libmceliece: Verification

libmceliece

libmceliece is intended to become a central target for verification of full functional correctness of implementations of Classic McEliece. This file tracks what has been verified so far and what has not.

The Classic McEliece specification is in English and is not currently computer-comprehensible. However, there is a Sage package that has been manually checked line by line against the specification. Full functional correctness of software X means that X computes the same output as the Sage package for all possible inputs.

Changes in C compilers and in assemblers often change the behavior of software and might introduce bugs where no bugs existed before. Some tools address this by verifying correctness at the machine-language level. It is important to re-run these tools whenever new binaries are produced.

Changes in CPUs can also introduce bugs where no bugs existed before. Verification is always relative to a model of CPU behavior, and physical CPUs often deviate from these models, sometimes in problematic ways.

There is also a risk that current or future versions of Sage do not correctly compute the documented Sage functions used by the Sage package. This can interfere with falsifiability: even if the CPU matches the model, a bug in a Classic McEliece implementation could be hidden by a bug in Sage.

The `supercop-20221025/crypto_kem/mceliece*` checksums match the checksums produced by libmceliece and checksums produced by the Sage package. These checksums are hashes of outputs for various pseudorandomly generated inputs (with randomness treated as another input). However, there could be bugs for other inputs.

Separate tests on the official software have specifically checked bad public-key padding and bad ciphertext padding (not just the specified return value rejecting these inputs, but also the documented handling of output buffers). The SUPERCOP checksum inputs also include bad ciphertext padding and invalid ciphertexts. However, there could be other types of inputs triggering bugs.

Various runs under `valgrind` and `asan` have not detected any abnormal use of RAM. However, `valgrind` will not notice overflows from a C variable into an adjacent C variable; `asan` will not notice overflows in assembly language; and neither tool addresses the risk of bugs for rare inputs.

https://cr.yp.to/papers.html#goppadecoding presents detailed proofs of the correctness of a basic decoding algorithm for classical binary Goppa codes. However, these proofs have not yet been computer-verified. Furthermore, the decoding algorithm used in the official Classic McEliece software includes many speedups beyond the basic decoding algorithm.

https://cr.yp.to/papers.html#controlbits presents computer-verified HOL Light proofs for the main formulas used to compute control bits. However, there could be a mismatch between the software and these formulas.

https://github.com/linesthatinterlace/verif-cb/blob/main/src/verif-cb.lean presents computer-verified Lean proofs for the same formulas.

https://sorting.cr.yp.to/verif.html presents tools that have been used to computer-verify machine-language sorting software for all sizes of interest. However, there could be bugs in these tools.

https://github.com/linesthatinterlace/pqc-verification/tree/main/classic-mceliece/verification presents computer verification, using the SAW tool, that various C functions match shorter descriptions in the Cryptol language. However, this verification covers only some C functions, mainly finite-field arithmetic and root-finding.

Version: This is version 2023.02.18 of the "Verification" web page.