tHere is some code using openssl and libsrtp native api
#define SRTP_MASTER_KEY_KEY_LEN 16
#define SRTP_MASTER_KEY_SALT_LEN 14
static void dtls_srtp_init( struct transport_dtls *dtls )
{
/*
When SRTP mode is in effect, different keys are used for ordinary
DTLS record protection and SRTP packet protection. These keys are
generated using a TLS exporter [RFC5705] to generate
2 * (SRTPSecurityParams.master_key_len +
SRTPSecurityParams.master_salt_len) bytes of data
which are assigned as shown below. The per-association context value
is empty.
client_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
server_write_SRTP_master_key[SRTPSecurityParams.master_key_len];
client_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
server_write_SRTP_master_salt[SRTPSecurityParams.master_salt_len];
*/
int code;
err_status_t err;
srtp_policy_t policy;
char dtls_buffer[SRTP_MASTER_KEY_KEY_LEN * 2 + SRTP_MASTER_KEY_SALT_LEN * 2];
char client_write_key[SRTP_MASTER_KEY_KEY_LEN + SRTP_MASTER_KEY_SALT_LEN];
char server_write_key[SRTP_MASTER_KEY_KEY_LEN + SRTP_MASTER_KEY_SALT_LEN];
size_t offset = 0;
/*
The exporter label for this usage is "EXTRACTOR-dtls_srtp". (The
"EXTRACTOR" prefix is for historical compatibility.)
RFC 5764 4.2. Key Derivation
*/
const char * label = "EXTRACTOR-dtls_srtp";
SRTP_PROTECTION_PROFILE * srtp_profile= SSL_get_selected_srtp_profile( dtls->ssl );
/* SSL_export_keying_material exports a value derived from the master secret,
* as specified in RFC 5705. It writes |olen| bytes to |out| given a label and
* optional context. (Since a zero length context is allowed, the |use_context|
* flag controls whether a context is included.)
*
* It returns 1 on success and zero otherwise.
*/
code = SSL_export_keying_material(dtls->ssl,
dtls_buffer,
sizeof(dtls_buffer),
label,
strlen( label),
NULL,
0,
PJ_FALSE);
memcpy(&client_write_key[0], &dtls_buffer[offset], SRTP_MASTER_KEY_KEY_LEN);
offset += SRTP_MASTER_KEY_KEY_LEN;
memcpy(&server_write_key[0], &dtls_buffer[offset], SRTP_MASTER_KEY_KEY_LEN);
offset += SRTP_MASTER_KEY_KEY_LEN;
memcpy(&client_write_key[SRTP_MASTER_KEY_KEY_LEN], &dtls_buffer[offset], SRTP_MASTER_KEY_SALT_LEN);
offset += SRTP_MASTER_KEY_SALT_LEN;
memcpy(&server_write_key[SRTP_MASTER_KEY_KEY_LEN], &dtls_buffer[offset], SRTP_MASTER_KEY_SALT_LEN);
switch( srtp_profile->id )
{
case SRTP_AES128_CM_SHA1_80:
crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtp);
crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtcp);
break;
case SRTP_AES128_CM_SHA1_32:
crypto_policy_set_aes_cm_128_hmac_sha1_32(&policy.rtp); // rtp is 32,
crypto_policy_set_aes_cm_128_hmac_sha1_80(&policy.rtcp); // rtcp still 80
break;
default:
assert(0);
}
policy.ssrc.value = 0;
policy.next = NULL;
/* Init transmit direction */
policy.ssrc.type = ssrc_any_outbound;
policy.key = client_write_key;
err = srtp_create(&dtls->srtp_ctx_rx, &policy);
if (err != err_status_ok) {
printf("not working\n");
}
/* Init receive direction */
policy.ssrc.type = ssrc_any_inbound;
policy.key = server_write_key;
err = srtp_create(&dtls->srtp_ctx_tx, &policy);
if (err != err_status_ok) {
printf("not working\n");
}
}
