The a=crypto attribute in RFC 4568 has a separate section 9.2. for SRTP "Crypto" Attribute Grammar. What it basically includes is a list of attribute values required for encrypting media (crypto suite, method, session params, keys, MKI...).
However, DTLS-SRTP also does in its handshake protocol (RFC 5764 - SRTP Extension for DTLS) what is done via SDP in an offer-answer protocol. So, is it correct to say that where DTLS-SRTP is used, the a=crypto: attribute is not used. For example, does webRTC offer-answer SDP use the "a=crypto:" attribute as DTLS-SRTP is a must for webRTC?
Informational RFC "SDP for webRTC" also does not throw any light on this issue.
Please clarify.
correct, a=crypto is only used for SDES. When using DTLS-SRTP, the keys are derived from the DTLS handshake instead.