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LibCrypto: Simplify modular addition and subtraction

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Instead of building the REDUCE_PRIME constant on the fly from the carry
flag, we now simply use the constant in combination with select. This
improves the readablility of the functions significantly.
This commit is contained in:
Michiel Visser 2023-11-09 18:49:00 +01:00 • committed by Andrew Kaster
parent 5da070ba5e
commit d746c01716
1 changed files with 4 additions and 8 deletions
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12
Userland/Libraries/LibCrypto/Curves/SECP256r1.cpp
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@ -124,14 +124,12 @@ static constexpr u256 modular_add(u256 const& left, u256 const& right, bool carr
u256 output = left.addc(right, carry);
// If there is a carry, subtract p by adding 2^256 - p
u64 t = carry;
u256 addend = select(0u, REDUCE_PRIME, carry);
carry = false;
u256 addend { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
output = output.addc(addend, carry);
// If there is still a carry, subtract p by adding 2^256 - p
t = carry;
addend = { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
addend = select(0u, REDUCE_PRIME, carry);
return output + addend;
}
@ -141,14 +139,12 @@ static constexpr u256 modular_sub(u256 const& left, u256 const& right)
u256 output = left.subc(right, borrow);
// If there is a borrow, add p by subtracting 2^256 - p
u64 t = borrow;
u256 sub = select(0u, REDUCE_PRIME, borrow);
borrow = false;
u256 sub { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
output = output.subc(sub, borrow);
// If there is still a borrow, add p by subtracting 2^256 - p
t = borrow;
sub = { u128 { t, -(t << 32) }, u128 { -t, (t << 32) - (t << 1) } };
sub = select(0u, REDUCE_PRIME, borrow);
return output - sub;
}