CODEWITHSUNDEEP
python-3.xhashsha1
Robin van Leeuwen
Robin van Leeuwen

Reputation: 2703

Hashing with SHA1 in python3 does not produce same result

I'm trying to write some code preventing the update of modified lists. For this i'm calculating the SHA1 hash of the values, but the hexdigest() of this hash produces different result when i restart my ipython interpreter. Why is this?

In [1]: import hashlib

In [2]: hashid = hashlib.sha1()

In [3]: hashid.update(repr(frozenset(sorted(["a","b","c"]))).encode("utf-8"))

In [4]: hashid.hexdigest()
Out[4]: '53ca01b21fd7cb1996634bb45ad74851f73c45d3'

When reinitializing the hashid and doing the hash calculation again in the same ipython3 console it works:

In [5]: hashid = hashlib.sha1()

In [6]: hashid.update(repr(frozenset(sorted(["a","b","c"]))).encode("utf-8"))

In [7]: hashid.hexdigest()
Out[7]: '53ca01b21fd7cb1996634bb45ad74851f73c45d3'

But stopping my console and restarting it it produces different results:

In [7]: exit
rvl@laptop ~/ $ ipython3 
In [1]: import hashlib

In [2]: hashid = hashlib.sha1()

In [3]: hashid.update(repr(frozenset(sorted(["a","b","c"]))).encode("utf-8"))

In [4]: hashid.hexdigest()
Out[4]: '6e5813fcb173e35e81d6138eab4d21482885e7eb'

Why is this? And how can i produce the same SHA1 hash/hexdigest result when hasing identical sorted lists?

Upvotes: 0

Views: 1068

Answers (1)

ShadowRanger
ShadowRanger

Reputation: 155323

You can't rely on the ordering of the repr of a set/frozenset object, since the values have no guaranteed order (and in fact, as an anti-denial of service feature, the hash codes of strings will differ between different runs of the same version of Python, causing the set ordering to change).

Swap around your frozenset and sorted calls to get a consistently reproducible representation. Sorted lists have guaranteed ordering, while frozenset will get you uniqueness guarantees:

hashid.update(repr(sorted(frozenset(["a","b","c"]))).encode("utf-8"))

Upvotes: 1

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