Fix issues found in Quarkslab audit #276
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This issue was found by Laurent Grémy & Nicolas Surbayrole of Quarkslab.
pinkforest
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Jun 27, 2023
Eliminates the `patch.crates-io` directive by using the latest RC release of `curve25519-dalek` on crates.io
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A Quarkslab audit (to be published shortly) looked through the
curve25519-daleksource code; this PR fixes the issues they identified.The only serious one is that, although the rest of the
ScalarAPI works with canonical scalars, the API has an "invariant loophole",Scalar::from_bits, which allows constructing non-reduced scalars for use in, e.g., X/Ed25519, which mandate the use of non-reduced scalars.However, although the group operations (scalar, point) were designed to operate on non-reduced scalars, this was not true of (scalar, scalar) addition and subtraction, and so it was possible to use the API to explicitly construct large, unreduced
Scalars viafrom_bitsand then perform additions and subtractions which compute incorrect results. This is now fixed, with tests that ensure that addition and subtraction work correctly on very largeScalarvalues.A search of
Scalar::from_bitsof all Rust code on Github suggests that the function is mostly only used to implement X/Ed25519, which don't do those operations, so it's unlikely that this issue affects any existing code. This issue was noticed independently by @str4d in #238.Closes #238.