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Merged
tarcieri merged 1 commit into from
Oct 18, 2026
Merged

Rename integer serialization functions to i2osp* #718

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40 changes: 28 additions & 12 deletions src/algorithms/pad.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -25,23 +25,39 @@ fn be_pad_into(bytes: &[u8], out: &mut [u8]) {
out[start..].copy_from_slice(tail);
}

/// Converts input to the new vector of the given length, using BE and with 0s left padded.
/// Integer-to-Octet-String Primitive (I2OSP) from [RFC8017 §4.1].
///
/// `input` must fit in `padded_len` bytes.
/// Converts `input` to exactly `padded_len` big-endian bytes, left-padded with zeros.
/// `input` must fit in `padded_len` bytes. This is a caller-established precondition, checked by a
/// debug assertion.
///
/// Use [`i2osp_zeroizing`] when encoding secret values.
///
/// [RFC8017 §4.1]: https://www.rfc-editor.org/rfc/rfc8017.html#section-4.1
#[inline]
pub(crate) fn uint_to_be_pad(input: BoxedUint, padded_len: usize) -> Vec<u8> {
pub(crate) fn i2osp(input: BoxedUint, padded_len: usize) -> Vec<u8> {
let mut out = vec![0u8; padded_len];
be_pad_into(&input.to_be_bytes(), &mut out);
out
}

/// Converts input to the new vector of the given length, using BE and with 0s left padded.
/// Computes [`i2osp`] (Integer-to-Octet-String Primitive) with zeroization of the input and output
/// buffers.
///
/// The input integer, temporary byte buffer, and returned vector are each zeroized on drop.
/// The same input-width precondition as [`i2osp`] applies.
///
/// For RSA decryption, this computes `EM = I2OSP(m, k)`, where `padded_len` is `k`, the public
/// modulus length in octets.
///
/// `input` must fit in `padded_len` bytes.
/// [draft-irtf-cfrg-rsa-guidance-10 §7.2] requires this conversion to be constant-time with respect
/// to `m`. Serialization and copying in this function depend only on `padded_len` and the input's
/// storage precision. To satisfy that requirement, callers must ensure that both widths are
/// independent of `m`'s value.
///
/// For secret values: every intermediate buffer, including the returned one, is zeroized on drop.
/// [draft-irtf-cfrg-rsa-guidance-10 §7.2]: https://www.ietf.org/archive/id/draft-irtf-cfrg-rsa-guidance-10.html#section-7.2
#[inline]
pub(crate) fn uint_to_zeroizing_be_pad(input: BoxedUint, padded_len: usize) -> Zeroizing<Vec<u8>> {
pub(crate) fn i2osp_zeroizing(input: BoxedUint, padded_len: usize) -> Zeroizing<Vec<u8>> {
let input = Zeroizing::new(input);
let bytes = Zeroizing::new(input.to_be_bytes());
let mut out = Zeroizing::new(vec![0u8; padded_len]);
Expand All @@ -55,24 +71,24 @@ mod tests {
#[test]
fn be_pad() {
let x = BoxedUint::from(0x0102u64);
let p = uint_to_be_pad(x.clone(), 16);
let p = i2osp(x.clone(), 16);
assert_eq!(&p[..14], &[0u8; 14]);
assert_eq!(&p[14..], &[1, 2]);

let p = uint_to_zeroizing_be_pad(x.clone(), 3);
let p = i2osp_zeroizing(x.clone(), 3);
assert_eq!(&p[..], &[0, 1, 2]);

let p = uint_to_be_pad(x.clone(), 8);
let p = i2osp(x.clone(), 8);
assert_eq!(&p[..], &[0, 0, 0, 0, 0, 0, 1, 2]);

let p = uint_to_be_pad(BoxedUint::zero_with_precision(128), 4);
let p = i2osp(BoxedUint::zero_with_precision(128), 4);
assert_eq!(&p[..], &[0u8; 4]);
}

#[cfg(debug_assertions)]
#[test]
#[should_panic(expected = "integer exceeds output width")]
fn be_pad_rejects_broken_precondition() {
uint_to_zeroizing_be_pad(BoxedUint::from(0x0102u64), 1);
i2osp_zeroizing(BoxedUint::from(0x0102u64), 1);
}
}
10 changes: 5 additions & 5 deletions src/oaep.rs
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Expand Up @@ -18,7 +18,7 @@ use digest::{Digest, FixedOutputReset};
use rand_core::TryCryptoRng;

use crate::algorithms::oaep::*;
use crate::algorithms::pad::{uint_to_be_pad, uint_to_zeroizing_be_pad};
use crate::algorithms::pad::{i2osp, i2osp_zeroizing};
use crate::algorithms::rsa::{rsa_decrypt_and_check, rsa_encrypt};
use crate::errors::{Error, Result};
use crate::key::{self, RsaPrivateKey, RsaPublicKey};
Expand Down Expand Up @@ -215,7 +215,7 @@ where
let em = oaep_encrypt(rng, msg, digest, mgf_digest, label, pub_key.size())?;

let int = BoxedUint::from_be_slice(&em, pub_key.n_bits_precision())?;
Ok(uint_to_be_pad(rsa_encrypt(pub_key, &int)?, pub_key.size()))
Ok(i2osp(rsa_encrypt(pub_key, &int)?, pub_key.size()))
}

/// Encrypts the given message with RSA and the padding scheme from
Expand All @@ -241,7 +241,7 @@ where
let em = oaep_encrypt_digest::<_, D, MGD>(rng, msg, label, pub_key.size())?;

let int = BoxedUint::from_be_slice(&em, pub_key.n_bits_precision())?;
Ok(uint_to_be_pad(rsa_encrypt(pub_key, &int)?, pub_key.size()))
Ok(i2osp(rsa_encrypt(pub_key, &int)?, pub_key.size()))
}

/// Decrypts a plaintext using RSA and the padding scheme from [PKCS#1 OAEP].
Expand Down Expand Up @@ -277,7 +277,7 @@ where
let ciphertext = BoxedUint::from_be_slice(ciphertext, priv_key.n_bits_precision())?;

let em = rsa_decrypt_and_check(priv_key, rng, &ciphertext)?;
let mut em = uint_to_zeroizing_be_pad(em, priv_key.size());
let mut em = i2osp_zeroizing(em, priv_key.size());

oaep_decrypt(&mut em, digest, mgf_digest, label, priv_key.size())
}
Expand Down Expand Up @@ -314,7 +314,7 @@ where

let ciphertext = BoxedUint::from_be_slice(ciphertext, priv_key.n_bits_precision())?;
let em = rsa_decrypt_and_check(priv_key, rng, &ciphertext)?;
let mut em = uint_to_zeroizing_be_pad(em, priv_key.size());
let mut em = i2osp_zeroizing(em, priv_key.size());

oaep_decrypt_digest::<D, MGD>(&mut em, label, priv_key.size())
}
Expand Down
10 changes: 5 additions & 5 deletions src/pkcs1v15.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -44,7 +44,7 @@ use crypto_bigint::BoxedUint;
use digest::Digest;
use rand_core::TryCryptoRng;

use crate::algorithms::pad::{uint_to_be_pad, uint_to_zeroizing_be_pad};
use crate::algorithms::pad::{i2osp, i2osp_zeroizing};
use crate::algorithms::pkcs1v15::*;
use crate::algorithms::rsa::{rsa_decrypt_and_check, rsa_encrypt};
use crate::errors::{Error, Result};
Expand Down Expand Up @@ -156,7 +156,7 @@ fn encrypt<R: TryCryptoRng + ?Sized>(

let em = pkcs1v15_encrypt_pad(rng, msg, pub_key.size())?;
let int = BoxedUint::from_be_slice(&em, pub_key.n_bits_precision())?;
Ok(uint_to_be_pad(rsa_encrypt(pub_key, &int)?, pub_key.size()))
Ok(i2osp(rsa_encrypt(pub_key, &int)?, pub_key.size()))
}

/// Decrypts a plaintext using RSA and the padding scheme from PKCS#1 v1.5.
Expand All @@ -183,7 +183,7 @@ fn decrypt<R: TryCryptoRng + ?Sized>(

let ciphertext = BoxedUint::from_be_slice(ciphertext, priv_key.n_bits_precision())?;
let em = rsa_decrypt_and_check(priv_key, rng, &ciphertext)?;
let em = uint_to_zeroizing_be_pad(em, priv_key.size());
let em = i2osp_zeroizing(em, priv_key.size());

pkcs1v15_encrypt_unpad(&em, priv_key.size())
}
Expand Down Expand Up @@ -212,7 +212,7 @@ fn sign<R: TryCryptoRng + ?Sized>(

let em = BoxedUint::from_be_slice(&em, priv_key.n_bits_precision())?;
// The signature itself is public; only the computation is secret
Ok(uint_to_be_pad(
Ok(i2osp(
rsa_decrypt_and_check(priv_key, rng, &em)?,
priv_key.size(),
))
Expand All @@ -226,7 +226,7 @@ fn verify(pub_key: &RsaPublicKey, prefix: &[u8], hashed: &[u8], sig: &BoxedUint)
return Err(Error::Verification);
}

let em = uint_to_be_pad(rsa_encrypt(pub_key, sig)?, pub_key.size());
let em = i2osp(rsa_encrypt(pub_key, sig)?, pub_key.size());

pkcs1v15_sign_unpad(prefix, hashed, &em, pub_key.size())
}
Expand Down
10 changes: 5 additions & 5 deletions src/pss.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ use crypto_bigint::BoxedUint;
use digest::{Digest, FixedOutputReset};
use rand_core::TryCryptoRng;

use crate::algorithms::pad::uint_to_be_pad;
use crate::algorithms::pad::i2osp;
use crate::algorithms::pss::*;
use crate::algorithms::rsa::{rsa_decrypt_and_check, rsa_encrypt};
use crate::errors::{Error, Result};
Expand Down Expand Up @@ -157,7 +157,7 @@ where
return Err(Error::Verification);
}
let raw = rsa_encrypt(pub_key, sig)?;
let mut em = uint_to_be_pad(raw, pub_key.size());
let mut em = i2osp(raw, pub_key.size());

emsa_pss_verify(hashed, &mut em, salt_len, digest, pub_key.n().bits() as _)
}
Expand All @@ -176,7 +176,7 @@ where
return Err(Error::Verification);
}

let mut em = uint_to_be_pad(rsa_encrypt(pub_key, sig)?, pub_key.size());
let mut em = i2osp(rsa_encrypt(pub_key, sig)?, pub_key.size());

emsa_pss_verify_digest::<D>(hashed, &mut em, salt_len, pub_key.n().bits() as _)
}
Expand Down Expand Up @@ -243,7 +243,7 @@ where

let em = BoxedUint::from_be_slice(&em, priv_key.n_bits_precision())?;
let raw = rsa_decrypt_and_check(priv_key, blind_rng, &em)?;
Ok(uint_to_be_pad(raw, priv_key.size()))
Ok(i2osp(raw, priv_key.size()))
}

fn sign_pss_with_salt_digest<T, D>(
Expand All @@ -260,7 +260,7 @@ where
let em = emsa_pss_encode_digest::<D>(hashed, em_bits as _, salt)?;

let em = BoxedUint::from_be_slice(&em, priv_key.n_bits_precision())?;
Ok(uint_to_be_pad(
Ok(i2osp(
rsa_decrypt_and_check(priv_key, blind_rng, &em)?,
priv_key.size(),
))
Expand Down
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