SRTP Authentication

Hi,

I am trying to use srtp with Asterisk.
It fails with the error :res_srtp.c:384 ast_srtp_unprotect: SRTP unprotect: authentication failure

It might be bacause I am not providing any authentication tag in the payload.
Can you please tell me how to create the authentication tag.
I am using crypto “AES_CM_128_HMAC_SHA1_32 inline:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx”

Thanks in advance.

Saurav