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Data integrity with hash functions

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Data integrity with hash functions

Hash message authentication codes (HMAC) sign packets to verify that the information received is exactly the same as the information sent. This is called integrity. HMACs provide integrity through a keyed hash, the result of a mathematical calculation on a message using a hash function (algorithm) combined with a shared, secret key. A hash is commonly described as a signature on the packet. However, a hash differs from a digital signature. A hash uses a secret, shared key, and a digital signature uses public key technology and the private key of the sending computer. A digital signature provides nonrepudiation, and a hash does not. Nonrepudiation ensures that a communication can be proven to have originated from a specific person whose identity can be verified. It also ensures that the communication actually occurred.

Hash functions are also called one-way functions because it is easy to determine the hash from the message but mathematically infeasible to determine the message from the hash. Conversely, in two-way functions, the original message can be determined from its converted form. Encryption and decryption schemes are examples of two-way functions.

The hash is a cryptographic checksum or message integrity code (MIC) that each party must compute to verify the message. For example, the sending computer uses a hash function and shared key to compute the checksum for the message, including it with the packet. The receiving computer must perform the same hash function on the received message and shared key 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:

  • MD5

    Message Digest 5 (MD5) is based on RFC 1321. MD5 completes four passes over the data blocks, using a different numeric constant for each word in the message on each pass. The number of 32-bit constants used during the MD5 computation ultimately produces a 128-bit hash that is used for the integrity check.

  • SHA1

    Secure Hash Algorithm 1 (SHA1) was developed by the National Institute of Standards and Technology as described in Federal Information Processing Standard (FIPS) PUB 180-1. The SHA process is closely modeled after MD5. The SHA1 computation results in a 160-bit hash that is used for the integrity check. Because longer hash lengths provide greater security, SHA is stronger than MD5.