- djm@cvs.openbsd.org 2014/01/12 08:13:13

     [bufaux.c buffer.h kex.c kex.h kexc25519.c kexc25519c.c kexc25519s.c]
     [kexdhc.c kexdhs.c kexecdhc.c kexecdhs.c kexgexc.c kexgexs.c]
     avoid use of OpenSSL BIGNUM type and functions for KEX with
     Curve25519 by adding a buffer_put_bignum2_from_string() that stores
     a string using the bignum encoding rules. Will make it easier to
     build a reduced-feature OpenSSH without OpenSSL in the future;
     ok markus@

diff --git a/ChangeLog b/ChangeLog
index 99b846e3c9523583acdc3cd88884bd54f63a5b51..e30cec1a42a027abbe43d6f79b98ded4690557b0 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -3,6 +3,14 @@
    - djm@cvs.openbsd.org 2014/01/10 05:59:19
      [sshd_config]
      the /etc/ssh/ssh_host_ed25519_key is loaded by default too
+   - djm@cvs.openbsd.org 2014/01/12 08:13:13
+     [bufaux.c buffer.h kex.c kex.h kexc25519.c kexc25519c.c kexc25519s.c]
+     [kexdhc.c kexdhs.c kexecdhc.c kexecdhs.c kexgexc.c kexgexs.c]
+     avoid use of OpenSSL BIGNUM type and functions for KEX with
+     Curve25519 by adding a buffer_put_bignum2_from_string() that stores
+     a string using the bignum encoding rules. Will make it easier to
+     build a reduced-feature OpenSSH without OpenSSL in the future;
+     ok markus@
 
 20140110
  - (djm) OpenBSD CVS Sync

diff --git a/bufaux.c b/bufaux.c
index 3e70b8d90685d00099db625aad3ccf8b1ff5ed4f..9401fe1d09be83a1257c486ab651b74f9f455652 100644 (file)
--- a/bufaux.c
+++ b/bufaux.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bufaux.c,v 1.53 2013/11/08 11:15:19 dtucker Exp $ */
+/* $OpenBSD: bufaux.c,v 1.54 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -315,3 +315,76 @@ buffer_put_char(Buffer *buffer, int value)
 
        buffer_append(buffer, &ch, 1);
 }
+
+/* Pseudo bignum functions */
+
+void *
+buffer_get_bignum2_as_string_ret(Buffer *buffer, u_int *length_ptr)
+{
+       u_int len;
+       u_char *bin, *p, *ret;
+
+       if ((p = bin = buffer_get_string_ret(buffer, &len)) == NULL) {
+               error("%s: invalid bignum", __func__);
+               return NULL;
+       }
+
+       if (len > 0 && (bin[0] & 0x80)) {
+               error("%s: negative numbers not supported", __func__);
+               free(bin);
+               return NULL;
+       }
+       if (len > 8 * 1024) {
+               error("%s: cannot handle BN of size %d", __func__, len);
+               free(bin);
+               return NULL;
+       }
+       /* Skip zero prefix on numbers with the MSB set */
+       if (len > 1 && bin[0] == 0x00 && (bin[1] & 0x80) != 0) {
+               p++;
+               len--;
+       }
+       ret = xmalloc(len);
+       memcpy(ret, p, len);
+       memset(p, '\0', len);
+       free(bin);
+       return ret;
+}
+
+void *
+buffer_get_bignum2_as_string(Buffer *buffer, u_int *l)
+{
+       void *ret = buffer_get_bignum2_as_string_ret(buffer, l);
+
+       if (ret == NULL)
+               fatal("%s: buffer error", __func__);
+       return ret;
+}
+
+/*
+ * Stores a string using the bignum encoding rules (\0 pad if MSB set).
+ */
+void
+buffer_put_bignum2_from_string(Buffer *buffer, const u_char *s, u_int l)
+{
+       u_char *buf, *p;
+       int pad = 0;
+
+       if (l > 8 * 1024)
+               fatal("%s: length %u too long", __func__, l);
+       p = buf = xmalloc(l + 1);
+       /*
+        * If most significant bit is set then prepend a zero byte to
+        * avoid interpretation as a negative number.
+        */
+       if (l > 0 && (s[0] & 0x80) != 0) {
+               *p++ = '\0';
+               pad = 1;
+       }
+       memcpy(p, s, l);
+       buffer_put_string(buffer, buf, l + pad);
+       memset(buf, '\0', l + pad);
+       free(buf);
+}
+
+

diff --git a/buffer.h b/buffer.h
index 4fa2ca112613dc771634aeeb9f4487668e798ef5..7df8a38fa18d22cb94120c421648e22f9e184e7f 100644 (file)
--- a/buffer.h
+++ b/buffer.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: buffer.h,v 1.22 2013/07/12 00:19:58 djm Exp $ */
+/* $OpenBSD: buffer.h,v 1.23 2014/01/12 08:13:13 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -86,6 +86,10 @@ char *buffer_get_cstring_ret(Buffer *, u_int *);
 void   *buffer_get_string_ptr_ret(Buffer *, u_int *);
 int    buffer_get_char_ret(u_char *, Buffer *);
 
+void *buffer_get_bignum2_as_string_ret(Buffer *, u_int *);
+void *buffer_get_bignum2_as_string(Buffer *, u_int *);
+void  buffer_put_bignum2_from_string(Buffer *, const u_char *, u_int);
+
 #ifdef OPENSSL_HAS_ECC
 #include <openssl/ec.h>
 

diff --git a/kex.c b/kex.c
index dbb1a98167b3ba02507d23622ecfba48519dce9f..7d054cdcbb7396b92c60df943856dea955e63920 100644 (file)
--- a/kex.c
+++ b/kex.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.94 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kex.c,v 1.95 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -534,7 +534,7 @@ kex_choose_conf(Kex *kex)
 
 static u_char *
 derive_key(Kex *kex, int id, u_int need, u_char *hash, u_int hashlen,
-    BIGNUM *shared_secret)
+    const u_char *shared_secret, u_int slen)
 {
        Buffer b;
        struct ssh_digest_ctx *hashctx;
@@ -548,7 +548,7 @@ derive_key(Kex *kex, int id, u_int need, u_char *hash, u_int hashlen,
        digest = xmalloc(roundup(need, mdsz));
 
        buffer_init(&b);
-       buffer_put_bignum2(&b, shared_secret);
+       buffer_append(&b, shared_secret, slen);
 
        /* K1 = HASH(K || H || "A" || session_id) */
        if ((hashctx = ssh_digest_start(kex->hash_alg)) == NULL)
@@ -591,14 +591,15 @@ Newkeys *current_keys[MODE_MAX];
 
 #define NKEYS  6
 void
-kex_derive_keys(Kex *kex, u_char *hash, u_int hashlen, BIGNUM *shared_secret)
+kex_derive_keys(Kex *kex, u_char *hash, u_int hashlen,
+    const u_char *shared_secret, u_int slen)
 {
        u_char *keys[NKEYS];
        u_int i, mode, ctos;
 
        for (i = 0; i < NKEYS; i++) {
                keys[i] = derive_key(kex, 'A'+i, kex->we_need, hash, hashlen,
-                   shared_secret);
+                   shared_secret, slen);
        }
 
        debug2("kex_derive_keys");
@@ -613,6 +614,18 @@ kex_derive_keys(Kex *kex, u_char *hash, u_int hashlen, BIGNUM *shared_secret)
        }
 }
 
+void
+kex_derive_keys_bn(Kex *kex, u_char *hash, u_int hashlen, const BIGNUM *secret)
+{
+       Buffer shared_secret;
+
+       buffer_init(&shared_secret);
+       buffer_put_bignum2(&shared_secret, secret);
+       kex_derive_keys(kex, hash, hashlen,
+           buffer_ptr(&shared_secret), buffer_len(&shared_secret));
+       buffer_free(&shared_secret);
+}
+
 Newkeys *
 kex_get_newkeys(int mode)
 {

diff --git a/kex.h b/kex.h
index fbe4940e8807827c56a25862424e02ce1d3d2079..7e2878f70aae940462587df9096b53af8fadd66e 100644 (file)
--- a/kex.h
+++ b/kex.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.h,v 1.59 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kex.h,v 1.60 2014/01/12 08:13:13 djm Exp $ */
 
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
@@ -153,7 +153,8 @@ void         kex_finish(Kex *);
 
 void    kex_send_kexinit(Kex *);
 void    kex_input_kexinit(int, u_int32_t, void *);
-void    kex_derive_keys(Kex *, u_char *, u_int, BIGNUM *);
+void    kex_derive_keys(Kex *, u_char *, u_int, const u_char *, u_int);
+void    kex_derive_keys_bn(Kex *, u_char *, u_int, const BIGNUM *);
 
 Newkeys *kex_get_newkeys(int);
 
@@ -182,14 +183,14 @@ kex_ecdh_hash(int, const EC_GROUP *, char *, char *, char *, int,
 void
 kex_c25519_hash(int, char *, char *, char *, int,
     char *, int, u_char *, int, const u_char *, const u_char *,
-    const BIGNUM *, u_char **, u_int *);
+    const u_char *, u_int, u_char **, u_int *);
 
 #define CURVE25519_SIZE 32
 void   kexc25519_keygen(u_char[CURVE25519_SIZE], u_char[CURVE25519_SIZE])
        __attribute__((__bounded__(__minbytes__, 1, CURVE25519_SIZE)))
        __attribute__((__bounded__(__minbytes__, 2, CURVE25519_SIZE)));
-BIGNUM *kexc25519_shared_key(const u_char[CURVE25519_SIZE],
-    const u_char[CURVE25519_SIZE])
+void kexc25519_shared_key(const u_char key[CURVE25519_SIZE],
+    const u_char pub[CURVE25519_SIZE], Buffer *out)
        __attribute__((__bounded__(__minbytes__, 1, CURVE25519_SIZE)))
        __attribute__((__bounded__(__minbytes__, 2, CURVE25519_SIZE)));
 

diff --git a/kexc25519.c b/kexc25519.c
index 8dd36399156cc4d6aa74416b49806ee5fbbddf36..48ca4aaa2182826961a855f5815e7493e8a8129a 100644 (file)
--- a/kexc25519.c
+++ b/kexc25519.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexc25519.c,v 1.3 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexc25519.c,v 1.4 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001, 2013 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -58,23 +58,19 @@ kexc25519_keygen(u_char key[CURVE25519_SIZE], u_char pub[CURVE25519_SIZE])
        crypto_scalarmult_curve25519(pub, key, basepoint);
 }
 
-BIGNUM *
+void
 kexc25519_shared_key(const u_char key[CURVE25519_SIZE],
-    const u_char pub[CURVE25519_SIZE])
+    const u_char pub[CURVE25519_SIZE], Buffer *out)
 {
        u_char shared_key[CURVE25519_SIZE];
-       BIGNUM *shared_secret;
 
        crypto_scalarmult_curve25519(shared_key, key, pub);
 #ifdef DEBUG_KEXECDH
        dump_digest("shared secret", shared_key, CURVE25519_SIZE);
 #endif
-       if ((shared_secret = BN_new()) == NULL)
-               fatal("%s: BN_new failed", __func__);
-       if (BN_bin2bn(shared_key, sizeof(shared_key), shared_secret) == NULL)
-               fatal("%s: BN_bin2bn failed", __func__);
+       buffer_clear(out);
+       buffer_put_bignum2_from_string(out, shared_key, CURVE25519_SIZE);
        memset(shared_key, 0, CURVE25519_SIZE); /* XXX explicit_bzero() */
-       return (shared_secret);
 }
 
 void
@@ -87,7 +83,7 @@ kex_c25519_hash(
     u_char *serverhostkeyblob, int sbloblen,
     const u_char client_dh_pub[CURVE25519_SIZE],
     const u_char server_dh_pub[CURVE25519_SIZE],
-    const BIGNUM *shared_secret,
+    const u_char *shared_secret, u_int secretlen,
     u_char **hash, u_int *hashlen)
 {
        Buffer b;
@@ -108,7 +104,7 @@ kex_c25519_hash(
        buffer_put_string(&b, serverhostkeyblob, sbloblen);
        buffer_put_string(&b, client_dh_pub, CURVE25519_SIZE);
        buffer_put_string(&b, server_dh_pub, CURVE25519_SIZE);
-       buffer_put_bignum2(&b, shared_secret);
+       buffer_append(&b, shared_secret, secretlen);
 
 #ifdef DEBUG_KEX
        buffer_dump(&b);

diff --git a/kexc25519c.c b/kexc25519c.c
index 4655c2542c55253ccef5e0fe5d4e2010604a502c..a80678af6c01c9e000e2d50bf2737747278cbd6a 100644 (file)
--- a/kexc25519c.c
+++ b/kexc25519c.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexc25519c.c,v 1.3 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexc25519c.c,v 1.4 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -45,7 +45,6 @@
 void
 kexc25519_client(Kex *kex)
 {
-       BIGNUM *shared_secret;
        Key *server_host_key;
        u_char client_key[CURVE25519_SIZE];
        u_char client_pubkey[CURVE25519_SIZE];
@@ -53,6 +52,7 @@ kexc25519_client(Kex *kex)
        u_char *server_host_key_blob = NULL, *signature = NULL;
        u_char *hash;
        u_int slen, sbloblen, hashlen;
+       Buffer shared_secret;
 
        kexc25519_keygen(client_key, client_pubkey);
 
@@ -93,7 +93,8 @@ kexc25519_client(Kex *kex)
        signature = packet_get_string(&slen);
        packet_check_eom();
 
-       shared_secret = kexc25519_shared_key(client_key, server_pubkey);
+       buffer_init(&shared_secret);
+       kexc25519_shared_key(client_key, server_pubkey, &shared_secret);
 
        /* calc and verify H */
        kex_c25519_hash(
@@ -105,7 +106,7 @@ kexc25519_client(Kex *kex)
            server_host_key_blob, sbloblen,
            client_pubkey,
            server_pubkey,
-           shared_secret,
+           buffer_ptr(&shared_secret), buffer_len(&shared_secret),
            &hash, &hashlen
        );
        free(server_host_key_blob);
@@ -121,8 +122,8 @@ kexc25519_client(Kex *kex)
                kex->session_id = xmalloc(kex->session_id_len);
                memcpy(kex->session_id, hash, kex->session_id_len);
        }
-
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
-       BN_clear_free(shared_secret);
+       kex_derive_keys(kex, hash, hashlen,
+           buffer_ptr(&shared_secret), buffer_len(&shared_secret));
+       buffer_free(&shared_secret);
        kex_finish(kex);
 }

diff --git a/kexc25519s.c b/kexc25519s.c
index dc4f56c8049b514f056f3426d4061742146ad355..2b8e8efa175c940aa281e7eb15e413f5c611cfe2 100644 (file)
--- a/kexc25519s.c
+++ b/kexc25519s.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexc25519s.c,v 1.3 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexc25519s.c,v 1.4 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -42,7 +42,6 @@
 void
 kexc25519_server(Kex *kex)
 {
-       BIGNUM *shared_secret;
        Key *server_host_private, *server_host_public;
        u_char *server_host_key_blob = NULL, *signature = NULL;
        u_char server_key[CURVE25519_SIZE];
@@ -50,6 +49,7 @@ kexc25519_server(Kex *kex)
        u_char server_pubkey[CURVE25519_SIZE];
        u_char *hash;
        u_int slen, sbloblen, hashlen;
+       Buffer shared_secret;
 
        /* generate private key */
        kexc25519_keygen(server_key, server_pubkey);
@@ -76,7 +76,8 @@ kexc25519_server(Kex *kex)
        dump_digest("client public key:", client_pubkey, CURVE25519_SIZE);
 #endif
 
-       shared_secret = kexc25519_shared_key(server_key, client_pubkey);
+       buffer_init(&shared_secret);
+       kexc25519_shared_key(server_key, client_pubkey, &shared_secret);
 
        /* calc H */
        key_to_blob(server_host_public, &server_host_key_blob, &sbloblen);
@@ -89,7 +90,7 @@ kexc25519_server(Kex *kex)
            server_host_key_blob, sbloblen,
            client_pubkey,
            server_pubkey,
-           shared_secret,
+           buffer_ptr(&shared_secret), buffer_len(&shared_secret),
            &hash, &hashlen
        );
 
@@ -117,7 +118,9 @@ kexc25519_server(Kex *kex)
        free(server_host_key_blob);
        /* have keys, free server key */
        free(client_pubkey);
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
-       BN_clear_free(shared_secret);
+
+       kex_derive_keys(kex, hash, hashlen,
+           buffer_ptr(&shared_secret), buffer_len(&shared_secret));
+       buffer_free(&shared_secret);
        kex_finish(kex);
 }

diff --git a/kexdhc.c b/kexdhc.c
index ccd137cacbf36e33498590bc28f64a93e394f487..78509af211443bde0373c56bed34d1e50c3b78c6 100644 (file)
--- a/kexdhc.c
+++ b/kexdhc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexdhc.c,v 1.13 2013/05/17 00:13:13 djm Exp $ */
+/* $OpenBSD: kexdhc.c,v 1.14 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
@@ -155,7 +155,7 @@ kexdh_client(Kex *kex)
                memcpy(kex->session_id, hash, kex->session_id_len);
        }
 
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
        kex_finish(kex);
 }

diff --git a/kexdhs.c b/kexdhs.c
index 39b9aba54ed2a1f3d4c78b6867376dc1ba49c77a..d2c7adc9636e05f1f5a3b3f2733aad39625eb131 100644 (file)
--- a/kexdhs.c
+++ b/kexdhs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexdhs.c,v 1.16 2013/11/02 22:24:24 markus Exp $ */
+/* $OpenBSD: kexdhs.c,v 1.17 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
@@ -154,7 +154,7 @@ kexdh_server(Kex *kex)
        /* have keys, free DH */
        DH_free(dh);
 
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
        kex_finish(kex);
 }

diff --git a/kexecdhc.c b/kexecdhc.c
index fc62cec5542d340a6ed2a13394aa5ea1d8b9c883..e3d1cf5f9c6d67a279c9361f53169a0482ac8241 100644 (file)
--- a/kexecdhc.c
+++ b/kexecdhc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexecdhc.c,v 1.5 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexecdhc.c,v 1.6 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -152,7 +152,7 @@ kexecdh_client(Kex *kex)
                memcpy(kex->session_id, hash, kex->session_id_len);
        }
 
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
        kex_finish(kex);
 }

diff --git a/kexecdhs.c b/kexecdhs.c
index d1dd8c7fbff40f3507d23793bcd1bbb5b4805ba6..6fbb79c9dfa8a07aa150f0843f9e8c4bb63e8e43 100644 (file)
--- a/kexecdhs.c
+++ b/kexecdhs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexecdhs.c,v 1.8 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexecdhs.c,v 1.9 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -148,7 +148,7 @@ kexecdh_server(Kex *kex)
        /* have keys, free server key */
        EC_KEY_free(server_key);
 
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
        kex_finish(kex);
 }

diff --git a/kexgexc.c b/kexgexc.c
index ca771edfe7dffdeaefd8a268d4bd291db2ba6395..a69ff27056c080a72c0cba7a4731cc6b031e5085 100644 (file)
--- a/kexgexc.c
+++ b/kexgexc.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgexc.c,v 1.14 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexgexc.c,v 1.15 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -200,7 +200,7 @@ kexgex_client(Kex *kex)
                kex->session_id = xmalloc(kex->session_id_len);
                memcpy(kex->session_id, hash, kex->session_id_len);
        }
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
 
        kex_finish(kex);

diff --git a/kexgexs.c b/kexgexs.c
index 90853403e328f789b6745d6d57aed9f0c2faf830..8773778ed3a0286008ec474a414d526c25f5ab5f 100644 (file)
--- a/kexgexs.c
+++ b/kexgexs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgexs.c,v 1.17 2014/01/09 23:20:00 djm Exp $ */
+/* $OpenBSD: kexgexs.c,v 1.18 2014/01/12 08:13:13 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -201,7 +201,7 @@ kexgex_server(Kex *kex)
        /* have keys, free DH */
        DH_free(dh);
 
-       kex_derive_keys(kex, hash, hashlen, shared_secret);
+       kex_derive_keys_bn(kex, hash, hashlen, shared_secret);
        BN_clear_free(shared_secret);
 
        kex_finish(kex);