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Keyed-MD5, and HTTP-NG
There are a few different ways to add key material to MD5 to
make it suitable as a shared-secret authenticator function. Some of these
are less resistant to attacks than others. For example, the keyed MD5
mechanism that is part of the current IPsec specifications can be
attacked using 2**60 chosen messages. Fortunately, the IPsec specs
also require that the shared MD5 key be changed every 2**32 messages,
so this attack is unlikely to succeed. Specifically, IPsec uses
MD5 as follows: X = MD5(key | keypad | Message), where "|" means
concatenation and the "keypad" pads out the key to 512 bits.
Basically, this function is the same as standard MD5 with a
different initialization vector for the compression operation
on the first block of the message.
RSA Labs recommends that a people use an authenticator like
X = MD5(key1, MD5(key2, Message)). This resists the chosen plaintext
attacks that were published at the crypto conference in Spring 1995.
There are also some very fast MAC algorithms being proposed
these days. As Phill Hallam mentioned, you may want to look at the
work of Phil Rogaway. At a minimum, make sure that your standard
allows people to migrate from a current solution based on MD5 to a
future solution based on new functions. However, I would be relunctant
to require new functions until they have had a chance to be tested
by the cryptographic research community.