+
+/* Unhide an avp.
+ *
+ * This unencodes the AVP using the L2TP CHAP secret and the
+ * previously stored random vector. It replaces the hidden data with
+ * the cleartext data and returns the length of the cleartext data
+ * (including the AVP "header" of 6 bytes).
+ *
+ * Based on code from rp-l2tpd by Roaring Penguin Software Inc.
+ */
+int unhide_avp(u8 *avp, tunnelidt t, sessionidt s, u16 length)
+{
+ MD5_CTX ctx;
+ u8 *cursor;
+ u8 digest[16];
+ u8 working_vector[16];
+ uint16_t hidden_length;
+ u8 type[2];
+ size_t done, todo;
+ u8 *output;
+
+ // Find the AVP type.
+ type[0] = *(avp + 4);
+ type[1] = *(avp + 5);
+
+ // Line up with the hidden data
+ cursor = output = avp + 6;
+
+ // Compute initial pad
+ MD5Init(&ctx);
+ MD5Update(&ctx, type, 2);
+ MD5Update(&ctx, config->l2tpsecret, strlen(config->l2tpsecret));
+ MD5Update(&ctx, session[s].random_vector, session[s].random_vector_length);
+ MD5Final(digest, &ctx);
+
+ // Get hidden length
+ hidden_length = ((uint16_t) (digest[0] ^ cursor[0])) * 256 + (uint16_t) (digest[1] ^ cursor[1]);
+
+ // Keep these for later use
+ working_vector[0] = *cursor;
+ working_vector[1] = *(cursor + 1);
+ cursor += 2;
+
+ if (hidden_length > length - 8)
+ {
+ log(1, 0, s, t, "Hidden length %d too long in AVP of length %d\n", (int) hidden_length, (int) length);
+ return 0;
+ }
+
+ /* Decrypt remainder */
+ done = 2;
+ todo = hidden_length;
+ while (todo)
+ {
+ working_vector[done] = *cursor;
+ *output = digest[done] ^ *cursor;
+ ++output;
+ ++cursor;
+ --todo;
+ ++done;
+ if (done == 16 && todo)
+ {
+ // Compute new digest
+ done = 0;
+ MD5Init(&ctx);
+ MD5Update(&ctx, config->l2tpsecret, strlen(config->l2tpsecret));
+ MD5Update(&ctx, &working_vector, 16);
+ MD5Final(digest, &ctx);
+ }
+ }
+
+ return hidden_length + 6;
+}
+
projects / l2tpns.git / blobdiff