May 26, 2018

Perl5 interface to DMAC Double Message-Digest Algorithms

This is Encrypted MAC EMAC, formerly known as Double MAC DMAC. Unlike HMAC, which reuses an existing one-way hash function, such as MD5, SHA-1 or RIPEMD-160, EMAC reuses an existing block cipher to produce a secure message authentication code MAC.

Using the block cipher, a message is encrypted in CBC mode. The last block is taken as the MAC of the message. For fixed-length messages, this method is provably secure. In reality, however, messages have arbitrary lengths, and this method is not secure. To make secure MACs for variable length messages, the last block is encrypted once again with a different key. The security of this construction has been proved in the paper, ``CBC MAC for Real-Time Data Sources’’ by Erez Petrank and Charles Rackoff. The security can be proved on the assumption that the underlying block cipher is pseudo-random.

The performance and key-agility of EMAC are reasonable. EMAC is preferable for short messages because the block length is smaller compared to the schemes based on a hash function. EMAC is also chosen as one of the NESSIE winners for Message Authentication Codes, along with UMAC, TTMAC and HMAC. The current NESSIE specification chooses the AES as block cipher.

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