# Module Stdlib.Digest

    module Digest: Digest

## 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

The type of digests: 16-byte strings.

    val compare : t -> t -> int

The comparison function for 16-byte digests, with the same specification as `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

The equal function for 16-byte digests.

- Since 4.03

    val string : string -> t

Return the digest of the given string.

    val bytes : bytes -> t

Return the digest of the given byte sequence.

- Since 4.02

    val 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

`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

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

Return the digest of the file whose name is given.

    val output : out_channel -> t -> unit

Write a digest on the given output channel.

    val input : in_channel -> t

Read a digest from the given input channel.

    val 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

Convert a hexadecimal representation back into the corresponding digest.

- Since 5.2
- Raises `Invalid_argument` if the argument is not exactly 32 hexadecimal characters.

    val from_hex : string -> t

Same function as `Digest.of_hex`.

- Since 4.00

## Generic interface

    module type S = sig .. end

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

`BLAKE128` is the BLAKE2b hash function producing 128-bit (16-byte) digests.

    module BLAKE256: S

`BLAKE256` is the BLAKE2b hash function producing 256-bit (32-byte) digests.

    module BLAKE512: S

`BLAKE512` is the BLAKE2b hash function producing 512-bit (64-byte) digests.

    module MD5: S

`MD5` is the MD5 hash function.