initial import.

Makefile

@@ -0,0 +1,7 @@
+CFLAGS=-g -Os -Wall
+
+all: srtp.o srtp-decrypt.o
+	$(CC) -o srtp-decrypt srtp-decrypt.o srtp.o -lpcap -lgcrypt
+
+check:
+	./srtp-decrypt -k aSBrbm93IGFsbCB5b3VyIGxpdHRsZSBzZWNyZXRz < ./marseillaise-srtp.pcap | text2pcap -t "%M:%S." -u 10000,10000 - - > ./marseillaise-rtp.pcap

srtp-decrypt.c

@@ -0,0 +1,152 @@
+#include <sys/types.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <arpa/inet.h>
+#include <getopt.h>
+#include <assert.h>
+
+#include "srtp.h"
+
+#include <pcap.h>
+
+static const char b64chars[] = 
+  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+static unsigned char shiftb64(unsigned char c) {
+  char *p = strchr(b64chars, c);
+  assert(p);
+  return p-b64chars;
+}
+
+static void decode_block(unsigned char *in, unsigned char *out) {
+  unsigned char shifts[4];
+  int i;
+
+  for (i = 0; i < 4; i++) {
+    shifts[i] = shiftb64(in[i]);
+  }
+
+  out[0] = (shifts[0]<<2)|(shifts[1]>>4);
+  out[1] = (shifts[1]<<4)|(shifts[2]>>2);
+  out[2] = (shifts[2]<<6)|shifts[3];
+}
+
+static void decode_sdes(unsigned char *in,
+  unsigned char *key, unsigned char *salt) {
+  int i;
+  size_t len = strlen((char *) in);
+  assert(len == 40);
+  unsigned char raw[30];
+
+  for (i = 0; 4*i < len; i++) {
+    decode_block(in+4*i, raw+3*i);
+  }
+
+  memcpy(key, raw, 16);
+  memcpy(salt, raw+16, 14);
+}
+
+static srtp_session_t *s = NULL;
+
+static void hexdump(const void *ptr, size_t size) {
+  int i, j;
+  const unsigned char *cptr = ptr;
+
+  for (i = 0; i < size; i += 16) {
+    printf("%04x ", i);
+    for (j = 0; j < 16 && i+j < size; j++) {
+      printf("%02x ", cptr[i+j]);
+    }
+    printf("\n");
+  }
+}
+
+static int rtp_offset = 42 /* 14 + 20 + 8 */;
+static int frame_nr = -1;
+static struct timeval start_tv = {0, 0};
+
+static void handle_pkt(u_char *arg, const struct pcap_pkthdr *hdr,
+  const u_char *bytes) {
+  unsigned char buffer[2048];
+  size_t pktsize;
+  int ret;
+  struct timeval delta;
+
+  frame_nr += 1;
+  if (start_tv.tv_sec == 0 && start_tv.tv_sec == 0) {
+    start_tv = hdr->ts;
+  } 
+
+  if (hdr->caplen < rtp_offset) {
+    fprintf(stderr, "frame %d dropped: too short\n", frame_nr);
+    return;
+  }
+
+  memcpy(buffer, bytes + rtp_offset, hdr->caplen - rtp_offset);
+  pktsize = hdr->caplen - rtp_offset;
+
+  ret = srtp_recv(s, buffer, &pktsize);
+  if (ret != 0) {
+    fprintf(stderr, "frame %d dropped: decoding failed '%s'\n", frame_nr,
+      strerror(ret));
+    return;
+  }
+
+  timersub(&hdr->ts, &start_tv, &delta);
+  printf("%02ld:%02ld.%06lu\n", delta.tv_sec/60, delta.tv_sec%60, delta.tv_usec);
+
+  hexdump(buffer, pktsize);
+}
+
+static void usage(const char *arg0) {
+  fprintf(stderr, "usage: %s -k <base64 SDES key> [-d <rtp offset in frames>]\n", arg0);
+  exit(1);
+}
+
+int main(int argc, char **argv) {
+  unsigned char key[16], salt[14];
+  int c;
+  char errbuf[PCAP_ERRBUF_SIZE];
+  pcap_t *pcap;
+  unsigned char *sdes = NULL;
+
+  while ((c = getopt(argc, argv, "k:d:")) != -1) {
+    switch (c) {
+    case 'k':
+      sdes = (unsigned char *) optarg;
+      break;
+    case 'd':
+      rtp_offset = atoi(optarg);
+      break;
+    default:
+      usage(argv[0]);
+    }
+  }
+
+  if (sdes == NULL) {
+    usage(argv[0]);
+  }
+
+  decode_sdes(sdes, key, salt);
+
+  s = srtp_create(SRTP_ENCR_AES_CM, SRTP_AUTH_HMAC_SHA1, 10,
+    SRTP_PRF_AES_CM, 0);
+  assert(s != NULL);
+  srtp_setkey(s, key, sizeof(key), salt, sizeof(salt));
+
+  pcap = pcap_open_offline("-", errbuf);
+  if (!pcap) {
+    fprintf(stderr, "libpcap failed to open file '%s'\n", errbuf);
+    exit(1);
+  }
+  assert(pcap != NULL);
+
+  pcap_loop(pcap, 0, handle_pkt, NULL);
+
+  srtp_destroy(s);
+
+  return 0;
+}