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Change Scalar32 and Scalar64 to not mask the high 3 bits of a Scalar.

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This should not cause overflow, since this just lets the high limb have the
same bounds as the other limbs, but we should check this carefully.
This commit is contained in:
Henry de Valence 2017-10-30 14:34:36 -07:00 committed by Isis Lovecruft
parent b05c2c30aa
commit 3c085d264c
Failed to extract signature
2 changed files with 4 additions and 12 deletions
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8
src/scalar_32bit.rs
View file

@ -9,9 +9,6 @@
//! depend on how the limbs are combined, but will stay within
//! -0x1ffffffe00000008 (62 bits with sign bit) to
//! 0x43fffffbc0000011 (63 bits), which is still safe.
//!
//! (the 9th limb will never exceed 21 bits, so the actual
//! ranges are slightly smaller)
use core::fmt::Debug;
use core::ops::{Index, IndexMut};
@ -53,7 +50,7 @@ impl Scalar32 {
Scalar32([0,0,0,0,0,0,0,0,0])
}
/// Unpack a 32 byte / 512 bit scalar into 9 29-bit limbs, ignoring the upper 3 bits.
/// Unpack a 32 byte / 256 bit scalar into 9 29-bit limbs.
pub fn from_bytes(bytes: &[u8; 32]) -> Scalar32 {
let mut words = [0u32; 8];
for i in 0..8 {
@ -63,7 +60,7 @@ impl Scalar32 {
}
let mask = (1u32 << 29) - 1;
let top_mask = (1u32 << 21) - 1;
let top_mask = (1u32 << 24) - 1;
let mut s = Scalar32::zero();
s[ 0] = words[0] & mask;
@ -377,7 +374,6 @@ impl Scalar32 {
}
}
#[cfg(test)]
mod test {
use super::*;

8
src/scalar_64bit.rs
View file

@ -7,10 +7,6 @@
//! To see that this is safe for intermediate results, note that
//! the largest limb in a 5 by 5 product of 52-bit limbs will be
//! (0xfffffffffffff^2) * 5 = 0x4ffffffffffff60000000000005 (107 bits).
//!
//! (the 5th limb will never exceed 45 bits, so the actual
//! ranges are slightly smaller)
use core::fmt::Debug;
use core::ops::{Index, IndexMut};
@ -52,7 +48,7 @@ impl Scalar64 {
Scalar64([0,0,0,0,0])
}
/// Unpack a 32 byte / 256 bit scalar into 5 52-bit limbs, ignoring the upper 3 bits
/// Unpack a 32 byte / 256 bit scalar into 5 52-bit limbs.
pub fn from_bytes(bytes: &[u8; 32]) -> Scalar64 {
let mut words = [0u64; 8];
for i in 0..4 {
@ -62,7 +58,7 @@ impl Scalar64 {
}
let mask = (1u64 << 52) - 1;
let top_mask = (1u64 << 45) - 1;
let top_mask = (1u64 << 48) - 1;
let mut s = Scalar64::zero();
s[ 0] = words[0] & mask;