API cleanup

libs/srtp/recv.c

@@ -51,7 +51,8 @@ int main (void)
     static const uint8_t salt[14] =
         "\x12\x34\x56\x78\x90" "\x12\x34\x56\x78\x90" "\x12\x34\x56\x78";
 
-    srtp_session_t *s = srtp_create ("AES_CM_128_HMAC_SHA1_80", 0, 0);
+    srtp_session_t *s = srtp_create (SRTP_ENCR_AES_CM, SRTP_AUTH_HMAC_SHA1, 10,
+                                     SRTP_PRF_AES_CM, 0);
     if (s == NULL)
         return 1;
     if (srtp_setkey (s, key, 16, salt, 14))

libs/srtp/srtp.c

@@ -61,6 +61,7 @@ struct srtp_session_t
     uint32_t rtcp_index;
     uint32_t rtp_roc;
     uint16_t rtp_seq;
+    uint16_t rtp_rcc;
     uint8_t  tag_len;
 };
 
@@ -74,6 +75,7 @@ enum
     SRTCP_SALT
 };
 
+
 #ifdef WIN32
 # include <winsock2.h>
 #else
@@ -159,33 +161,52 @@ static int proto_create (srtp_proto_t *p, int gcipher, int gmd)
  * internal cryptographic counters; it is however of course feasible to open
  * multiple simultaneous sessions with the same master key.
  *
- * @param name cipher-suite name
- * @param kdr key derivation rate
+ * @param encr encryption algorithm number
+ * @param auth authentication algortihm number
+ * @param tag_len authentication tag byte length (NOT including RCC)
  * @param flags OR'ed optional flags.
  *
  * @return NULL in case of error
  */
 srtp_session_t *
-srtp_create (const char *name, unsigned flags, unsigned kdr)
+srtp_create (int encr, int auth, unsigned tag_len, int prf, unsigned flags)
 {
-    assert (name != NULL);
+    if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
+        return NULL;
 
-    if (kdr != 0)
-        return NULL; // FIXME: KDR not implemented yet
+    int cipher, md;
+    switch (encr)
+    {
+        case SRTP_ENCR_NULL:
+            cipher = GCRY_CIPHER_NONE;
+            break;
 
-    uint8_t tag_len;
-    int cipher = GCRY_CIPHER_AES, md = GCRY_MD_SHA1;
+        case SRTP_ENCR_AES_CM:
+            cipher = GCRY_CIPHER_AES;
+            break;
 
-    if (strcmp (name, "AES_CM_128_HMAC_SHA1_80") == 0)
-        tag_len = 10;
-    else
-    if (strcmp (name, "AES_CM_128_HMAC_SHA1_32") == 0)
-        tag_len = 4;
-    else
-    // F8_128_HMAC_SHA1_80 is not implemented
+        default:
             return NULL;
+    }
 
-    if ((flags & ~SRTP_FLAGS_MASK) || init_libgcrypt ())
+    switch (auth)
+    {
+        case SRTP_AUTH_NULL:
+            md = GCRY_MD_NONE;
+            break;
+
+        case SRTP_AUTH_HMAC_SHA1:
+            md = GCRY_MD_SHA1;
+            break;
+
+        default:
+            return NULL;
+    }
+
+    if (tag_len > gcry_md_get_algo_dlen (auth))
+        return NULL;
+
+    if (prf != SRTP_PRF_AES_CM)
         return NULL;
 
     srtp_session_t *s = malloc (sizeof (*s));
@@ -194,7 +215,6 @@ srtp_create (const char *name, unsigned flags, unsigned kdr)
 
     memset (s, 0, sizeof (*s));
     s->flags = flags;
-    s->kdr = kdr;
     s->tag_len = tag_len;
 
     if (proto_create (&s->rtp, cipher, md) == 0)
@@ -336,6 +356,33 @@ srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
 }
 
 
+/**
+ * Sets Roll-over-Counter Carry (RCC) rate for the SRTP session. If not
+ * specified (through this function), the default rate of ONE is assumed
+ * (i.e. every RTP packets will carry the RoC). RCC rate is ignored if none
+ * of the RCC mode has been selected.
+ *
+ * The RCC mode is selected through one of these flags for srtp_create():
+ *  SRTP_RCC_MODE1: integrity protection only for RoC carrying packets
+ *  SRTP_RCC_MODE2: integrity protection for all packets
+ *  SRTP_RCC_MODE3: no integrity protection
+ *
+ * RCC mode 3 is insecure. Compared to plain RTP, it provides confidentiality
+ * (through encryption) but is much more prone to DoS. It can only be used if
+ * anti-spoofing protection is provided by lower network layers (e.g. IPsec,
+ * or trusted routers and proper source address filtering).
+ *
+ * If RCC rate is 1, RCC mode 1 and 2 are functionally identical.
+ *
+ * @param rate RoC Carry rate (MUST NOT be zero)
+ */
+void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate)
+{
+    assert (rate != 0);
+    s->rtp_rcc = rate;
+}
+
+
 /** AES-CM encryption/decryption (ctr length = 16 bytes) */
 static int
 ctr_crypt (gcry_cipher_hd_t hd, uint32_t *ctr, uint8_t *data, size_t len)

libs/srtp/srtp.h

@@ -26,20 +26,47 @@ enum
 {
     SRTP_UNENCRYPTED=0x1,   // do not encrypt SRTP packets
     SRTCP_UNENCRYPTED=0x2,  // do not encrypt SRTCP packets
-	SRTP_NULL_CIPHER=0x3, // use NULL cipher (encrypt nothing)
-	SRTP_UNAUTHENTICATED=0x4, // do not authenticated SRTP packets
-	SRTP_FLAGS_MASK=0x7
+    SRTP_UNAUTHENTICATED=0x4, // authenticate only SRTCP packets
+
+    SRTP_RCC_MODE1=0x10,    // use Roll-over-Counter Carry mode 1
+    SRTP_RCC_MODE2=0x20,    // use Roll-over-Counter Carry mode 2
+    SRTP_RCC_MODE3=0x30,    // use Roll-over-Counter Carry mode 3 (insecure)
+
+    SRTP_FLAGS_MASK=0x38
+};
+
+/* SRTP encryption algorithms (ciphers); same values as MIKEY */
+enum
+{
+    SRTP_ENCR_NULL=0,
+    SRTP_ENCR_AES_CM=1,
+    SRTP_ENCR_AES_F8=2 // not implemented
+};
+
+/* SRTP authenticaton algorithms; same values as MIKEY */
+enum
+{
+    SRTP_AUTH_NULL=0,
+    SRTP_AUTH_HMAC_SHA1=1
 };
 
+/* SRTP pseudo random function; same values as MIKEY */
+enum
+{
+    SRTP_PRF_AES_CM=0
+};
 
 # ifdef __cplusplus
 extern "C" {
 # endif
 
-srtp_session_t *srtp_create (const char *name, unsigned flags, unsigned kdr);
+srtp_session_t *srtp_create (int encr, int auth, unsigned tag_len, int prf,
+                             unsigned flags);
 void srtp_destroy (srtp_session_t *s);
+
 int srtp_setkey (srtp_session_t *s, const void *key, size_t keylen,
                  const void *salt, size_t saltlen);
+void srtp_setrcc_rate (srtp_session_t *s, uint16_t rate);
 
 int srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t maxsize);
 int srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp);