- 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@

14 files changed, 147 insertions, 48 deletions

diff --git a/ChangeLog b/ChangeLog
index 99b846e3c..e30cec1a4 100644
--- 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 3e70b8d90..9401fe1d0 100644
--- 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 4fa2ca112..7df8a38fa 100644
--- 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 dbb1a9816..7d054cdcb 100644
--- 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 fbe4940e8..7e2878f70 100644
--- 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 8dd363991..48ca4aaa2 100644
--- 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 4655c2542..a80678af6 100644
--- 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 dc4f56c80..2b8e8efa1 100644
--- 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 ccd137cac..78509af21 100644
--- 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 39b9aba54..d2c7adc96 100644
--- 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 fc62cec55..e3d1cf5f9 100644
--- 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 d1dd8c7fb..6fbb79c9d 100644
--- 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 ca771edfe..a69ff2705 100644
--- 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 90853403e..8773778ed 100644
--- 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);