Add PRF to derive key material from the ECDH shared secret.

[tinc] / src / openssl / prf.c

diff --git a/src/openssl/prf.c b/src/openssl/prf.c
new file mode 100644 (file)
index 0000000..2830d60
--- /dev/null
+++ b/src/openssl/prf.c
@@ -0,0 +1,76 @@
+/*
+    prf.c -- Pseudo-Random Function for key material generation
+    Copyright (C) 2011 Guus Sliepen <guus@tinc-vpn.org>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#include "system.h"
+
+#include "digest.h"
+#include "prf.h"
+
+/* Generate key material from a master secret and a seed, based on RFC 2246.
+   We use SHA512 and Whirlpool instead of MD5 and SHA1.
+ */
+
+static bool prf_xor(int nid, char *secret, size_t secretlen, char *seed, size_t seedlen, char *out, ssize_t outlen) {
+       digest_t digest;
+       
+       if(!digest_open_by_nid(&digest, nid, 0))
+               return false;
+
+       if(!digest_set_key(&digest, secret, secretlen))
+               return false;
+
+       size_t len = digest_length(&digest);
+
+       /* Data is what the "inner" HMAC function processes.
+          It consists of the previous HMAC result plus the seed.
+        */
+
+       char data[len + seedlen];
+       memset(data, 0, len);
+       memcpy(data + len, seed, seedlen);
+
+       char hash[len];
+
+       while(outlen > 0) {
+               /* Inner HMAC */
+               digest_create(&digest, data, len + seedlen, data);
+
+               /* Outer HMAC */
+               digest_create(&digest, data, len + seedlen, hash);
+
+               /* XOR the results of the outer HMAC into the out buffer */
+               for(int i = 0; i < len && i < outlen; i++)
+                       *out++ ^= hash[i];
+
+               outlen -= len;
+       }
+
+       digest_close(&digest);
+       return true;
+}
+
+bool prf(char *secret, size_t secretlen, char *seed, size_t seedlen, char *out, size_t outlen) {
+       /* Split secret in half, generate outlen bits with two different hash algorithms,
+          and XOR the results. */
+
+       memset(out, 0, outlen);
+
+       return prf_xor(NID_sha512, secret, secretlen / 2, seed, seedlen, out, outlen)
+               && prf_xor(NID_whirlpool, secret, secretlen / 2, seed, seedlen, out, outlen);
+}