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Hash Functions and Message Digests

Hash Functions and Message Digests provide a means of generating a unique "fingerprint" of a message, and are a vital building block in many security protocols...

Authentication protocols are used to ensure that a message was sent by the entity claiming to have sent it. Most protocols also provide message integrity guarantees, because of the methods used for authentication; therefore it is common to see both of these services bundled together in applications such as IPSec.

Message authentication ad integrity is different to end point authentication or user authentication; this typically happens when a session is initiated, using some login process, and is a prerequisite of any secure communication. The authentication and integrity protocols discussed in this section are concerned with providing these services for an individual message.

In pure secret key systems, complete trust is placed in the key management process, so that integrity and authentication is implicit in successful decryption.

In public/private key system, because the public key used to encrypt a message is widely available, it is not possible to provide authentication of the sender by a simple encryption/decryption of the message. Instead a secure hash of the message is typically sent along with the message.

A has function is an algorithm which provides a fixed length “fingerprint” of the message using one of the common polynomial functions standardised for this purpose. Examples of common hash functions include Message Digest 4 (MD4), Message Digest 5 (MD5), and various Secure Hash Algorithms (SHA-1, SHA-256, etc). Hash functions must have certain key properties:

  • A small change in the message should produce at least a 1bit change in the fingerprint
  • The likelihood of two different messages producing the same fingerprint value (known as a collision) should be very small, and consistent with the length of the fingerprint
  • It should be practically impossible to recover the message from the fingerprint file

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