Update Kyber poly_tomsg to fix timing leak (w/ -Os)#320
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This (partially) addresses #319. The function poly_tomsg from the reference implementation of Kyber (which was copied into the M4-optimized implementations) would result in a variable-time udiv instruction operating on secret data when compiled with gcc using -Os. I tried a couple of versions from gcc 11 to gcc 13, but did not see any difference. This commit updates the m4-specific code to use the patch from pq-crystals/kyber@dda29cc. Note that the code in PQClean has not yet been updated and hence the clean implementation within pqm4 is still vulnerable.
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No difference between the GCC 11-13, with |
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This is a port of issue PQClean/PQClean#534 Description from mupq/pqm4#320: The function poly_tomsg from the reference implementation of Kyber (which was copied into the M4-optimized implementations) would result in a variable-time udiv instruction operating on secret data when compiled with gcc using -Os. I tried a couple of versions from gcc 11 to gcc 13, but did not see any difference. Description from mupq/pqm4#319 This bit of code or similar is used in your various implementations of Kyber to compress a polynomial ring element into a (secret) message: ``` t = (((a->coeffs[8 * i + j] << 1) + KYBER_Q / 2) / KYBER_Q) & 1; ``` To do so, it performs a division by Q that might not necessarily compile to a multiplication instruction: looking at the output of some C compilers using https://godbolt.org/z/sKn3TKKGq and https://godbolt.org/z/8GqKoTfYh for example, a division instruction is emitted even when -O3 is specified. Should a division instruction be emitted, its execution time would likely be variable and leak information about its secret input. Signed-off-by: Stephan Mueller <smueller@chronox.de>
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This is a port of issue PQClean/PQClean#534 Description from mupq/pqm4#320: The function poly_tomsg from the reference implementation of Kyber (which was copied into the M4-optimized implementations) would result in a variable-time udiv instruction operating on secret data when compiled with gcc using -Os. I tried a couple of versions from gcc 11 to gcc 13, but did not see any difference. Description from mupq/pqm4#319 This bit of code or similar is used in your various implementations of Kyber to compress a polynomial ring element into a (secret) message: ``` t = (((a->coeffs[8 * i + j] << 1) + KYBER_Q / 2) / KYBER_Q) & 1; ``` To do so, it performs a division by Q that might not necessarily compile to a multiplication instruction: looking at the output of some C compilers using https://godbolt.org/z/sKn3TKKGq and https://godbolt.org/z/8GqKoTfYh for example, a division instruction is emitted even when -O3 is specified. Should a division instruction be emitted, its execution time would likely be variable and leak information about its secret input. Signed-off-by: Stephan Mueller <smueller@chronox.de>
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This (partially) addresses #319.
The function poly_tomsg from the reference implementation of Kyber (which was copied into the M4-optimized implementations) would result in a variable-time udiv instruction operating on secret data when compiled with gcc using -Os. I tried a couple of versions from gcc 11 to gcc 13, but did not see any difference.
This commit updates the m4-specific code to use the patch from pq-crystals/kyber@dda29cc. Note that the code in PQClean has not yet been updated and hence the clean implementation within pqm4 is still vulnerable.