Rehash: Resumable Hashlib

Rehash is a resumable interface to the OpenSSL-based hashers (message digest objects) in the CPython hashlib standard library. Rehash provides hashers that can be pickled, persisted and reconstituted from their repr(), and otherwise serialized. The rest of the Rehash API is identical to hashlib.

Rehash hashers can be used to checkpoint and restore progress when hashing large byte streams:

import pickle, rehash
hasher = rehash.sha256(b"foo")
state = pickle.dumps(hasher)

hasher2 = pickle.loads(state)
hasher2.update(b"bar")

assert hasher2.hexdigest() == rehash.sha256(b"foobar").hexdigest()

Installation

pip install rehash

Applications

Rehash is useful in any situation when your VM is short-lived or preemptible, and the object you’re hashing is huge. For example, Rehash can be used to hand off the hashing state of large objects between AWS Lambda functions or Google Cloud Functions, which have runtime limits of 15 and 9 minutes, respectively.

Non-openssl hashers

sha3 and blake2 hash algorithms in Python 3.6 are not OpenSSL-based and not supported by rehash.

PyPy

PyPy uses its own hasher implementations. Those are not serializable using rehash.

Security note

By default, rehash objects present themselves with a repr() that exposes their internal state. This allows one to resume the hashing from the point where it stopped. If exposed through an untrusted channel under specific conditions, this could potentially allow an attacker to use an extension attack. If you are unsure about the implications of this, set rehash.opaque_repr = True after importing rehash.

License

Licensed under the terms of the Apache License, Version 2.0.

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