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Use a full-length key even with null ciphers #559

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merged 5 commits into from
Sep 9, 2021
Merged

Use a full-length key even with null ciphers #559

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89a5f70
Use a full-length key even with null ciphers
Sep 8, 2021
15e64b9
Accommodate shorter GCM keys
Sep 8, 2021
f383031
Formatting
Sep 8, 2021
c4a5455
Set full-size key length in crypto policies
Sep 9, 2021
812a683
Set full-size key length in crypto policies
Sep 9, 2021
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44 changes: 37 additions & 7 deletions srtp/srtp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -757,22 +757,40 @@ static inline int base_key_length(const srtp_cipher_type_t *cipher,
int key_length)
{
switch (cipher->id) {
case SRTP_NULL_CIPHER:
return 0;
case SRTP_AES_ICM_128:
case SRTP_AES_ICM_192:
case SRTP_AES_ICM_256:
/* The legacy modes are derived from
* the configured key length on the policy */
return key_length - SRTP_SALT_LEN;
break;
case SRTP_AES_GCM_128:
return key_length - SRTP_AEAD_SALT_LEN;
break;
case SRTP_AES_GCM_256:
return key_length - SRTP_AEAD_SALT_LEN;
break;
default:
return key_length;
break;
}
}

/* Get the key length that the application should supply for the given cipher */
static inline int full_key_length(const srtp_cipher_type_t *cipher)
{
switch (cipher->id) {
case SRTP_NULL_CIPHER:
case SRTP_AES_ICM_128:
return SRTP_AES_ICM_128_KEY_LEN_WSALT;
case SRTP_AES_ICM_192:
return SRTP_AES_ICM_192_KEY_LEN_WSALT;
case SRTP_AES_ICM_256:
return SRTP_AES_ICM_256_KEY_LEN_WSALT;
case SRTP_AES_GCM_128:
return SRTP_AES_GCM_128_KEY_LEN_WSALT;
case SRTP_AES_GCM_256:
return SRTP_AES_ICM_256_KEY_LEN_WSALT;
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@lminiero lminiero Sep 13, 2021 •
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Shouldn't this be SRTP_AES_GCM_256_KEY_LEN_WSALT? SRTP_AES_ICM_256_KEY_LEN_WSALT is 32+14 while SRTP_AES_GCM_256_KEY_LEN_WSALT is 32+12. This is causing SRTP/SRTCP errors in our WebRTC servers when GCM-256 is used.

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Yes.

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Ack, thanks, I created a mini-PR in #562 that addresses the typo.

default:
return 0;
}
}

Expand Down Expand Up @@ -870,6 +888,7 @@ srtp_err_status_t srtp_stream_init_keys(srtp_stream_ctx_t *srtp,
srtp_err_status_t stat;
srtp_kdf_t kdf;
uint8_t tmp_key[MAX_SRTP_KEY_LEN];
int input_keylen, input_keylen_rtcp;
int kdf_keylen = 30, rtp_keylen, rtcp_keylen;
int rtp_base_key_len, rtp_salt_len;
int rtcp_base_key_len, rtcp_salt_len;
Expand Down Expand Up @@ -906,6 +925,12 @@ srtp_err_status_t srtp_stream_init_keys(srtp_stream_ctx_t *srtp,

session_keys->mki_size = master_key->mki_size;

input_keylen = full_key_length(session_keys->rtp_cipher->type);
input_keylen_rtcp = full_key_length(session_keys->rtcp_cipher->type);
if (input_keylen_rtcp > input_keylen) {
input_keylen = input_keylen_rtcp;
}

rtp_keylen = srtp_cipher_get_key_length(session_keys->rtp_cipher);
rtcp_keylen = srtp_cipher_get_key_length(session_keys->rtcp_cipher);
rtp_base_key_len =
Expand All @@ -920,6 +945,11 @@ srtp_err_status_t srtp_stream_init_keys(srtp_stream_ctx_t *srtp,
kdf_keylen = 46; /* AES-CTR mode is always used for KDF */
}

if (input_keylen > kdf_keylen) {
kdf_keylen = 46; /* AES-CTR mode is always used for KDF */
}

debug_print(mod_srtp, "input key len: %d", input_keylen);
debug_print(mod_srtp, "srtp key len: %d", rtp_keylen);
debug_print(mod_srtp, "srtcp key len: %d", rtcp_keylen);
debug_print(mod_srtp, "base key len: %d", rtp_base_key_len);
Expand All @@ -932,7 +962,7 @@ srtp_err_status_t srtp_stream_init_keys(srtp_stream_ctx_t *srtp,
* the legacy CTR mode KDF, which uses a 112 bit master SALT.
*/
memset(tmp_key, 0x0, MAX_SRTP_KEY_LEN);
memcpy(tmp_key, key, (rtp_base_key_len + rtp_salt_len));
memcpy(tmp_key, key, input_keylen);

/* initialize KDF state */
#if defined(OPENSSL) && defined(OPENSSL_KDF)
Expand Down Expand Up @@ -3247,7 +3277,7 @@ void srtp_crypto_policy_set_null_cipher_hmac_sha1_80(srtp_crypto_policy_t *p)
*/

p->cipher_type = SRTP_NULL_CIPHER;
p->cipher_key_len = 0;
p->cipher_key_len = 16;
p->auth_type = SRTP_HMAC_SHA1;
p->auth_key_len = 20;
p->auth_tag_len = 10;
Expand All @@ -3261,7 +3291,7 @@ void srtp_crypto_policy_set_null_cipher_hmac_null(srtp_crypto_policy_t *p)
*/

p->cipher_type = SRTP_NULL_CIPHER;
p->cipher_key_len = 0;
p->cipher_key_len = 16;
p->auth_type = SRTP_NULL_AUTH;
p->auth_key_len = 0;
p->auth_tag_len = 0;
Expand Down
171 changes: 171 additions & 0 deletions test/srtp_driver.c
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Original file line number Diff line number Diff line change
Expand Up @@ -61,6 +61,8 @@

srtp_err_status_t srtp_validate(void);

srtp_err_status_t srtp_validate_null(void);

#ifdef GCM
srtp_err_status_t srtp_validate_gcm(void);
#endif
Expand Down Expand Up @@ -442,6 +444,15 @@ int main(int argc, char *argv[])
exit(1);
}

printf("testing srtp_protect and srtp_unprotect against "
"reference packet using null cipher and HMAC\n");
if (srtp_validate_null() == srtp_err_status_ok) {
printf("passed\n\n");
} else {
printf("failed\n");
exit(1);
}

#ifdef GCM
printf("testing srtp_protect and srtp_unprotect against "
"reference packet using GCM\n");
Expand Down Expand Up @@ -1806,6 +1817,166 @@ srtp_err_status_t srtp_validate()
return srtp_err_status_ok;
}

/*
* srtp_validate_null() verifies the correctness of libsrtp by comparing
* some computed packets against some pre-computed reference values.
* These packets were made with a policy that applies null encryption
* and HMAC authentication.
*/

srtp_err_status_t srtp_validate_null()
{
// clang-format off
uint8_t srtp_plaintext_ref[28] = {
0x80, 0x0f, 0x12, 0x34, 0xde, 0xca, 0xfb, 0xad,
0xca, 0xfe, 0xba, 0xbe, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab
};
uint8_t srtp_plaintext[38] = {
0x80, 0x0f, 0x12, 0x34, 0xde, 0xca, 0xfb, 0xad,
0xca, 0xfe, 0xba, 0xbe, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
uint8_t srtp_ciphertext[38] = {
0x80, 0x0f, 0x12, 0x34, 0xde, 0xca, 0xfb, 0xad,
0xca, 0xfe, 0xba, 0xbe, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xa1, 0x36, 0x27,
0x0b, 0x67, 0x91, 0x34, 0xce, 0x9b
};
uint8_t rtcp_plaintext_ref[24] = {
0x81, 0xc8, 0x00, 0x0b, 0xca, 0xfe, 0xba, 0xbe,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
};
uint8_t rtcp_plaintext[38] = {
0x81, 0xc8, 0x00, 0x0b, 0xca, 0xfe, 0xba, 0xbe,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
uint8_t srtcp_ciphertext[38] = {
0x81, 0xc8, 0x00, 0x0b, 0xca, 0xfe, 0xba, 0xbe,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab, 0xab,
0x00, 0x00, 0x00, 0x01, 0xfe, 0x88, 0xc7, 0xfd,
0xfd, 0x37, 0xeb, 0xce, 0x61, 0x5d,
};
// clang-format on

srtp_t srtp_snd, srtp_recv;
srtp_err_status_t status;
int len;
srtp_policy_t policy;

/*
* create a session with a single stream using the default srtp
* policy and with the SSRC value 0xcafebabe
*/
memset(&policy, 0, sizeof(policy));
srtp_crypto_policy_set_null_cipher_hmac_sha1_80(&policy.rtp);
srtp_crypto_policy_set_null_cipher_hmac_sha1_80(&policy.rtcp);
policy.ssrc.type = ssrc_specific;
policy.ssrc.value = 0xcafebabe;
policy.key = test_key;
policy.deprecated_ekt = NULL;
policy.window_size = 128;
policy.allow_repeat_tx = 0;
policy.next = NULL;

status = srtp_create(&srtp_snd, &policy);
if (status) {
return status;
}

/*
* protect plaintext, then compare with ciphertext
*/
len = 28;
status = srtp_protect(srtp_snd, srtp_plaintext, &len);
if (status || (len != 38)) {
return srtp_err_status_fail;
}

debug_print(mod_driver, "ciphertext:\n %s",
octet_string_hex_string(srtp_plaintext, len));
debug_print(mod_driver, "ciphertext reference:\n %s",
octet_string_hex_string(srtp_ciphertext, len));

if (srtp_octet_string_is_eq(srtp_plaintext, srtp_ciphertext, len)) {
return srtp_err_status_fail;
}

/*
* protect plaintext rtcp, then compare with srtcp ciphertext
*/
len = 24;
status = srtp_protect_rtcp(srtp_snd, rtcp_plaintext, &len);
if (status || (len != 38)) {
return srtp_err_status_fail;
}

debug_print(mod_driver, "srtcp ciphertext:\n %s",
octet_string_hex_string(rtcp_plaintext, len));
debug_print(mod_driver, "srtcp ciphertext reference:\n %s",
octet_string_hex_string(srtcp_ciphertext, len));

if (srtp_octet_string_is_eq(rtcp_plaintext, srtcp_ciphertext, len)) {
return srtp_err_status_fail;
}

/*
* create a receiver session context comparable to the one created
* above - we need to do this so that the replay checking doesn't
* complain
*/
status = srtp_create(&srtp_recv, &policy);
if (status) {
return status;
}

/*
* unprotect ciphertext, then compare with plaintext
*/
status = srtp_unprotect(srtp_recv, srtp_ciphertext, &len);
if (status || (len != 28)) {
return status;
}

if (srtp_octet_string_is_eq(srtp_ciphertext, srtp_plaintext_ref, len)) {
return srtp_err_status_fail;
}

/*
* unprotect srtcp ciphertext, then compare with rtcp plaintext
*/
len = 38;
status = srtp_unprotect_rtcp(srtp_recv, srtcp_ciphertext, &len);
if (status || (len != 24)) {
return status;
}

if (srtp_octet_string_is_eq(srtcp_ciphertext, rtcp_plaintext_ref, len)) {
return srtp_err_status_fail;
}

status = srtp_dealloc(srtp_snd);
if (status) {
return status;
}

status = srtp_dealloc(srtp_recv);
if (status) {
return status;
}

return srtp_err_status_ok;
}

#ifdef GCM
/*
* srtp_validate_gcm() verifies the correctness of libsrtp by comparing
Expand Down