Derive Hash based on Password. This predicate is similar to
crypto_data_hash/3 in that it derives a hash from given data.
However, it is tailored for the specific use case of
passwords. One essential distinction is that for this use case,
the derivation of a hash should be as slow as possible to
counteract brute-force attacks over possible passwords.
Another important distinction is that equal passwords must yield,
with very high probability, different hashes. For this reason,
cryptographically strong random numbers are automatically added to
the password before a hash is derived.
Hash is unified with an atom that contains the computed hash and all
parameters that were used, except for the password. Instead of
storing passwords, store these hashes. Later, you can verify the
validity of a password with crypto_password_hash/2, comparing the
then entered password to the stored hash. If you need to export this
atom, you should treat it as opaque ASCII data with up to 255 bytes
of length. The maximal length may increase in the future.
Admissible options are:
- algorithm(+Algorithm)
- The algorithm to use. Currently, the only available algorithms
are
pbkdf2-sha512 (the default) and bcrypt.
- cost(+C)
- C is an integer, denoting the binary logarithm of the number
of iterations used for the derivation of the hash. This
means that the number of iterations is set to 2^C. Currently,
the default is 17, and thus more than one hundred thousand
iterations. You should set this option as high as your server
and users can tolerate. The default is subject to change and
will likely increase in the future or adapt to new algorithms.
- salt(+Salt)
- Use the given list of bytes as salt. By default,
cryptographically secure random numbers are generated for this
purpose. The default is intended to be secure, and constitutes
the typical use case of this predicate.
Currently, PBKDF2 with SHA-512 is used as the hash derivation
function, using 128 bits of salt. All default parameters, including
the algorithm, are subject to change, and other algorithms will also
become available in the future. Since computed hashes store all
parameters that were used during their derivation, such changes will
not affect the operation of existing deployments. Note though that
new hashes will then be computed with the new default parameters.
- See also
- - crypto_data_hkdf/4 for generating keys from Hash.
