This module provides a generic high-level API to the message digest algorithms found in OpenSSL's crypto library. There are two functions of particular interest: digestByName and digest. The former can be used to retrieve an Algorithm, i.e. an OpenSSL object that implements a particular algorithm. That type can then be used to compute actual message digests with the latter function:

>>> import Data.ByteString.Char8 ( pack )
>>> digest (digestByName "md5") (pack "Hello, world.")
"\b\n\239\131\155\149\250\207s\236Y\147u\233-G"

Neat pretty-printing can be achieved with toHex, which converts the binary representation of a message digest into the common hexadecimal one:

>>> toHex $ digest (digestByName "md5") (pack "Hello, world.")
"080aef839b95facf73ec599375e92d47"
>>> toHex $ digest (digestByName "sha1") (pack "Hello, world.")
"2ae01472317d1935a84797ec1983ae243fc6aa28"

The precise set of available digest algorithms provided by OpenSSL depends on the version of the library installed into the system, obviously, but it's reasonable to expect the following algorithms to be present: MD5, RIPEMD160, SHA1, SHA224, SHA256, SHA384, and SHA512. If an algorithm is not available, digestByName will throw an DigestAlgorithmNotAvailableInOpenSSL exception. If you don't like exceptions, use the tamer digestByName' variant:

>>> digestByName' "i bet this algorithm won't exist"
Nothing

Algorithm is an instance of IsString, so with the proper GHC extensions enabled it's possible to simplify the call to digest even further:

>>> :set -XOverloadedStrings
>>> toHex $ digest "sha256" (pack "The 'Through the Universe' podcast rules.")
"73624694a9435095c8fdaad711273a23c02226196c452f817cfd86f965895614"

Last but not least, digest is actually a class method of Digestable, which collects things we can compute digests of. The defaults are conservative, i.e. we support all things that correspond roughly to C's construct of "void pointer plus a length". digest can use with any of the following signatures:

>>> let shape1 = digest :: Algorithm -> (Ptr (),    CSize) -> MessageDigest
>>> let shape2 = digest :: Algorithm -> (Ptr Word8, CSize) -> MessageDigest
>>> let shape3 = digest :: Algorithm -> (Ptr Word8, CUInt) -> MessageDigest
>>> let shape4 = digest :: Algorithm -> (Ptr (),    Int)   -> MessageDigest
>>> let shape5 = digest :: Algorithm -> StrictByteString   -> MessageDigest
>>> let shape6 = digest :: Algorithm -> LazyByteString     -> MessageDigest

StrictByteString and LazyByteString are also instances of IsString and therefore subject to implicit construction from string literals:

>>> shape5 "sha256" "hello" == shape6 "sha256" "hello"
True

Note that this code offers no overloaded digest version for String, because that function would produce non-deterministic results for Unicode characters. There is an instance for [Word8], though, so strings can be hashed after a proper encoding has been applied. For those who don't care about determinism, there is the following specialized function:

>>> toHex $ digestString "md5" "no Digestable instance for this sucker"
"a74827f849005794565f83fbd68ad189"

If you don't mind orphaned instances, however, feel free to shoot yourself in the foot:

>>> :set -XFlexibleInstances
>>> instance Digestable String where updateChunk ctx str = withCStringLen str (updateChunk ctx)
>>> toHex $ digest "sha256" ("now we can hash strings" :: String)
"7f2989f173125810aa917c4ffe0e26ae1b5f7fb852274829c210297a43dfc7f9"