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GeezerVison is showing github.com — `hmac.digest()` and `HMAC(...).digest()` disagree when blocksize is smaller than 16 · Issue #155428 · python/cpython

hmac.digest() and HMAC(...).digest() disagree when blocksize is smaller than 16 #155428

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hmac.digest() and HMAC(...).digest() disagree when blocksize is smaller than 16#155428
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picnixz
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stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

Description

@picnixz
picnixz
opened on Aug 9, 2026
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Bug report

Bug description:

hmac.digest() and HMAC(...).digest() return DIFFERENT MACs for any digest whose block_size is below 16, because hmac._compute_digest_fallback reads block_size raw while hmac.HMAC._init_old clamps it to 64.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

  • gh-155428: align hmac.digest and hmac.HMAC.digest outputs when using small block sizes #155429
  • [3.15] gh-155428: align hmac.digest and hmac.HMAC.digest outputs when using small block sizes (GH-155429) #160060
  • [3.14] gh-155428: align hmac.digest and hmac.HMAC.digest outputs when using small block sizes (GH-155429) #160061
Reacted by Gregory P. Smith

Activity

  1. picnixz
    self-assigned this
    on Aug 9, 2026
  2. picnixz
    added
    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error
    stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory
    on Aug 9, 2026
  3. picnixz
    added a parent issue on Aug 9, 2026
    • Meta-issue for bugs in Lib/hmac.py and Lib/hashlib.py #155413
  4. picnixz
    added 3 commits that reference this issue on Oct 21, 2026

    gh-155428: align `hmac.digest` and `hmac.HMAC.digest` outputs when us…

    815dc47

    [3.14] gh-155428: align `hmac.digest` and `hmac.HMAC.digest` outputs …

    1a80008

    [3.15] gh-155428: align `hmac.digest` and `hmac.HMAC.digest` outputs …

    eb930ba
  5. picnixz
    closed this as completedon Oct 21, 2026
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  • @picnixz
    picnixz

Labels

stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

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