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One-way string hashing for salted passwords using the Unix crypt format

crypt #

One-way string hashing for salted passwords using the Unix crypt format.

This package implements the SHA-256 crypt hash and SHA-512 crypt hash, cryptographic hash formats that are specified in "Unix crypt using SHA-256 and SHA-512" (version: 0.6 2016-08-31).

These hashes are commonly used in Unix and POSIX systems, and in LDAP entries as authentication credentials for POSIX accounts.

Crypt format strings #

It can produce crypt formatted string like:




The format is a sequence of fields, leading and separated by a dollar sign ($):

  • algorithm indicator;
  • number of rounds (optional if it is the default number of rounds);
  • the salt; and
  • the hash value.

The formated string starts with $5$ for a SHA-256 crypt and $6$ for a SHA-512 crypt. There are other (less secure) algorithms, but they are currently not supported by this package.

When SHA-256 or SHA-512 is being used, the default number of rounds is 5000 (as defined by the specification).

Note: different systems use the crypt formatted string differently. For example, as the value of the userPassword attribute in an LDAP posixAccount entry, {crypt} needs to be prepended to it. For example,


Usage #

Checking against a crypt format hash #

To test if an entered password matches the password that was used to create a crypt format hash, create a Crypt object from the crypt format has and invoke the match method on it.

bool isValid(String cryptFormatHash, String enteredPassword) =>

Generating a crypt format hash #

To generate a crypt format hash, use Crypt.sha256 or Crypt.sha512 constructors, and convert it to a String.

import 'package:crypt/crypt.dart';

void main() {
  // Creating crypt strings

  // Default rounds and random salt generated
  final c1 = Crypt.sha256('p@ssw0rd');

  // Random salt generated
  final c2 = Crypt.sha256('p@ssw0rd', rounds: 10000);

  // Default rounds
  final c3 = Crypt.sha256('p@ssw0rd', salt: 'abcdefghijklmnop');

  // No defaults used
  final c4 = Crypt.sha256('p@ssw0rd', rounds: 10000,
                          salt: 'abcdefghijklmnop');

  // SHA-512
  final d1 = Crypt.sha512('p@ssw0rd');


  // Comparing a value to a crypt hash

  for (final hashString in [
  ]) {
    // Parse the crypt string: this extracts the type, rounds and salt
    final h = Crypt(hashString);

    const correctValue = 'p@ssw0rd';
    const wrongValue = '123456';

    if (!h.match(correctValue)) {
      print('Error: unexpected non-match: $correctValue');

    if (h.match(wrongValue)) {
      print('Error: unexpected match: $wrongValue');

The above example produced the following output:


Features and bugs #

Random number generators #

Salt generation uses a cryptographically secure random number generator, if one is available. If one is not available, it falls back to using a cryptographically insecure one.

Set Crypt.cryptographicallySecureSalts to true to prevent a cryptographically insecure random number from being used. An exception will then be thrown if attempting to generate a salt on platforms that don't support a cryptographically secure random number generator.

Explicitly set it to false to allow this fallback behaviour in future releases. The default is currently set to false, for backward compatibility. But a future release may set the default to true for improved security. Explicitly setting it to false will ensure code will still work when that breaking change is made.

Dependency on the crypto package #

The current release depends on the Dart crypto package version 3.0.0, which has support for SHA-512. If you need to use an older version of crypto, use version 2.0.0 of this package -- but that older version won't have support for SHA-512 crypt strings and is not null safe.

Please file feature requests and bugs at the GitHub issue tracker.

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One-way string hashing for salted passwords using the Unix crypt format

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