_DH_GEX_NEW
         key_state->request[0] = SSH_MSG_KEX_DH_GEX_REQUEST;

src/libgcrypt.h

@@ -149,6 +149,7 @@
 #define _libssh2_bn_ctx_new() 0
 #define _libssh2_bn_ctx_free(bnctx) ((void)0)
 #define _libssh2_bn_init() gcry_mpi_new(0)
+#define _libssh2_bn_init_from_bin() NULL /* because gcry_mpi_scan() creates a new bignum */
 #define _libssh2_bn_rand(bn, bits, top, bottom) gcry_mpi_randomize (bn, bits, GCRY_WEAK_RANDOM)
 #define _libssh2_bn_mod_exp(r, a, p, m, ctx) gcry_mpi_powm (r, a, p, m)
 #define _libssh2_bn_set_word(bn, val) gcry_mpi_set_ui(bn, val)

src/openssl.h

@@ -175,6 +175,7 @@ void libssh2_md5(const unsigned char *message, unsigned long len, unsigned char
 #define _libssh2_bn_ctx_new() BN_CTX_new()
 #define _libssh2_bn_ctx_free(bnctx) BN_CTX_free(bnctx)
 #define _libssh2_bn_init() BN_new()
+#define _libssh2_bn_init_from_bin() _libssh2_bn_init()
 #define _libssh2_bn_rand(bn, bits, top, bottom) BN_rand(bn, bits, top, bottom)
 #define _libssh2_bn_mod_exp(r, a, p, m, ctx) BN_mod_exp(r, a, p, m, ctx)
 #define _libssh2_bn_set_word(bn, val) BN_set_word(bn, val)

src/wincng.h

@@ -316,6 +316,8 @@ _libssh2_bn *_libssh2_wincng_bignum_init(void);
 
 #define _libssh2_bn_init() \
   _libssh2_wincng_bignum_init()
+#define _libssh2_bn_init_from_bin() \
+  _libssh2_bn_init()
 #define _libssh2_bn_rand(bn, bits, top, bottom) \
   _libssh2_wincng_bignum_rand(bn, bits, top, bottom)
 #define _libssh2_bn_mod_exp(r, a, p, m, ctx) \