A message digest is a short (fixed size) summary of a long message which is cryptographically secure against tampering. Use a message digest if all you care about is integrity: If d is the digest of a message m, then a computationally bound adversary cannot produce another message m' for which the digest is also d.

Here is a simple application for computing and verifying the digest of a file.

-- Program to compute the message digest of a file

import Raaz
import System.Environment

main = getArgs >>= digestFile . head >>= print

-- Program to verify the integrity of a file

import Raaz
import System.Environment

main = do [d,file] <- getArgs
          dp       <- digestFile file
          if fromString d == dp
             then putStrLn "OK"
             else putStrLn "FAILED"

There are three variants for computing the digest of a message. digest, digestFile and digestSource.

Serialisation

Digests are instances of Encodable and hence can be serialised directly to byte string.

Warning

Message digests DO NOT provide any authentication; any one could have compute the digest of a given message m and hence says nothing about the peer from whom the digest has been received. If you want some guarantee on who the digest came from consider using a message authenticator (see Raaz.Auth). In addition if you also want secrecy consider using encrypted authenticator (see Raaz.AuthEncrypt)

Message digest can also be computed incrementally using a digest context captured by the DigestCxt data type. The three functions relevant for this style of operation are startDigest, updateDigest, and finaliseDigest which respectively prepares the context for a new incremental processing, updates the context with an additional chunk of data, and finalises the context to recover the digest. The type DigestCxt is an instance of the class Memory and hence any IO action that requires a DigestCxt as argument can be run using withMemory.

If the entire input is with you either as a file or a string, the digest and digestFile is a much more high level interface and should be preferred.

To inter-operate with other libraries and applications, one might want to compute the digest using specific cryptographic hash. In such a situation, import one of the more specific module instead of this one.

• Raaz.Digest.Blake2b
• Raaz.Digest.Blake2s
• Raaz.Digest.Sha512
• Raaz.Digest.Sha256

Here is an example that uses sha512 to compute the digest.

import Raaz.Digest.Sha512
import System.Environment

main = getArgs >>= digestFile . head >>= print