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pkcs1v15: use ctutils::CtGt to address TODO #726

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Merged
tarcieri merged 1 commit into from
Oct 19, 2026
Merged

pkcs1v15: use ctutils::CtGt to address TODO #726

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12 changes: 4 additions & 8 deletions src/algorithms/pkcs1v15.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@

use alloc::vec::Vec;
use const_oid::AssociatedOid;
use crypto_bigint::{Choice, CtAssign, CtEq, CtSelect};
use crypto_bigint::{Choice, CtAssign, CtEq, CtGt, CtSelect};
use digest::Digest;
use rand_core::TryCryptoRng;
use zeroize::Zeroizing;
Expand Down Expand Up @@ -103,13 +103,9 @@ fn decrypt_inner(em: &[u8], k: usize) -> Result<(u8, u32)> {
looking_for_index &= !equals0;
}

// The PS padding must be at least 8 bytes long, and it starts two
// bytes into em.
// TODO: WARNING: THIS MUST BE CONSTANT TIME CHECK:
// Ref: https://github.com/dalek-cryptography/subtle/issues/20
// This is currently copy & paste from the constant time impl in
// go, but very likely not sufficient.
let valid_ps = Choice::from_u8_lsb((((2i32 + 8i32 - index as i32 - 1i32) >> 31) & 1) as u8);
// PS starts at offset 2 and must contain at least 8 nonzero octets,
// so the first zero separator must occur at offset 10 or later.
let valid_ps = index.ct_gt(&9);

@tarcieri tarcieri Oct 19, 2026

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This unfortunately doesn't exist but would be a clearer way to write it:

Suggested change
let valid_ps = index.ct_gt(&9);
let valid_ps = index.ct_ge(&10);

Thought about leaving a comment to that effect.

let valid = first_byte_is_zero & second_byte_is_two & !looking_for_index & valid_ps;
index = u32::ct_select(&0, &(index + 1), valid);

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