Sha_256 (webs.Webs_hash.Sha

SHA-256 hashes

type t

The type for SHA-256 hashes.

val length : t -> int

length h is the length of h in bytes (i.e. 32).

val hash : string -> t

hash s is the SHA-256 hash of s.

HMAC-SHA-256

val hmac : key:string -> string -> t

hmac ~key msg is the RFC 2104 HMAC-SHA-256 for key key and message msg. key should not be less than 32 bytes.

PBKDF2-HMAC-SHA-256

type salt = string

The type for password salt bytes.

type key = string

The type for password derived keys bytes.

val random_salt : 
  ?random_state:Stdlib.Random.State.t ->
  length:int ->
  unit ->
  salt

random_salt ~length () are length bytes sourced from the given PRNG state (defaults to Stdlib.Random.make_self_init).

val pbkdf2_hmac : 
  key_length:int ->
  iterations:int ->
  salt:salt ->
  password:string ->
  unit ->
  key

pbkdf2_hmac ~key_length ~iterations ~salt ~password () derives a key for password password with a salt salt and iterations iterations to generate a key of length key_length using RFC 8018's PBKFD2-HMAC-SHA-256.

Important. Use equal_key to compare derived keys, not String.equal or ( = ).

In 2023, here is a good baseline of parameters:

A key_length of 32 bytes.
A number of iterations of 600_000.
A salt length of 16 bytes.

See also the OWASP password storage cheet.

Raises Invalid_argument if key_len or iterations are smaller or equal to 0 or if key_len is greater than 2³² - 1 * 32 or max_int.

val equal_key : key -> key -> bool

equal_key k0 k1 is a constant time string equality for k0 and k1 of the same length. Do not use to compare strings of different lengths this raises Invalid_argument in that case.

Predicates

val equal : t -> t -> bool

equal h0 h1 is a constant time equality comparison function between hashes h0 and h1.

Converting

val to_binary_string : t -> string

to_binary_string h is the sequence of bytes of h.

val of_binary_string : string -> (t, unit) Stdlib.result

of_binary_string s is the sequence of bytes of s as a hash value. An error is returned if the length of s in not 32.

val to_hex : t -> string

to_hex h is the sequence of bytes of h as US-ASCII lowercase hexadecimal digits.

val of_hex' : string -> (t, int) Stdlib.result

of_hex s parses a sequence of US-ASCII (lower or upper cased) hexadecimal digits to its hash value. Errors with an offending index or the length of the string in case s was not exactly made of 64 US-ASCII hex digits.

val of_hex : string -> (t, string) Stdlib.result

of_hex is of of_hex' but errors with an english error message.

val pp : Stdlib.Format.formatter -> t -> unit

pp formats digests with to_hex.