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Document fixed Argon2id header key size

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Argon2id includes the requested output length in its computation, so deriving 192 bytes and using a prefix is not equivalent to deriving only the selected cipher's key material length. This differs from PBKDF2, where the prefix property made this detail invisible.

VeraCrypt derives the maximum header key material currently needed by the supported cipher/cascade set, which is 192 bytes, and then uses the required prefix for the selected encryption algorithm. For AES-XTS this means the first 64 bytes of the 192-byte Argon2id output are used.

Make this design rule explicit in code and documentation by introducing ARGON2_HEADER_KEYDATA_SIZE instead of relying implicitly on GetMaxPkcs5OutSize. If a future cipher or cascade requires more than 192 bytes, that must be handled as an explicit format/design change.

Document the 192-byte Argon2id header KDF output requirement so third-party implementations derive the same header key material.

References: https://github.com/veracrypt/VeraCrypt/issues/1614
This commit is contained in:
Mounir IDRASSI
2026-05-21 17:19:23 +09:00
parent c4acb6a0be
commit c3ce2db9ac
12 changed files with 81 additions and 24 deletions
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src/Common/Crypto.c
+2 -2
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@@ -801,7 +801,8 @@ int EAGetLargestKeyForMode (int mode)
return key;
}
// Returns the maximum number of bytes necessary to be generated by the PBKDF2 (PKCS #5)
// Returns the maximum number of bytes necessary to be generated by PBKDF2 (PKCS #5).
// Argon2id header key material uses the fixed ARGON2_HEADER_KEYDATA_SIZE value.
int GetMaxPkcs5OutSize (void)
{
int size = 32;
@@ -1488,4 +1489,3 @@ void VcUnprotectKeys (PCRYPTO_INFO pCryptoInfo, uint64 encID)
#endif
#endif