Module Stdlib.Digest

Message digest.

This module provides functions to compute 'digests', also known as 'hashes', of arbitrary-length strings or files. The supported hashing algorithms are BLAKE2 and MD5.

Basic functions

The functions in this section use the MD5 hash function to produce 128-bit digests (16 bytes). MD5 is not cryptographically secure. Hence, these functions should not be used for security-sensitive applications. The BLAKE2 functions below are cryptographically secure.

type t = string

type t = string;

The type of digests: 16-byte strings.

val compare : t -> t -> int

let compare: t => t => int;

The comparison function for 16-byte digests, with the same specification as Stdlib.compare and the implementation shared with String.compare. Along with the type t, this function compare allows the module Digest to be passed as argument to the functors Set.Make and Map.Make.

since 4.00

val equal : t -> t -> bool

let equal: t => t => bool;

The equal function for 16-byte digests.

since 4.03

val string : string -> t

let string: string => t;

Return the digest of the given string.

val bytes : bytes -> t

let bytes: bytes => t;

Return the digest of the given byte sequence.

since 4.02

val substring : string -> int -> int -> t

let substring: string => int => int => t;

Digest.substring s ofs len returns the digest of the substring of s starting at index ofs and containing len characters.

val subbytes : bytes -> int -> int -> t

let subbytes: bytes => int => int => t;

Digest.subbytes s ofs len returns the digest of the subsequence of s starting at index ofs and containing len bytes.

since 4.02

val channel : in_channel -> int -> t

let channel: in_channel => int => t;

If len is nonnegative, Digest.channel ic len reads len characters from channel ic and returns their digest, or raises End_of_file if end-of-file is reached before len characters are read. If len is negative, Digest.channel ic len reads all characters from ic until end-of-file is reached and return their digest.

val file : string -> t

let file: string => t;

Return the digest of the file whose name is given.

val output : out_channel -> t -> unit

let output: out_channel => t => unit;

Write a digest on the given output channel.

val input : in_channel -> t

let input: in_channel => t;

Read a digest from the given input channel.

val to_hex : t -> string

let to_hex: t => string;

Return the printable hexadecimal representation of the given digest.

raises Invalid_argument if the argument is not exactly 16 bytes.

val of_hex : string -> t

let of_hex: string => t;

Convert a hexadecimal representation back into the corresponding digest.

raises Invalid_argument if the argument is not exactly 32 hexadecimal characters. since 5.2

val from_hex : string -> t

let from_hex: string => t;

Same function as Digest.of_hex.

since 4.00

Generic interface

module type S = sig ... end

module type S = { ... };

The signature for a hash function that produces digests of length hash_length from character strings, byte arrays, and files.

Specific hash functions

module BLAKE128 : S

module BLAKE128: S;

BLAKE128 is the BLAKE2b hash function producing 128-bit (16-byte) digests. It is cryptographically secure. However, the small size of the digests enables brute-force attacks in 2{^64} attempts.

module BLAKE256 : S

module BLAKE256: S;

BLAKE256 is the BLAKE2b hash function producing 256-bit (32-byte) digests. It is cryptographically secure, and the digests are large enough to thwart brute-force attacks.

module BLAKE512 : S

module BLAKE512: S;

BLAKE512 is the BLAKE2b hash function producing 512-bit (64-byte) digests. It is cryptographically secure, and the digests are large enough to thwart brute-force attacks.

module MD5 : S

module MD5: S;

MD5 is the MD5 hash function. It produces 128-bit (16-byte) digests and is not cryptographically secure at all. It should be used only for compatibility with earlier designs that mandate the use of MD5.