Windows: Password Cache Causes Excessive Argon2 KDF Delays with High PIM Values

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
22/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
c
Domain
performance, security

Research direction

Start with ReadVolumeHeaderWCacheWithAbort() in src/Common/Cache.c, which loops over cached passwords and PIMs, and with its caller in src/Driver/Ntvol.c. Trace how ReadVolumeHeaderWithAbort() picks the PRF and reaches the Argon2 path when the PIM is above 31. Done means cached attempts no longer trigger the expensive derivation on Windows, with the cache behaviour checked against the Linux/macOS work in PR #1972. This touches crypto and kernel-driver code, so it is not a good first task without maintainer guidance.

Written by the indexing model from the issue text.

Description

bug
Source verification against VeraCrypt 1.26.30

The Windows password-cache path still contains the behavior described in this issue.

  1. In [src/Common/Cache.c](https://github.com/veracrypt/VeraCrypt/blob/VeraCrypt_1.26.30/src/Common/Cache.c#L58-L151), ReadVolumeHeaderWCacheWithAbort() stores cached passwords and PIM values, but does not retain the PRF/KDF that successfully opened each volume.

  2. When the supplied password is empty, the function iterates through cached passwords and calls ReadVolumeHeaderWithAbort() for each candidate using the same pkcs5_prf and the candidate's cached PIM.

  3. The Windows driver invokes this function through [src/Driver/Ntvol.c](https://github.com/veracrypt/VeraCrypt/blob/VeraCrypt_1.26.30/src/Driver/Ntvol.c#L622-L652).

  4. With KDF autodetection enabled and PIM values above 31, unsuccessful cached-password attempts can enter the expensive Argon2 derivation path. This can require approximately 1 GiB of memory per attempt, with computational cost increasing alongside PIM.

The Linux/macOS cache work in [PR #1972](https://github.com/veracrypt/VeraCrypt/pull/1972) does not cover the Windows driver. Its description explicitly identifies src/Common/Cache.c as unchanged. The same Windows-side limitation is also documented in [#1969](https://github.com/veracrypt/VeraCrypt/issues/1969).

Version reviewed: VeraCrypt 1.26.30
Tag: [VeraCrypt_1.26.30](https://github.com/veracrypt/VeraCrypt/tree/VeraCrypt_1.26.30)
Runtime qualification: The original Windows behavior was reported on 1.26.29. The persistence in 1.26.30 is confirmed by source inspection, not by a new runtime reproduction on that release.

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