Internet Protocol Security
Internet Protocol Security |
|
Hash message authentication codes (HMAC) "sign" packets to verify that information received is exactly the same as the information sent (integrity). This is critical when data is exchanged over unsecured media.
HMACs provide integrity by means of a hash function (algorithm), combined with a shared, secret key. A hash is more commonly described as a signature on the packet. This is somewhat inaccurate, because a hash differs from a digital signature: a hash uses a secret, shared key; a digital signature uses public key technology and the sending computer's key. Hash functions are also sometimes referred to as message digests or one-way transforms. One-way transforms or functions are so named for two reasons: each party must perform the computation on their respective end, and because it is easy to go from message to digest but mathematically infeasible to go from digest to message. Conversely, two-way functions can go either way; encryption schemes are examples of two-way functions.
The hash signature itself is actually a cryptographic checksum or Message Integrity Code (MIC) that each party must compute to verify the message. For example, the sending computer uses an HMAC algorithm and shared key to compute the checksum for the message and includes it with the packet. The receiving computer must perform an HMAC computation on the received message, and compare it to the original (included in the packet from the sender). If the message has changed in transit, the hash values are different and the packet is rejected.
For integrity, you can choose between two hash functions when setting policy:
Message Digest 5 (MD5) is based on RFC 1321. It was a response to a weakness found in MD4, the previous incarnation of the original MD. MD5 is a little slower, but stronger.
MD5 makes four passes over the data blocks (whereas MD4 made three passes), using a different number constant for each message word on every pass. This equates to 64, 32-bit constants used during the MD5 computation.
Ultimately, this produces a 128-bit key used for integrity check.
Secure Hash Algorithm (SHA) was developed by the National Institute of Standards and Technology as described in FIPS PUB 180-1. The SHA process is closely modeled after MD5.
SHA uses 79, 32-bit constants during the computation, which results in a 160-bit key that is used for integrity check. Longer key lengths provide greater security, so SHA is considered the stronger of the two.
For clarity and brevity, "sign" or "signature" is used for the remainder of this chapter when discussing how the hash function provides integrity.