Overview +of hash algorithms

What is a cryptographic hash?
What is an HMAC?
The base class and its derived classes

What is a cryptographic +hash?

A cryptographic hash algorithm (also known as a message digest, +a one-way function, or simply a hash) takes a variable-length input (the message) +and produce a fixed length output known as the hash (or digest) of the input. +It is often useful to think of the resulting output as a representation, or +fingerprint, of the original input.

There are two properties that +are important to cryptographic hashes:

it must be hard to find +collisions, i.e. it is highly unlikely that two distinct strings will hash +to the same output
it is extremely difficult +to determine the original input given only the output. Even very small changes +to the input will give rise to radical changes in the output.

These properties make hash functions useful in cryptography and other +applications as they allow the representation of objects in a known fixed +size.

The hash algorithms supported by the Symbian platform +are:

+ + + +Hash algorithms +Further information + + + + +

MD4

RFC +1320

+ + +

MD5

RFC +1321

+ + +

SHA-1

FIPS +180-1 and RFC +3174

+ + +

HMAC

RFC +2104

+ + + +

There is also support for MD2 (see RFC 1319) and SHA, which is there for backward compatibility, +but it is not to be used in new code.

What is an +HMAC?

Unlike the hashes mentioned above, HMAC (Hashed Message Authentication +Code) is a key-dependant hash. It allows a key to be specified at creation +of the HMAC. Only people with that key can verify the hash. HMACs are useful +when authentication but not secrecy of a message is required.

The +sender appends to the message data an authentication tag (which is a function +of the data and the shared key). The recipient recomputes the authentication +tag on the received message using the shared key. The integrity of the message +is deemed valid only if the two authentication tags match.

For further +details see RFC +2104.

The Hash API

CMessageDigest is +the base class for all hash algorithms.

The diagram below show the +main classes used in the hash framework. For information on each class see +the Cryptography API Reference material.

+ Inheritance diagram for the Hash API + +

The hash API is used internally by the Security components: AppInst, +Certman (Certificate Management) and Cryptography. Networking/TLS uses a pseudo-random +function (PRF) based on HMAC. Both MD5 and SHA-1 are used in TLS.