Scramp

A Python implementation of the SCRAM authentication protocol. Scramp supports the following mechanisms:

• SCRAM-SHA-1
• SCRAM-SHA-1-PLUS
• SCRAM-SHA-256
• SCRAM-SHA-256-PLUS
• SCRAM-SHA-512
• SCRAM-SHA-512-PLUS
• SCRAM-SHA3-512
• SCRAM-SHA3-512-PLUS

Table Of Contents

• Installation
• Examples
  • Client and Server
  • Client only
  • Server only
  • Server Error
  • Avoiding User Enumeration
• Standards
• Missing tls-exporter channel binding support
• Key Derivation Function
• API Docs
  • scramp.MECHANISMS
  • scramp.ScramClient
  • scramp.ScramMechanism
  • scramp.ScramServer
  • scramp.make_channel_binding(name, ssl_socket)
• Testing
• OpenSSF Scorecard
• Doing A Release Of Scramp
• Release Notes
  • Version 1.4.18, 2026-10-22
  • Version 1.4.17, 2026-08-07
  • Version 1.4.16, 2026-08-06
  • Version 1.4.15, 2026-07-26
  • Version 1.4.14, 2026-07-25
  • Version 1.4.13, 2026-07-23
  • Version 1.4.12, 2026-07-05
  • Version 1.4.11, 2026-07-04
  • Version 1.4.10, 2026-06-27
  • Version 1.4.9, 2026-06-19
  • Version 1.4.8, 2026-01-06
  • Version 1.4.7, 2026-01-04
  • Version 1.4.6, 2025-07-05
  • Version 1.4.5, 2024-04-13
  • Version 1.4.4, 2022-11-01
  • Version 1.4.3, 2022-10-26
  • Version 1.4.2, 2022-10-22
  • Version 1.4.1, 2021-08-25
  • Version 1.4.0, 2021-03-28
  • Version 1.3.0, 2021-03-28
  • Version 1.2.2, 2021-02-13
  • Version 1.2.1, 2021-02-07
  • Version 1.2.0, 2020-05-30
  • Version 1.1.1, 2020-03-28
  • Version 1.1.0, 2019-02-24
  • Version 1.0.0, 2019-02-17
  • Version 0.0.0, 2019-02-10

Installation

• Create a virtual environment: python3 -m venv venv
• Activate the virtual environment: source venv/bin/activate
• Install: pip install scramp

Examples

Client and Server

Here's an example using both the client and the server. It's a bit contrived as normally you'd be using either the client or server on its own.

>>> from scramp import ScramClient, ScramMechanism
>>>
>>> USERNAME = 'user'
>>> PASSWORD = 'pencil'
>>> MECHANISMS = ['SCRAM-SHA-256']
>>>
>>>
>>> # Choose a mechanism for our server
>>> m = ScramMechanism()  # Default is SCRAM-SHA-256
>>>
>>> # On the server side we create the authentication information for each user
>>> # and store it in an authentication database. We'll use a dict:
>>> db = {}
>>>
>>> salt, stored_key, server_key, iteration_count = m.make_auth_info(PASSWORD)
>>>
>>> db[USERNAME] = salt, stored_key, server_key, iteration_count
>>>
>>> # Define your own function for retrieving the authentication information
>>> # from the database given a username
>>>
>>> def auth_fn(username):
...     return db[username]
>>>
>>> # Make the SCRAM server
>>> s = m.make_server(auth_fn)
>>>
>>> # Now set up the client and carry out authentication with the server
>>> c = ScramClient(MECHANISMS, USERNAME, PASSWORD)
>>> cfirst = c.get_client_first()
>>>
>>> s.set_client_first(cfirst)
>>> sfirst = s.get_server_first()
>>>
>>> c.set_server_first(sfirst)
>>> cfinal = c.get_client_final()
>>>
>>> s.set_client_final(cfinal)
>>> sfinal = s.get_server_final()
>>>
>>> c.set_server_final(sfinal)
>>>
>>> # If it all runs through without raising an exception, the authentication
>>> # has succeeded

Client only

Here's an example using just the client. The client nonce is specified in order to give a reproducible example, but in production you'd omit the c_nonce parameter and let ScramClient generate a client nonce:

>>> from scramp import ScramClient
>>>
>>> USERNAME = 'user'
>>> PASSWORD = 'pencil'
>>> C_NONCE = 'rOprNGfwEbeRWgbNEkqO'
>>> MECHANISMS = ['SCRAM-SHA-256']
>>>
>>> # Normally the c_nonce would be omitted, in which case ScramClient will
>>> # generate the nonce itself.
>>>
>>> c = ScramClient(MECHANISMS, USERNAME, PASSWORD, c_nonce=C_NONCE)
>>>
>>> # Get the client first message and send it to the server
>>> cfirst = c.get_client_first()
>>> print(cfirst)
n,,n=user,r=rOprNGfwEbeRWgbNEkqO
>>>
>>> # Set the first message from the server
>>> c.set_server_first(
...     'r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,'
...     's=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096')
>>>
>>> # Get the client final message and send it to the server
>>> cfinal = c.get_client_final()
>>> print(cfinal)
c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=
>>>
>>> # Set the final message from the server
>>> c.set_server_final('v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=')
>>>
>>> # If it all runs through without raising an exception, the authentication
>>> # has succeeded

Server only

Here's an example using just the server. The server nonce and salt is specified in order to give a reproducible example, but in production you'd omit the s_nonce and salt parameters and let Scramp generate them:

>>> from scramp import ScramMechanism
>>>
>>> USERNAME = 'user'
>>> PASSWORD = 'pencil'
>>> S_NONCE = '%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0'
>>> SALT = b'[m\x99h\x9d\x125\x8e\xec\xa0K\x14\x126\xfa\x81'
>>>
>>> db = {}
>>>
>>> m = ScramMechanism()
>>>
>>> salt, stored_key, server_key, iteration_count = m.make_auth_info(
...     PASSWORD, salt=SALT)
>>>
>>> db[USERNAME] = salt, stored_key, server_key, iteration_count
>>>
>>> # Define your own function for getting a password given a username
>>> def auth_fn(username):
...     return db[username]
>>>
>>> # Normally the s_nonce parameter would be omitted, in which case the
>>> # server will generate the nonce itself.
>>>
>>> s = m.make_server(auth_fn, s_nonce=S_NONCE)
>>>
>>> # Set the first message from the client
>>> s.set_client_first('n,,n=user,r=rOprNGfwEbeRWgbNEkqO')
>>>
>>> # Get the first server message, and send it to the client
>>> sfirst = s.get_server_first()
>>> print(sfirst)
r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096
>>>
>>> # Set the final message from the client
>>> s.set_client_final(
...     'c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,'
...     'p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=')
>>>
>>> # Get the final server message and send it to the client
>>> sfinal = s.get_server_final()
>>> print(sfinal)
v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=
>>>
>>> # If it all runs through without raising an exception, the authentication
>>> # has succeeded

Server Error

Here's an example of when setting a message from the client causes an error. The server nonce and salt is specified in order to give a reproducible example, but in production you'd omit the s_nonce and salt parameters and let Scramp generate them:

>>> from scramp import ScramException, ScramMechanism
>>>
>>> USERNAME = 'user'
>>> PASSWORD = 'pencil'
>>> S_NONCE = '%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0'
>>> SALT = b'[m\x99h\x9d\x125\x8e\xec\xa0K\x14\x126\xfa\x81'
>>>
>>> db = {}
>>>
>>> m = ScramMechanism()
>>>
>>> salt, stored_key, server_key, iteration_count = m.make_auth_info(
...     PASSWORD, salt=SALT)
>>>
>>> db[USERNAME] = salt, stored_key, server_key, iteration_count
>>>
>>> # Define your own function for getting a password given a username
>>> def auth_fn(username):
...     return db[username]
>>>
>>> # Normally the s_nonce parameter would be omitted, in which case the
>>> # server will generate the nonce itself.
>>>
>>> s = m.make_server(auth_fn, s_nonce=S_NONCE)
>>>
>>> try:
...     # Set the first message from the client
...     s.set_client_first('p=tls-unique,,n=user,r=rOprNGfwEbeRWgbNEkqO')
... except ScramException as e:
...     print(e)
...     # Get the final server message and send it to the client
...     sfinal = s.get_server_final()
...     print(sfinal)
Received GS2 flag 'p' which indicates that the client requires channel binding,
but the server does not: channel-binding-not-supported
e=channel-binding-not-supported

Avoiding User Enumeration

User enumeration is where an attacker tries to find valid user names by trying lots of different ones and distinguishing valid ones from invalid ones by the different responses the server. In Scramp, if the auth_fn returns None then the client receives an unknown-user response, which enables user enumeration. This is allowed within the Scram specification.

To avoid user enumeration, instead of auth_fn returning None make it return fake credentials. Then the error returned will be the same as if the password were wrong. Here's an example:

>>> from hmac import digest
>>> from secrets import DEFAULT_ENTROPY
>>> from scramp import ScramException, ScramMechanism
>>>
>>> S_NONCE = '%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0'
>>> S_SECRET = b"long-lived-server-secret"
>>>
>>> db = {}
>>>
>>> m = ScramMechanism()
>>>
>>> def auth_fn(username):
...     if username in db:
...         return db[username]
...     else:
...         algo = m.hf().name
...         u = username.encode()
...         salt = digest(S_SECRET, b"salt:" + u, 'sha256')[:DEFAULT_ENTROPY]
...         stored_key = digest(S_SECRET, b"sk:" + u, algo)
...         server_key = digest(S_SECRET, b"vk:" + u, algo)
...         i = int(m.iteration_count)
...         return salt, stored_key, server_key, i
>>>
>>>
>>> # Normally the s_nonce parameter would be omitted, in which case the
>>> # server will generate the nonce itself.
>>>
>>> s = m.make_server(auth_fn, s_nonce=S_NONCE)
>>>
>>> # Now set up the client and carry out authentication with the server
>>> MECHANISMS = ['SCRAM-SHA-256']
>>> USERNAME = 'user'
>>> PASSWORD = 'pencil'
>>>
>>> c = ScramClient(MECHANISMS, USERNAME, PASSWORD)
>>> cfirst = c.get_client_first()
>>>
>>> s.set_client_first(cfirst)
>>> sfirst = s.get_server_first()
>>>
>>> c.set_server_first(sfirst)
>>> cfinal = c.get_client_final()
>>>
>>> try:
...    s.set_client_final(cfinal)
... except ScramException as e:
...     print(e)
The client keys don't match.: invalid-proof
>>>
>>> # Get the final server message and send it to the client
>>> sfinal = s.get_server_final()
>>> print(sfinal)
e=invalid-proof
>>>
>>> try:
...    c.set_server_final(sfinal)
... except ScramException as e:
...     print(e)
The server returned an error: invalid-proof

When you create your fake inputs to make_auth_info, make sure that:

• Salt: HMAC(server_secret, username) truncated to the real salt length. It must be stable across requests, or an attacker spots fake users because their salt changes on every attempt while real salts don’t.
• Iteration count: use what real users have. If counts vary per user, that’s a leak you can’t fully close, so pick the default.
• Mechanism: respond to whatever was requested as if the user had it.
• Server secret: keep it stable across restarts.

Standards

• RFC 5802 Describes SCRAM.
• RFC 7677 Registers SCRAM-SHA-256 and SCRAM-SHA-256-PLUS.
• draft-melnikov-scram-sha-512 Registers SCRAM-SHA-512 and SCRAM-SHA-512-PLUS.
• draft-melnikov-scram-sha3-512 Registers SCRAM-SHA3-512 and SCRAM-SHA3-512-PLUS.
• RFC 5929 Channel Bindings for TLS.
• RFC 9266 Defines the tls-exporter channel binding, which is not yet supported by Scramp.

Missing tls-exporter channel binding support

RFC 9266 defines tls-exporter as the channel binding type for TLS 1.3, using the keying-material exporter from RFC 5705 (TLS ≤1.2) / RFC 8446 §7.5 (TLS 1.3), i.e. OpenSSL's SSL_export_keying_material().

Scramp can't implement tls-exporter because Python doesn't support it in SSLSocket.get_channel_binding(). Only the older tls-unique/tls-server-end-point are supported.

• Relevant Python issue: python/cpython#25255
• Relevant Scramp issue.

Once tls-exporter is supported by SSLSocket.get_channel_binding(), we can support tls-exporter in Scramp.

Key Derivation Function

If it's present, then Scramp uses the standard hashlib.pbkdf2_hmac key derivation function, but if it isn't present, then Scramp uses it's own pure-Python implementation.

API Docs

scramp.MECHANISMS

A tuple of the supported mechanism names.

scramp.ScramClient

ScramClient(mechanisms, username, password, channel_binding=None, c_nonce=None)

Constructor of the scramp.ScramClient class, with the following parameters:

• mechanisms - A list or tuple of mechanism names. ScramClient will choose the most secure. If cbind_data is None, the '-PLUS' variants will be filtered out first. The chosen mechanism is available as the property mechanism_name.
• username
• password
• channel_binding - Providing a value for this parameter allows channel binding to be used (ie. it lets you use mechanisms ending in '-PLUS'). The value for channel_binding is a tuple consisting of the channel binding name and the channel binding data. For example, if the channel binding name is tls-unique, the channel_binding parameter would be ('tls-unique', data), where data is obtained by calling SSLSocket.get_channel_binding() The convenience function scramp.make_channel_binding() can be used to create a channel binding tuple.
• c_nonce - The client nonce. It's sometimes useful to set this when testing / debugging, but in production this should be omitted, in which case ScramClient will generate a client nonce.

The ScramClient object has the following methods and properties:

• get_client_first() - Get the client first message.
• set_server_first(message) - Set the first message from the server.
• get_client_final() - Get the final client message.
• set_server_final(message) - Set the final message from the server.
• mechanism_name - The mechanism chosen from the list given in the constructor.

scramp.ScramMechanism

ScramMechanism(mechanism='SCRAM-SHA-256')

Constructor of the ScramMechanism class, with the following parameter:

• mechanism - The SCRAM mechanism to use.

The ScramMechanism object has the following methods and properties:

• make_auth_info(password, iteration_count=None, salt=None) - Returns the tuple (salt, stored_key, server_key, iteration_count) which is stored in the authentication database on the server side. It has the following parameters:
  • password - The user's password as a str.
  • iteration_count - The rounds as an int. If None then use the minimum associated with the mechanism.
  • salt - It's sometimes useful to set this binary parameter when testing / debugging, but in production this should be omitted, in which case a salt will be generated.
• make_server(auth_fn, channel_binding=None, s_nonce=None) - returns a ScramServer object. It takes the following parameters:
  • auth_fn This is a function provided by the programmer that has one parameter, a username of type str and returns returns the tuple (salt, stored_key, server_key, iteration_count). Where salt, stored_key and server_key are of a binary type, and iteration_count is an int. It can optionally return None if a user isn't found.
  • channel_binding - Providing a value for this parameter allows channel binding to be used (ie. it lets you use mechanisms ending in -PLUS). The value for channel_binding is a tuple consisting of the channel binding name and the channel binding data. For example, if the channel binding name is 'tls-unique', the channel_binding parameter would be ('tls-unique', data), where data is obtained by calling SSLSocket.get_channel_binding(). The convenience function scramp.make_channel_binding() can be used to create a channel binding tuple. If channel_binding is provided and the mechanism isn't a -PLUS variant, then the server will negotiate with the client to use the -PLUS variant if the client supports it, or otherwise to use the mechanism without channel binding.
  • s_nonce - The server nonce as a str. It's sometimes useful to set this when testing / debugging, but in production this should be omitted, in which case ScramServer will generate a server nonce.
  • salted_password - A binary object representing the hashed password.
• iteration_count - The minimum iteration count recommended for this mechanism.

scramp.ScramServer

The ScramServer object has the following methods:

• set_client_first(message) - Set the first message from the client.
• get_server_first() - Get the server first message.
• set_client_final(message) - Set the final client message.
• get_server_final() - Get the server final message.

Note that ScramServer doesn't enforce a maximum message or field length, so that's something you might want to consider doing if you're embedding it in a network-facing server.

scramp.make_channel_binding(name, ssl_socket)

A helper function that makes a channel_binding tuple when given a channel binding name and an SSL socket. The parameters are:

• name - A channel binding name such as 'tls-unique' or 'tls-server-end-point'.
• ssl_socket - An instance of ssl.SSLSocket.

Testing

• Activate the virtual environment: source venv/bin/activate
• Install tox: pip install tox
• Run tox: tox

OpenSSF Scorecard

It might be worth running the OpenSSF Scorecard:

podman run -e GITHUB_AUTH_TOKEN=<auth_token> gcr.io/openssf/scorecard:stable --repo=github.com/tlocke/scramp

Doing A Release Of Scramp

Run tox to make sure all tests pass, then update the release notes, then do:

• git tag -a x.y.z -m "version x.y.z"
• rm -r dist
• python -m build
• twine upload dist/*

Release Notes

Version 1.4.18, 2026-10-22

• Add a pure-Python fallback for pbkdf2_hmac since hashlib.pbkdf2_hmac is only available if Python has been compiled with OpenSSL.
• Strengthen validation of iteration counts and salts.
• Improve SCRAM message parsing and validation.

Version 1.4.17, 2026-08-07

• Allow auth_fn to be any callable.

Version 1.4.16, 2026-08-06

• Encapsulate auth_fn with AuthFn, a class that parses the output.

Version 1.4.15, 2026-07-26

• Check message attribute values don't have invalid characters.

Version 1.4.14, 2026-07-25

• Make sure all Salt and Nonce errors can be handled with the protocol.
• Add note in README on tls-exporter.

Version 1.4.13, 2026-07-23

• Have a Nonce class for validation.
• Have a Salt class for validation.
• Handle exceptions from the auth function.
• Optional extensions should be ignored.
• Check for invalid GS2 header.

Version 1.4.12, 2026-07-05

• Check for invalid escape chars in username.

Version 1.4.11, 2026-07-04

• Make sure all errors detected by the server get returned to the client as valid errors.

Version 1.4.10, 2026-06-27

• Implement max and min guards for iteration count.
• The xor() function should fail if the two values are of unequal lengths
• Handle the case of an error in first server message
• Fix formatting in channel binding error message
• SHA3-512 didn't work, but it should do now

Version 1.4.9, 2026-06-19

• Fix typo in error message.
• Strictly validate on base 64 decoding
• Handle escaping in usernames according to the spec.
• SASLprep: Raise exception rather than using assert

Version 1.4.8, 2026-01-06

• Use the secrets module from the Standard Library for generating the salt and nonce.

Version 1.4.7, 2026-01-04

• The main change in this release is that we now use hashlib.pbkdf2_hmac() from the standard library, rather than our own Python implementation. Since this is fast, we no longer need to provide a way of using a custom key derivation function, and so make_stored_server_keys() has been removed.
• To avoid timing attacks we now use hmac.compare_digest() for digest comparisons.

Version 1.4.6, 2025-07-05

• Drop support for Python 3.8, and add support for 3.13

Version 1.4.5, 2024-04-13

• Drop support for Python 3.7, which means we're not dependent on importlib-metadata anymore.
• Various changes to build system, docs and automated testing.

Version 1.4.4, 2022-11-01

• Tighten up parsing of messages to make sure that a ScramException is raised if a message is malformed.

Version 1.4.3, 2022-10-26

• The client now sends a gs2-cbind-flag of 'y' if the client supports channel binding, but thinks the server does not.

Version 1.4.2, 2022-10-22

• Switch to using the MIT-0 licence https://choosealicense.com/licenses/mit-0/
• When creating a ScramClient, allow non -PLUS variants, even if a channel_binding parameter is provided. Previously this would raise and exception.

Version 1.4.1, 2021-08-25

• When using make_channel_binding() to create a tls-server-end-point channel binding, support certificates with hash algorithm of sha512.

Version 1.4.0, 2021-03-28

• Raise an exception if the client receives an error from the server.

Version 1.3.0, 2021-03-28

• As the specification allows, server errors are now sent to the client in the server_final message, an exception is still thrown as before.

Version 1.2.2, 2021-02-13

• Fix bug in generating the AuthMessage. It was incorrect when channel binding was used. So now Scramp supports channel binding.

Version 1.2.1, 2021-02-07

• Add support for channel binding.
• Add support for SCRAM-SHA-512 and SCRAM-SHA3-512 and their channel binding variants.

Version 1.2.0, 2020-05-30

• This is a backwardly incompatible change on the server side, the client side will work as before. The idea of this change is to make it possible to have an authentication database. That is, the authentication information can be stored, and then retrieved when needed to authenticate the user.
• In addition, it's now possible on the server side to use a third party hashing library such as passlib as the hashing implementation.

Version 1.1.1, 2020-03-28

• Add the README and LICENCE to the distribution.

Version 1.1.0, 2019-02-24

• Add support for the SCRAM-SHA-1 mechanism.

Version 1.0.0, 2019-02-17

• Implement the server side as well as the client side.

Version 0.0.0, 2019-02-10

• Copied SCRAM implementation from pg8000. The idea is to make it a general SCRAM implemtation. Credit to the Scrampy project which I read through to help with this project. Also credit to the passlib project from which I copied the saslprep function.