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Implement RFC8017 rules for PSS message width #725

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

Implement RFC8017 rules for PSS message width #725

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12 changes: 2 additions & 10 deletions src/algorithms/pss.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -272,18 +272,14 @@ pub(crate) fn emsa_pss_verify<D>(
em: &mut [u8],
s_len: Option<usize>,
hash: &mut D,
key_bits: usize,
em_bits: usize,
) -> Result<()>
where
D: Digest + FixedOutputReset,
{
let em_bits = key_bits - 1;
let em_len = em_bits.div_ceil(8);
let key_len = key_bits.div_ceil(8);
let h_len = <D as Digest>::output_size();

let em = &mut em[key_len - em_len..];

let (db, h) = emsa_pss_verify_pre(m_hash, em, em_bits, s_len, h_len)?;

// 7. Let dbMask = MGF(H, em_len - h_len - 1)
Expand Down Expand Up @@ -328,18 +324,14 @@ pub(crate) fn emsa_pss_verify_digest<D>(
m_hash: &[u8],
em: &mut [u8],
s_len: Option<usize>,
key_bits: usize,
em_bits: usize,
) -> Result<()>
where
D: Digest + FixedOutputReset,
{
let em_bits = key_bits - 1;
let em_len = em_bits.div_ceil(8);
let key_len = key_bits.div_ceil(8);
let h_len = <D as Digest>::output_size();

let em = &mut em[key_len - em_len..];

let (db, h) = emsa_pss_verify_pre(m_hash, em, em_bits, s_len, h_len)?;

let mut hash = D::new();
Expand Down
23 changes: 17 additions & 6 deletions src/pss.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -145,7 +145,8 @@ where
D: Digest + FixedOutputReset,
{
let mut em = decode_signature(pub_key, sig)?;
emsa_pss_verify(hashed, &mut em, salt_len, digest, pub_key.n().bits() as _)
let em_bits = pub_key.n().bits() as usize - 1;
emsa_pss_verify(hashed, &mut em, salt_len, digest, em_bits)
}

pub(crate) fn verify_digest<D>(
Expand All @@ -158,24 +159,34 @@ where
D: Digest + FixedOutputReset,
{
let mut em = decode_signature(pub_key, sig)?;
emsa_pss_verify_digest::<D>(hashed, &mut em, salt_len, pub_key.n().bits() as _)
let em_bits = pub_key.n().bits() as usize - 1;
emsa_pss_verify_digest::<D>(hashed, &mut em, salt_len, em_bits)
}

/// Checks signature length and range before applying the public-key operation.
/// Performs the length, RSAVP1, and I2OSP steps of RFC8017 §8.1.2.
fn decode_signature(pub_key: &RsaPublicKey, signature: &[u8]) -> Result<Vec<u8>> {
// Step 1: S must be exactly k octets, independently of integer storage precision.
// Step 1: `S` must be exactly `k` octets, independently of integer storage precision.
if signature.len() != pub_key.size() {
return Err(Error::Verification);
}

// Steps 2a and 2b: s = OS2IP(S), m = RSAVP1((n, e), s).
// Steps 2a and 2b: `s = OS2IP(S), m = RSAVP1((n, e), s)`.
let s = os2ip(signature, pub_key.n_bits_precision()).map_err(|_| Error::Verification)?;
if s >= *pub_key.n().as_ref() {
return Err(Error::Verification);
}
let m = rsa_encrypt(pub_key, &s)?;

Ok(i2osp(m, pub_key.size()))
// Step 2c: EM has `emLen` octets, which can be `k - 1`.
//
// Check the public representative's width before calling infallible I2OSP; discarding a nonzero
// leading octet is invalid.
let em_bits = pub_key.n().bits() as usize - 1;
let em_len = em_bits.div_ceil(8);
if m.bits() as usize > 8 * em_len {
return Err(Error::Verification);
}
Ok(i2osp(m, em_len))
}

/// SignPSS calculates the signature of hashed using RSASSA-PSS.
Expand Down
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