I'm having a look at IPsec standard and I realized that integrity and data authentication check is based on 96-bit truncated version of HMAC algorithms. For example, in IPsec v3 we can use HMAC-MD5-96, HMAC-SHA-1-96, AES-XCBC-MAC-96. What is the reason behind this choice?

  • What RFCs have you checked so far? – cremefraiche Feb 8 '16 at 2:58

For the answer to this check out RFC 2104, em mine:

A well-known practice with message authentication codes is to
truncate the output of the MAC and output only part of the bits
(e.g., [MM, ANSI]). Preneel and van Oorschot [PV] show some
analytical advantages of truncating the output of hash-based MAC
functions. The results in this area are not absolute as for the
overall security advantages of truncation. It has advantages (less
information on the hash result available to an attacker)
disadvantages (less bits to predict for the attacker). Applications
of HMAC can choose to truncate the output of HMAC by outputting the t leftmost bits of the HMAC computation for some parameter t (namely,
the computation is carried in the normal way as defined in section 2
above but the end result is truncated to t bits). We recommend that
the output length t be not less than half the length of the hash
output (to match the birthday attack bound) and not less than 80 bits (a suitable lower bound on the number of bits that need to be
predicted by an attacker). We propose denoting a realization of HMAC that uses a hash function H with t bits of output as HMAC-H-t. For
example, HMAC-SHA1-80 denotes HMAC computed using the SHA-1 function
and with the output truncated to 80 bits. (If the parameter t is not specified, e.g. HMAC-MD5, then it is assumed that all the bits of the hash are output.)

Truncation also reduces the amount of data transmitted over the wire so bandwidth requirements are reduced.

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