Password hashing

Password hashing and password based key derivation mechanisms in actual use are all based on the idea of iterating a hash function many times on a combination of the password and a random salt, which is stored along with the hash, and allows verifying a proposed password while avoiding clear-text storage.

The latest developments in password hashing have been memory-hard and tunable mechanisms, pioneered by scrypt [SD2012], and followed-on by the schemes submitted to the Password Hashing Competition [PHC].

The nacl.pwhash module exposes both the PHC recommended partially data dependent argon2id and the data independent argon2i mechanisms alongside to the scrypt one.

In the case of password storage, it’s usually suggested to give preference to data dependent mechanisms, therefore the default mechanism suggested by libsodium since version 1.0.15, and therefore by PyNaCl since version 1.2 is argon2id.

If you think in your use-case the risk of potential timing-attacks stemming from data-dependency is greater than the potential time/memory trade-offs stemming out of data-independency, you should prefer argon2i to argon2id or scrypt

Hashers and parameters

PyNaCl exposes the functions and the associated parameters needed to exploit the password hashing constructions in a uniform way in the modules argon2id, argon2i and scrypt, therefore, if you need to change your choice of construction, you simply need to replace one module name with another in the example below.

Further, if you just want to use a default choosen construction, you can directly call nacl.pwhash.str() or nacl.pwhash.kdf() to use the preferred construct in modular crypt password hashing or key derivation mode.

Password storage and verification

All implementations of the modular crypt hasher str function internally generate a random salt, and return a hash encoded in ascii modular crypt format, which can be stored in a shadow-like file

>>> import nacl.pwhash
>>> password = b'my password'
>>> for i in range(4):
...     print(nacl.pwhash.str(password))
>>> # if needed, each hasher is exposed
... # in just the same way:
... for i in range(4):
...     print(nacl.pwhash.scrypt.str(password))
>>> for i in range(4):
...     print(nacl.pwhash.argon2i.str(password))
>>> # and
>>> for i in range(4):
...     print(nacl.pwhash.argon2id.str(password))

To verify a user-proposed password, the verify() function checks the stored hash prefix, and dispatches verification to the correct checker, which in turn extracts the used salt, memory and operation count parameters from the modular format string and checks the compliance of the proposed password with the stored hash

>>> import nacl.pwhash
>>> hashed = (b'$7$C6..../....qv5tF9KG2WbuMeUOa0TCoqwLHQ8s0TjQdSagne'
...           b'9NvU0$3d218uChMvdvN6EwSvKHMASkZIG51XPIsZQDcktKyN7'
...           )
>>> correct = b'my password'
>>> wrong = b'My password'
>>> # while the result will be True on password match,
... # on mismatch an exception will be raised
... res = nacl.pwhash.verify(hashed, correct)
>>> print(res)
>>> res2 = nacl.pwhash.verify_scryptsalsa208sha256(hashed, wrong)
Traceback (most recent call last):
nacl.exceptions.InvalidkeyError: Wrong password
>>> # the verify function raises an exception
... # also when it is run against a password hash
... # starting with a prefix it doesn't know
... wrong_hash = (b'$?$C6..../....qv5tF9KG2WbuMeUOa0TCoqwLHQ8s0TjQdSagne'
...               b'9NvU0$3d218uChMvdvN6EwSvKHMASkZIG51XPIsZQDcktKyN7'
... )
>>> res = nacl.pwhash.verify(wrong_hash, correct)
Traceback (most recent call last):
nacl.exceptions.InvalidkeyError: given password_hash is not in a supported format

Key derivation

Alice needs to send a secret message to Bob, using a shared password to protect the content. She generates a random salt, combines it with the password using one of the kdf functions and sends the message along with the salt and key derivation parameters.

from nacl import pwhash, secret, utils

password = b'password shared between Alice and Bob'
message = b"This is a message for Bob's eyes only"

kdf = pwhash.argon2i.kdf
salt = utils.random(pwhash.argon2i.SALTBYTES)
ops = pwhash.argon2i.OPSLIMIT_SENSITIVE
mem = pwhash.argon2i.MEMLIMIT_SENSITIVE

# or, if there is a need to use scrypt:
# kdf = pwhash.scrypt.kdf
# salt = utils.random(pwhash.scrypt.SALTBYTES)
# ops = pwhash.scrypt.OPSLIMIT_SENSITIVE
# mem = pwhash.scrypt.MEMLIMIT_SENSITIVE

Alices_key = kdf(secret.SecretBox.KEY_SIZE, password, salt,
                 opslimit=ops, memlimit=mem)
Alices_box = secret.SecretBox(Alices_key)
nonce = utils.random(secret.SecretBox.NONCE_SIZE)

encrypted = Alices_box.encrypt(message, nonce)

# now Alice must send to Bob both the encrypted message
# and the KDF parameters: salt, opslimit and memlimit;
# using the same kdf mechanism, parameters **and password**
# Bob is able to derive the correct key to decrypt the message

Bobs_key = kdf(secret.SecretBox.KEY_SIZE, password, salt,
               opslimit=ops, memlimit=mem)
Bobs_box = secret.SecretBox(Bobs_key)
received = Bobs_box.decrypt(encrypted)
This is a message for Bob's eyes only

if Eve manages to get the encrypted message, and tries to decrypt it with a incorrect password, even if she does know all of the key derivation parameters, she would derive a different key. Therefore the decryption would fail and an exception would be raised

>>> # ops, mem and salt are the same used by Alice
>>> guessed_pw = b'I think Alice shared this password with Bob'
>>> Eves_key = pwhash.argon2i.kdf(secret.SecretBox.KEY_SIZE,
...                               guessed_pw, salt,
...                               opslimit=ops, memlimit=mem)
>>> Eves_box = secret.SecretBox(Eves_key)
>>> intercepted = Eves_box.decrypt(encrypted)
Traceback (most recent call last):
nacl.exceptions.CryptoError: Decryption failed. Ciphertext failed ...

Contrary to the hashed password storage case where a serialization format is well-defined, in the raw key derivation case the library user must take care to store (and retrieve) both a reference to the kdf used to derive the secret key and all the derivation parameters. These parameters are needed to later generate the same secret key from the password.

Module level constants for operation and memory cost tweaking

To help in selecting the correct values for the tweaking parameters for the used construction, all the implementation modules provide suggested values for the opslimit and memlimit parameters with the names:


and the corresponding minimum and maximum allowed values in:


Further, for each construction, pwhash modules expose the following constants:


In general, the _INTERACTIVE values are recommended in the case of hashes stored for interactive password checking, and lead to a sub-second password verification time, with a memory consumption in the tens of megabytes range, while the _SENSITIVE values are meant to store hashes for password protecting sensitive data, and lead to hashing times exceeding one second, with memory consumption in the hundred of megabytes range. The _MODERATE values, suggested for argon2 mechanisms are meant to run the construct at a runtime and memory cost intermediate between _INTERACTIVE and _SENSITIVE.

[SD2012]A nice overview of password hashing history is available in Solar Designer’s presentation Password security: past, present, future
[PHC]The Argon2 recommendation is prominently shown in the Password Hashing Competition site, along to the special recognition shortlist and the original call for submissions.