Fix HPKE decryption ciphertext length validation#14391
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reaperhulk merged 1 commit intomainfrom Feb 27, 2026
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The minimum ciphertext length check only validated against X25519_NENC (32 bytes) but didn't account for the 16-byte AES-GCM authentication tag. This allowed 32-47 byte ciphertexts to pass the early check and proceed through the full DH exchange and HKDF key schedule before AES-GCM correctly rejected them. Add AES_128_GCM_NT constant and include it in the check to avoid wasted computation on trivially invalid inputs. https://claude.ai/code/session_01EGLxxnHUgfiStRV9BzFJxo
reaperhulk
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Feb 27, 2026
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Summary
This PR fixes the ciphertext length validation in HPKE decryption to account for the authentication tag size.
Key Changes
AES_128_GCM_NTconstant (16 bytes) toaead_paramsmodule to represent the AES-128-GCM authentication tag lengthSuite::decrypt()to validate that the ciphertext is at leastX25519_NENC + AES_128_GCM_NTbytes, rather than justX25519_NENCbytesDetails
The previous validation was insufficient as it only checked for the encapsulated key size but did not account for the authentication tag that must be present in the ciphertext. This could allow invalid ciphertexts to pass the length check and potentially cause issues during decryption. The fix ensures the ciphertext contains both the encapsulated key and the authentication tag before attempting decryption.
https://claude.ai/code/session_01EGLxxnHUgfiStRV9BzFJxo