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3.1.3 Use a checked_add for the modulus in secp256k1/P-256

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Luke Parker 2023-02-23 00:57:41 -05:00
parent ac0f5e9b2d
commit cb4ce5e354
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1 changed files with 6 additions and 2 deletions
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8
crypto/ciphersuite/src/kp256.rs
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@ -6,7 +6,7 @@ use group::ff::{Field, PrimeField};
use elliptic_curve::{
generic_array::GenericArray,
bigint::{Encoding, U384},
bigint::{CheckedAdd, Encoding, U384},
hash2curve::{Expander, ExpandMsg, ExpandMsgXmd},
};
@ -64,7 +64,11 @@ macro_rules! kp_curve {
// The byte repr of scalars will be 32 big-endian bytes
// Set the lower 32 bytes of our 48-byte array accordingly
modulus[16 ..].copy_from_slice(&(Self::F::zero() - Self::F::one()).to_bytes());
let modulus = U384::from_be_slice(&modulus).wrapping_add(&U384::ONE);
// Use a checked_add + unwrap since this addition cannot fail (being a 32-byte value with
// 48-bytes of space)
// While a non-panicking saturating_add/wrapping_add could be used, they'd likely be less
// performant
let modulus = U384::from_be_slice(&modulus).checked_add(&U384::ONE).unwrap();
// The defined P-256 and secp256k1 ciphersuites both use expand_message_xmd
let mut wide = U384::from_be_bytes({