Submitting patches 

• Always make a new branch for your work.

• Patches should be small to facilitate easier review. Studies have shown that review quality falls off as patch size grows. Sometimes this will result in many small PRs to land a single large feature.

• Larger changes should be discussed on our mailing list before submission.

• New features and significant bug fixes should be documented in the Changelog .

• You must have legal permission to distribute any code you contribute to cryptography , and it must be available under both the BSD and Apache Software License Version 2.0 licenses.

If you believe you’ve identified a security issue in cryptography , please follow the directions on the security page .

Code 

When in doubt, refer to PEP 8 for Python code. You can check if your code meets our automated requirements by formatting it with ruff format and running ruff against it. If you’ve installed the development requirements this will automatically use our configuration. You can also run the nox job with nox -e flake .

Write comments as complete sentences.

Class names which contains acronyms or initialisms should always be capitalized. A class should be named HTTPClient , not HttpClient .

Every code file must start with the boilerplate licensing notice:

# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.
											

API considerations 

Most projects’ APIs are designed with a philosophy of “make easy things easy, and make hard things possible”. One of the perils of writing cryptographic code is that secure code looks just like insecure code, and its results are almost always indistinguishable. As a result, cryptography has, as a design philosophy: “make it hard to do insecure things”. Here are a few strategies for API design that should be both followed, and should inspire other API choices:

If it is necessary to compare a user provided value with a computed value (for example, verifying a signature), there should be an API provided that performs the verification in a secure way (for example, using a constant time comparison), rather than requiring the user to perform the comparison themselves.

If it is incorrect to ignore the result of a method, it should raise an exception, and not return a boolean True / False flag. For example, a method to verify a signature should raise InvalidSignature , and not return whether the signature was valid.

# This is bad.
def verify(sig: bytes) -> bool:
    # ...
    return is_valid
# Good!
def verify(sig: bytes) -> None:
    # ...
    if not is_valid:
        raise InvalidSignature
											

Every recipe should include a version or algorithmic marker of some sort in its output in order to allow transparent upgrading of the algorithms in use, as the algorithms or parameters needed to achieve a given security margin evolve.

C bindings 

More information on C bindings can be found in the dedicated section of the documentation .