302bb3964a
Per Brian, x25519_ge_frombytes_vartime does not match the usual BoringSSL return value convention, and we're slightly inconsistent about whether to mask the last byte with 63 or 127. (It then gets ANDed with 64, so it doesn't matter which.) Use 127 to align with the curve25519 RFC. Finally, when we invert the transformation, use the same constants inverted so that they're parallel. Bug: 243, 244 Change-Id: I0e3aca0433ead210446c58d86b2f57526bde1eac Reviewed-on: https://boringssl-review.googlesource.com/27984 Reviewed-by: Adam Langley <agl@google.com> |
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|---|---|---|
| .. | ||
| BUILD.gn | ||
| CMakeLists.txt | ||
| curve25519_tables.h | ||
| curve25519.c | ||
| internal.h | ||
| LICENSE | ||
| make_curve25519_tables.py | ||
| METADATA | ||
| p256.c | ||
| README.chromium | ||
| README.md | ||
Fiat
Some of the code in this directory is generated by Fiat and thus these files are licensed under the MIT license. (See LICENSE file.)
Curve25519
To generate the field arithmetic procedures in curve25519.c from a fiat-crypto
checkout (as of 7892c66d5e0e5770c79463ce551193ceef870641), run
make src/Specific/solinas32_2e255m19_10limbs/femul.c (replacing femul with
the desired field operation). The "source" file specifying the finite field and
referencing the desired implementation strategy is
src/Specific/solinas32_2e255m19_10limbs/CurveParameters.v, specifying roughly
"unsaturated arithmetic modulo 2^255-19 using 10 limbs of radix 2^25.5 in 32-bit
unsigned integers with a single carry chain and two wraparound carries" where
only the prime is considered normative and everything else is treated as
"compiler hints".
The 64-bit implementation uses 5 limbs of radix 2^51 with instruction scheduling
taken from curve25519-donna-c64. It is found in
src/Specific/solinas64_2e255m19_5limbs_donna.
P256
To generate the field arithmetic procedures in p256.c from a fiat-crypto
checkout, run
make src/Specific/montgomery64_2e256m2e224p2e192p2e96m1_4limbs/femul.c.
The corresponding "source" file is
src/Specific/montgomery64_2e256m2e224p2e192p2e96m1_4limbs/CurveParameters.v,
specifying roughly "64-bit saturated word-by-word Montgomery reduction modulo
2^256 - 2^224 + 2^192 + 2^96 - 1". Again, everything except for the prime is
untrusted. There is currently a known issue where fesub.c for p256 does not
manage to complete the build (specialization) within a week on Coq 8.7.0.
https://github.com/JasonGross/fiat-crypto/tree/3e6851ddecaac70d0feb484a75360d57f6e41244/src/Specific/montgomery64_2e256m2e224p2e192p2e96m1_4limbs
does manage to build that file, but the work on that branch was never finished
(the correctness proofs of implementation templates still apply, but the
now abandoned prototype specialization facilities there are unverified).
Working With Fiat Crypto Field Arithmetic
The fiat-crypto readme https://github.com/mit-plv/fiat-crypto#arithmetic-core contains an overview of the implementation templates followed by a tour of the specialization machinery. It may be helpful to first read about the less messy parts of the system from chapter 3 of http://adam.chlipala.net/theses/andreser.pdf. There is work ongoing to replace the entire specialization mechanism with something much more principled https://github.com/mit-plv/fiat-crypto/projects/4.