4.2.1. HMAC-SHA1

The pre-defined authentication transform for SRTP is HMAC-SHA1 [RFC2104]. With HMAC-SHA1, the SRTP_PREFIX_LENGTH (Figure 3) SHALL be 0. For SRTP (respectively SRTCP), the HMAC SHALL be applied to the session authentication key and M as specified above, i.e., HMAC(k_a, M). The HMAC output SHALL then be truncated to the n_tag left-most bits.