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Diffstat (limited to 'plugins/crypto-vala/src/srtp.c')
| -rw-r--r-- | plugins/crypto-vala/src/srtp.c | 836 |
1 files changed, 836 insertions, 0 deletions
diff --git a/plugins/crypto-vala/src/srtp.c b/plugins/crypto-vala/src/srtp.c new file mode 100644 index 00000000..708244d9 --- /dev/null +++ b/plugins/crypto-vala/src/srtp.c @@ -0,0 +1,836 @@ +/* + * Secure RTP with libgcrypt + * Copyright (C) 2007 Rémi Denis-Courmont + * + * This library is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2.1 of the License, or (at your option) any later version. + * + * This library 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 + * Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this library; if not, write to the Free Software + * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA + */ + +/* TODO: + * Useless stuff (because nothing depends on it): + * - non-nul key derivation rate + * - MKI payload + */ + +#ifdef HAVE_CONFIG_H +# include <config.h> +#endif + +#include <stdint.h> +#include <stddef.h> + +#include "srtp.h" + +#include <stdbool.h> +#include <stdlib.h> +#include <assert.h> +#include <errno.h> + +#include <gcrypt.h> + +#ifdef _WIN32 +# include <winsock2.h> +#else +# include <netinet/in.h> +#endif + +#define debug( ... ) (void)0 + +typedef struct srtp_proto_t +{ + gcry_cipher_hd_t cipher; + gcry_md_hd_t mac; + uint64_t window; + uint32_t salt[4]; +} srtp_proto_t; + +struct srtp_session_t +{ + srtp_proto_t rtp; + srtp_proto_t rtcp; + unsigned flags; + unsigned kdr; + uint32_t rtcp_index; + uint32_t rtp_roc; + uint16_t rtp_seq; + uint16_t rtp_rcc; + uint8_t tag_len; +}; + +enum +{ + SRTP_CRYPT, + SRTP_AUTH, + SRTP_SALT, + SRTCP_CRYPT, + SRTCP_AUTH, + SRTCP_SALT +}; + + +static inline unsigned rcc_mode (const srtp_session_t *s) +{ + return (s->flags >> 4) & 3; +} + + +static void proto_destroy (srtp_proto_t *p) +{ + gcry_md_close (p->mac); + gcry_cipher_close (p->cipher); +} + + +/** + * Releases all resources associated with a Secure RTP session. + */ +void srtp_destroy (srtp_session_t *s) +{ + assert (s != NULL); + + proto_destroy (&s->rtcp); + proto_destroy (&s->rtp); + free (s); +} + + +static int proto_create (srtp_proto_t *p, int gcipher, int gmd) +{ + if (gcry_cipher_open (&p->cipher, gcipher, GCRY_CIPHER_MODE_CTR, 0) == 0) + { + if (gcry_md_open (&p->mac, gmd, GCRY_MD_FLAG_HMAC) == 0) + return 0; + gcry_cipher_close (p->cipher); + } + return -1; +} + + +/** + * Allocates a Secure RTP one-way session. + * The same session cannot be used both ways because this would confuse + * internal cryptographic counters; it is however of course feasible to open + * multiple simultaneous sessions with the same master key. + * + * @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 (int encr, int auth, unsigned tag_len, int prf, unsigned flags) +{ + if ((flags & ~SRTP_FLAGS_MASK)) + return NULL; + + int cipher, md; + switch (encr) + { + case SRTP_ENCR_NULL: + cipher = GCRY_CIPHER_NONE; + break; + + case SRTP_ENCR_AES_CM: + cipher = GCRY_CIPHER_AES; + break; + + default: + return NULL; + } + + 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 (md)) + return NULL; + + if (prf != SRTP_PRF_AES_CM) + return NULL; + + srtp_session_t *s = malloc (sizeof (*s)); + if (s == NULL) + return NULL; + + memset (s, 0, sizeof (*s)); + s->flags = flags; + s->tag_len = tag_len; + s->rtp_rcc = 1; /* Default RCC rate */ + if (rcc_mode (s)) + { + if (tag_len < 4) + goto error; + } + + if (proto_create (&s->rtp, cipher, md) == 0) + { + if (proto_create (&s->rtcp, cipher, md) == 0) + return s; + proto_destroy (&s->rtp); + } + + error: + free (s); + return NULL; +} + + +/** + * Counter Mode encryption/decryption (ctr length = 16 bytes) + * with non-padded (truncated) text + */ +static int +do_ctr_crypt (gcry_cipher_hd_t hd, const void *ctr, uint8_t *data, size_t len) +{ + const size_t ctrlen = 16; + div_t d = div (len, ctrlen); + + if (gcry_cipher_setctr (hd, ctr, ctrlen) + || gcry_cipher_encrypt (hd, data, d.quot * ctrlen, NULL, 0)) + return -1; + + if (d.rem) + { + /* Truncated last block */ + uint8_t dummy[ctrlen]; + data += d.quot * ctrlen; + memcpy (dummy, data, d.rem); + memset (dummy + d.rem, 0, ctrlen - d.rem); + + if (gcry_cipher_encrypt (hd, dummy, ctrlen, data, ctrlen)) + return -1; + memcpy (data, dummy, d.rem); + } + + return 0; +} + + +/** + * AES-CM key derivation (saltlen = 14 bytes) + */ +static int +do_derive (gcry_cipher_hd_t prf, const void *salt, + const uint8_t *r, size_t rlen, uint8_t label, + void *out, size_t outlen) +{ + uint8_t iv[16]; + + memcpy (iv, salt, 14); + iv[14] = iv[15] = 0; + + assert (rlen < 14); + iv[13 - rlen] ^= label; + for (size_t i = 0; i < rlen; i++) + iv[sizeof (iv) - rlen + i] ^= r[i]; + + memset (out, 0, outlen); + return do_ctr_crypt (prf, iv, out, outlen); +} + + +/** + * Sets (or resets) the master key and master salt for a SRTP session. + * This must be done at least once before using srtp_send(), srtp_recv(), + * srtcp_send() or srtcp_recv(). Also, rekeying is required every + * 2^48 RTP packets or 2^31 RTCP packets (whichever comes first), + * otherwise the protocol security might be broken. + * + * @return 0 on success, in case of error: + * EINVAL invalid or unsupported key/salt sizes combination + */ +int +srtp_setkey (srtp_session_t *s, const void *key, size_t keylen, + const void *salt, size_t saltlen) +{ + /* SRTP/SRTCP cipher/salt/MAC keys derivation */ + gcry_cipher_hd_t prf; + uint8_t r[6], keybuf[20]; + + if (saltlen != 14) + return EINVAL; + + if (gcry_cipher_open (&prf, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CTR, 0) + || gcry_cipher_setkey (prf, key, keylen)) + return EINVAL; + + /* SRTP key derivation */ +#if 0 + if (s->kdr != 0) + { + uint64_t index = (((uint64_t)s->rtp_roc) << 16) | s->rtp_seq; + index /= s->kdr; + + for (int i = sizeof (r) - 1; i >= 0; i--) + { + r[i] = index & 0xff; + index = index >> 8; + } + } + else +#endif + memset (r, 0, sizeof (r)); + if (do_derive (prf, salt, r, 6, SRTP_CRYPT, keybuf, 16) + || gcry_cipher_setkey (s->rtp.cipher, keybuf, 16) + || do_derive (prf, salt, r, 6, SRTP_AUTH, keybuf, 20) + || gcry_md_setkey (s->rtp.mac, keybuf, 20) + || do_derive (prf, salt, r, 6, SRTP_SALT, s->rtp.salt, 14)) + return -1; + + /* SRTCP key derivation */ + memcpy (r, &(uint32_t){ htonl (s->rtcp_index) }, 4); + if (do_derive (prf, salt, r, 4, SRTCP_CRYPT, keybuf, 16) + || gcry_cipher_setkey (s->rtcp.cipher, keybuf, 16) + || do_derive (prf, salt, r, 4, SRTCP_AUTH, keybuf, 20) + || gcry_md_setkey (s->rtcp.mac, keybuf, 20) + || do_derive (prf, salt, r, 4, SRTCP_SALT, s->rtcp.salt, 14)) + return -1; + + (void)gcry_cipher_close (prf); + return 0; +} + +static int hexdigit (char c) +{ + if ((c >= '0') && (c <= '9')) + return c - '0'; + if ((c >= 'A') && (c <= 'F')) + return c - 'A' + 0xA; + if ((c >= 'a') && (c <= 'f')) + return c - 'a' + 0xa; + return -1; +} + +static ssize_t hexstring (const char *in, uint8_t *out, size_t outlen) +{ + size_t inlen = strlen (in); + + if ((inlen > (2 * outlen)) || (inlen & 1)) + return -1; + + for (size_t i = 0; i < inlen; i += 2) + { + int a = hexdigit (in[i]), b = hexdigit (in[i + 1]); + if ((a == -1) || (b == -1)) + return -1; + out[i / 2] = (a << 4) | b; + } + return inlen / 2; +} + +/** + * Sets (or resets) the master key and master salt for a SRTP session + * from hexadecimal strings. See also srtp_setkey(). + * + * @return 0 on success, in case of error: + * EINVAL invalid or unsupported key/salt sizes combination + */ +int +srtp_setkeystring (srtp_session_t *s, const char *key, const char *salt) +{ + uint8_t bkey[16]; /* TODO/NOTE: hard-coded for AES */ + uint8_t bsalt[14]; /* TODO/NOTE: hard-coded for the PRF-AES-CM */ + ssize_t bkeylen = hexstring (key, bkey, sizeof (bkey)); + ssize_t bsaltlen = hexstring (salt, bsalt, sizeof (bsalt)); + + if ((bkeylen == -1) || (bsaltlen == -1)) + return EINVAL; + return srtp_setkey (s, bkey, bkeylen, bsalt, bsaltlen) ? EINVAL : 0; +} + +/** + * 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 for RTP (salt = 14 bytes + 2 nul bytes) */ +static int +rtp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t roc, uint16_t seq, + const uint32_t *salt, uint8_t *data, size_t len) +{ + /* Determines cryptographic counter (IV) */ + uint32_t counter[4]; + counter[0] = salt[0]; + counter[1] = salt[1] ^ ssrc; + counter[2] = salt[2] ^ htonl (roc); + counter[3] = salt[3] ^ htonl (seq << 16); + + /* Encryption */ + return do_ctr_crypt (hd, counter, data, len); +} + + +/** Determines SRTP Roll-Over-Counter (in host-byte order) */ +static uint32_t +srtp_compute_roc (const srtp_session_t *s, uint16_t seq) +{ + uint32_t roc = s->rtp_roc; + + if (((seq - s->rtp_seq) & 0xffff) < 0x8000) + { + /* Sequence is ahead, good */ + if (seq < s->rtp_seq) + roc++; /* Sequence number wrap */ + } + else + { + /* Sequence is late, bad */ + if (seq > s->rtp_seq) + roc--; /* Wrap back */ + } + return roc; +} + + +/** Returns RTP sequence (in host-byte order) */ +static inline uint16_t rtp_seq (const uint8_t *buf) +{ + return (buf[2] << 8) | buf[3]; +} + + +/** Message Authentication and Integrity for RTP */ +static const uint8_t * +rtp_digest (gcry_md_hd_t md, const uint8_t *data, size_t len, + uint32_t roc) +{ + gcry_md_reset (md); + gcry_md_write (md, data, len); + gcry_md_write (md, &(uint32_t){ htonl (roc) }, 4); + return gcry_md_read (md, 0); +} + + +/** + * Encrypts/decrypts a RTP packet and updates SRTP context + * (CTR block cypher mode of operation has identical encryption and + * decryption function). + * + * @param buf RTP packet to be en-/decrypted + * @param len RTP packet length + * + * @return 0 on success, in case of error: + * EINVAL malformatted RTP packet + * EACCES replayed packet or out-of-window or sync lost + */ +static int srtp_crypt (srtp_session_t *s, uint8_t *buf, size_t len) +{ + assert (s != NULL); + assert (len >= 12u); + + if ((buf[0] >> 6) != 2) + return EINVAL; + + /* Computes encryption offset */ + uint16_t offset = 12; + offset += (buf[0] & 0xf) * 4; // skips CSRC + + if (buf[0] & 0x10) + { + uint16_t extlen; + + offset += 4; + if (len < offset) + return EINVAL; + + memcpy (&extlen, buf + offset - 2, 2); + offset += htons (extlen); // skips RTP extension header + } + + if (len < offset) + return EINVAL; + + /* Determines RTP 48-bits counter and SSRC */ + uint16_t seq = rtp_seq (buf); + uint32_t roc = srtp_compute_roc (s, seq), ssrc; + memcpy (&ssrc, buf + 8, 4); + + /* Updates ROC and sequence (it's safe now) */ + int16_t diff = seq - s->rtp_seq; + if (diff > 0) + { + /* Sequence in the future, good */ + s->rtp.window = s->rtp.window << diff; + s->rtp.window |= UINT64_C(1); + s->rtp_seq = seq, s->rtp_roc = roc; + } + else + { + /* Sequence in the past/present, bad */ + diff = -diff; + if ((diff >= 64) || ((s->rtp.window >> diff) & 1)) + return EACCES; /* Replay attack */ + s->rtp.window |= UINT64_C(1) << diff; + } + + /* Encrypt/Decrypt */ + if (s->flags & SRTP_UNENCRYPTED) + return 0; + + if (rtp_crypt (s->rtp.cipher, ssrc, roc, seq, s->rtp.salt, + buf + offset, len - offset)) + return EINVAL; + + return 0; +} + + +/** + * Turns a RTP packet into a SRTP packet: encrypt it, then computes + * the authentication tag and appends it. + * Note that you can encrypt packet in disorder. + * + * @param buf RTP packet to be encrypted/digested + * @param lenp pointer to the RTP packet length on entry, + * set to the SRTP length on exit (undefined on non-ENOSPC error) + * @param bufsize size (bytes) of the packet buffer + * + * @return 0 on success, in case of error: + * EINVAL malformatted RTP packet or internal error + * ENOSPC bufsize is too small to add authentication tag + * (<lenp> will hold the required byte size) + * EACCES packet would trigger a replay error on receiver + */ +int +srtp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize) +{ + size_t len = *lenp; + size_t tag_len; + size_t roc_len = 0; + + /* Compute required buffer size */ + if (len < 12u) + return EINVAL; + + if (!(s->flags & SRTP_UNAUTHENTICATED)) + { + tag_len = s->tag_len; + + if (rcc_mode (s)) + { + assert (tag_len >= 4); + assert (s->rtp_rcc != 0); + if ((rtp_seq (buf) % s->rtp_rcc) == 0) + { + roc_len = 4; + if (rcc_mode (s) == 3) + tag_len = 0; /* RCC mode 3 -> no auth*/ + else + tag_len -= 4; /* RCC mode 1 or 2 -> auth*/ + } + else + { + if (rcc_mode (s) & 1) + tag_len = 0; /* RCC mode 1 or 3 -> no auth */ + } + } + + *lenp = len + roc_len + tag_len; + } + else + tag_len = 0; + + if (bufsize < *lenp) + return ENOSPC; + + /* Encrypt payload */ + int val = srtp_crypt (s, buf, len); + if (val) + return val; + + /* Authenticate payload */ + if (!(s->flags & SRTP_UNAUTHENTICATED)) + { + uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)); + const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, roc); + + if (roc_len) + { + memcpy (buf + len, &(uint32_t){ htonl (s->rtp_roc) }, 4); + len += 4; + } + memcpy (buf + len, tag, tag_len); +#if 0 + printf ("Sent : 0x"); + for (unsigned i = 0; i < tag_len; i++) + printf ("%02x", tag[i]); + puts (""); +#endif + } + + return 0; +} + + +/** + * Turns a SRTP packet into a RTP packet: authenticates the packet, + * then decrypts it. + * + * @param buf RTP packet to be digested/decrypted + * @param lenp pointer to the SRTP packet length on entry, + * set to the RTP length on exit (undefined in case of error) + * + * @return 0 on success, in case of error: + * EINVAL malformatted SRTP packet + * EACCES authentication failed (spoofed packet or out-of-sync) + */ +int +srtp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp) +{ + size_t len = *lenp; + if (len < 12u) + return EINVAL; + + if (!(s->flags & SRTP_UNAUTHENTICATED)) + { + size_t tag_len = s->tag_len, roc_len = 0; + if (rcc_mode (s)) + { + if ((rtp_seq (buf) % s->rtp_rcc) == 0) + { + roc_len = 4; + if (rcc_mode (s) == 3) + tag_len = 0; + else + tag_len -= 4; + } + else + { + if (rcc_mode (s) & 1) + tag_len = 0; // RCC mode 1 or 3: no auth + } + } + + if (len < (12u + roc_len + tag_len)) + return EINVAL; + len -= roc_len + tag_len; + + uint32_t roc = srtp_compute_roc (s, rtp_seq (buf)), rcc; + if (roc_len) + { + assert (roc_len == 4); + memcpy (&rcc, buf + len, 4); + rcc = ntohl (rcc); + } + else + rcc = roc; + + const uint8_t *tag = rtp_digest (s->rtp.mac, buf, len, rcc); +#if 0 + printf ("Computed: 0x"); + for (unsigned i = 0; i < tag_len; i++) + printf ("%02x", tag[i]); + printf ("\nReceived: 0x"); + for (unsigned i = 0; i < tag_len; i++) + printf ("%02x", buf[len + roc_len + i]); + puts (""); +#endif + if (memcmp (buf + len + roc_len, tag, tag_len)) + return EACCES; + + if (roc_len) + { + /* Authenticated packet carried a Roll-Over-Counter */ + s->rtp_roc += rcc - roc; + assert (srtp_compute_roc (s, rtp_seq (buf)) == rcc); + } + *lenp = len; + } + + return srtp_crypt (s, buf, len); +} + + +/** AES-CM for RTCP (salt = 14 bytes + 2 nul bytes) */ +static int +rtcp_crypt (gcry_cipher_hd_t hd, uint32_t ssrc, uint32_t index, + const uint32_t *salt, uint8_t *data, size_t len) +{ + return rtp_crypt (hd, ssrc, index >> 16, index & 0xffff, salt, data, len); +} + + +/** Message Authentication and Integrity for RTCP */ +static const uint8_t * +rtcp_digest (gcry_md_hd_t md, const void *data, size_t len) +{ + gcry_md_reset (md); + gcry_md_write (md, data, len); + return gcry_md_read (md, 0); +} + + +/** + * Encrypts/decrypts a RTCP packet and updates SRTCP context + * (CTR block cypher mode of operation has identical encryption and + * decryption function). + * + * @param buf RTCP packet to be en-/decrypted + * @param len RTCP packet length + * + * @return 0 on success, in case of error: + * EINVAL malformatted RTCP packet + */ +static int srtcp_crypt (srtp_session_t *s, uint8_t *buf, size_t len) +{ + assert (s != NULL); + + /* 8-bytes unencrypted header, and 4-bytes unencrypted footer */ + if ((len < 12) || ((buf[0] >> 6) != 2)) + return EINVAL; + + uint32_t index; + memcpy (&index, buf + len, 4); + index = ntohl (index); + if (((index >> 31) != 0) != ((s->flags & SRTCP_UNENCRYPTED) == 0)) + return EINVAL; // E-bit mismatch + + index &= ~(1 << 31); // clear E-bit for counter + + /* Updates SRTCP index (safe here) */ + int32_t diff = index - s->rtcp_index; + if (diff > 0) + { + /* Packet in the future, good */ + s->rtcp.window = s->rtcp.window << diff; + s->rtcp.window |= UINT64_C(1); + s->rtcp_index = index; + } + else + { + /* Packet in the past/present, bad */ + diff = -diff; + if ((diff >= 64) || ((s->rtcp.window >> diff) & 1)) + return EACCES; // replay attack! + s->rtp.window |= UINT64_C(1) << diff; + } + + /* Crypts SRTCP */ + if (s->flags & SRTCP_UNENCRYPTED) + return 0; + + uint32_t ssrc; + memcpy (&ssrc, buf + 4, 4); + + if (rtcp_crypt (s->rtcp.cipher, ssrc, index, s->rtp.salt, + buf + 8, len - 8)) + return EINVAL; + return 0; +} + + +/** + * Turns a RTCP packet into a SRTCP packet: encrypt it, then computes + * the authentication tag and appends it. + * + * @param buf RTCP packet to be encrypted/digested + * @param lenp pointer to the RTCP packet length on entry, + * set to the SRTCP length on exit (undefined in case of error) + * @param bufsize size (bytes) of the packet buffer + * + * @return 0 on success, in case of error: + * EINVAL malformatted RTCP packet or internal error + * ENOSPC bufsize is too small (to add index and authentication tag) + */ +int +srtcp_send (srtp_session_t *s, uint8_t *buf, size_t *lenp, size_t bufsize) +{ + size_t len = *lenp; + if (bufsize < (len + 4 + s->tag_len)) + return ENOSPC; + + uint32_t index = ++s->rtcp_index; + if (index >> 31) + s->rtcp_index = index = 0; /* 31-bit wrap */ + + if ((s->flags & SRTCP_UNENCRYPTED) == 0) + index |= 0x80000000; /* Set Encrypted bit */ + memcpy (buf + len, &(uint32_t){ htonl (index) }, 4); + + int val = srtcp_crypt (s, buf, len); + if (val) + return val; + + len += 4; /* Digests SRTCP index too */ + + const uint8_t *tag = rtcp_digest (s->rtcp.mac, buf, len); + memcpy (buf + len, tag, s->tag_len); + *lenp = len + s->tag_len; + return 0; +} + + +/** + * Turns a SRTCP packet into a RTCP packet: authenticates the packet, + * then decrypts it. + * + * @param buf RTCP packet to be digested/decrypted + * @param lenp pointer to the SRTCP packet length on entry, + * set to the RTCP length on exit (undefined in case of error) + * + * @return 0 on success, in case of error: + * EINVAL malformatted SRTCP packet + * EACCES authentication failed (spoofed packet or out-of-sync) + */ +int +srtcp_recv (srtp_session_t *s, uint8_t *buf, size_t *lenp) +{ + size_t len = *lenp; + + if (len < (4u + s->tag_len)) + return EINVAL; + len -= s->tag_len; + + const uint8_t *tag = rtcp_digest (s->rtcp.mac, buf, len); + if (memcmp (buf + len, tag, s->tag_len)) + return EACCES; + + len -= 4; /* Remove SRTCP index before decryption */ + *lenp = len; + return srtcp_crypt (s, buf, len); +}
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