Description

assignee = None
closed_at = None
created_at = <Date 2016-10-07.11:50:44.279>
labels = ['type-bug', '3.9', '3.10', '3.11', 'extension-modules', 'docs']
title = '[doc] hmac cannot be used with shake algorithms'
updated_at = <Date 2022-01-17.23:14:46.268>
user = 'https://github.com/minrk'

activity = <Date 2022-01-17.23:14:46.268>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Extension Modules']
creation = <Date 2016-10-07.11:50:44.279>
creator = 'minrk'
dependencies = []
files = []
hgrepos = []
issue_num = 28384
keywords = []
message_count = 2.0
messages = ['278235', '278468']
nosy_count = 4.0
nosy_names = ['christian.heimes', 'docs@python', 'takluyver', 'minrk']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28384'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Activity

1. minrk commented on Oct 7, 2016

   

   minrkmannequin

   on Oct 7, 2016

   MannequinAuthor

   More actions

   HMAC digest methods call inner.digest() with no arguments, but new-in-3.6 shake algorithms require a length argument.

   possible solutions:

   1. add optional length argument to HMAC.[hex]digest, and pass through to inner hash object
   2. set hmac.digest_size, and use that to pass through to inner hash object if inner hash object has digest_size == 0
   3. give shake hashers a default value for length in digest methods (logically 32 for shake_256, 16 for shake_128, I think)

   test:

   import hmac, hashlib
   
   h = hmac.HMAC(b'secret', digestmod=hashlib.shake_256)
   h.hexdigest() # raises on self.inner.digest() requires length argument
2. minrk

   added

   3.7 (EOL)end of lifeend of life

   on Oct 7, 2016
3. SilentGhost

   added

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 7, 2016
4. tiran commented on Oct 11, 2016

   

   tiran

   on Oct 11, 2016

   Member

   More actions

   It's not a bug, but indented behavior. It does not make any sense to use SHAKE with the HMAC construct. In fact it does not make sense to combine Keccak sponge or Blake2 with HMAC at all. HMAC is only necessary for old, Merkle-Damgard hashing algorithms like MD5, SHA1 and SHA2, because they are subject to length extension attacks.

   The correct solution is
   4. improve documentation
5. iritkatriel

   added

   3.9 (EOL)end of lifeend of life

   3.10 (EOL)end of lifeend of life

   3.11only security fixesonly security fixes

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   and removed

   3.7 (EOL)end of lifeend of life

   on Jan 17, 2022
6. iritkatriel

   changed the title [-]hmac cannot be used with shake algorithms[/-] [+][doc] hmac cannot be used with shake algorithms[/+] on Jan 17, 2022
7. iritkatriel

   assigned

   docspython

   on Jan 17, 2022
8. ezio-melotti

   transferred this issue fromon Apr 10, 2022
9. picnixz

   removed

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   extension-modulesC modules in the Modules dirC modules in the Modules dir

   on Jul 15, 2025
10. 1 remaining item
11. picnixz

    removed

    3.10 (EOL)end of lifeend of life

    3.9 (EOL)end of lifeend of life

    on Jul 15, 2025
12. picnixz

    self-assigned this

    on Jul 15, 2025
13. github-project-automation

    moved this to Todo in docs issueson Jul 15, 2025
14. github-project-automation

    added this to docs issueson Jul 15, 2025
15. picnixz

    added a commit that references this issue on Jul 15, 2025

    gh-72570: mention the incompatibility of XOFs with HMAC (#136676)

    a02cf19
16. miss-islington

    added 2 commits that reference this issue on Jul 15, 2025

    pythongh-72570: mention the incompatibility of XOFs with HMAC (python…

    a4aea49

    pythongh-72570: mention the incompatibility of XOFs with HMAC (python…

    bc07959
17. picnixz commented on Jul 15, 2025

    

    picnixz

    on Jul 15, 2025

    Member

    More actions

    Now that I've improved the docs, I'll also need to make the interface consistent (because if we use the pure Python HMAC, we raise a TypeError instead of a ValueError) but this will be a separate issue.
18. picnixz

    added 2 commits that reference this issue on Jul 15, 2025

    [3.14] gh-72570: mention the incompatibility of XOFs with HMAC (GH-13…

    45b72c4

    [3.13] gh-72570: mention the incompatibility of XOFs with HMAC (GH-13…

    8517a89
19. picnixz

    closed this as completedon Jul 15, 2025
20. taegyunkim

    added a commit that references this issue on Aug 4, 2025

    pythongh-72570: mention the incompatibility of XOFs with HMAC (python…

    dc38a82
21. Agent-Hellboy

    added a commit that references this issue on Aug 19, 2025

    pythongh-72570: mention the incompatibility of XOFs with HMAC (python…

    8f670a1
22. kumaraditya303

    added a commit that references this issue on Sep 9, 2025

    [3.14] pythongh-72570: mention the incompatibility of XOFs with HMAC (p…

    a31ccb8

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