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ml-dsa: lower stack usage of KeyPair::from_seed by up to 112kb #1259

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Merged
tarcieri merged 1 commit into from
Mar 20, 2026
Merged

ml-dsa: lower stack usage of KeyPair::from_seed by up to 112kb #1259

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37 changes: 26 additions & 11 deletions ml-dsa/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -310,9 +310,8 @@ impl<P: MlDsaParams> SigningKey<P> {
s1: Vector<P::L>,
s2: Vector<P::K>,
t0: Vector<P::K>,
A_hat: Option<NttMatrix<P::K, P::L>>,
A_hat: NttMatrix<P::K, P::L>,
) -> Self {
let A_hat = A_hat.unwrap_or_else(|| expand_a(&rho));
let s1_hat = s1.ntt();
let s2_hat = s2.ntt();
let t0_hat = t0.ntt();
Expand All @@ -332,6 +331,19 @@ impl<P: MlDsaParams> SigningKey<P> {
}
}

#[inline]
fn new_expand_a(
rho: B32,
K: B32,
tr: B64,
s1: Vector<P::L>,
s2: Vector<P::K>,
t0: Vector<P::K>,
) -> Self {
let A_hat = expand_a(&rho);
Self::new(rho, K, tr, s1, s2, t0, A_hat)
}

/// Deterministically generate a signing key from the specified seed.
///
/// This method reflects the ML-DSA.KeyGen_internal algorithm from FIPS 204, but only returns a
Expand Down Expand Up @@ -523,14 +535,13 @@ impl<P: MlDsaParams> SigningKey<P> {
P: MlDsaParams,
{
let (rho, K, tr, s1_enc, s2_enc, t0_enc) = P::split_sk(enc);
Self::new(
Self::new_expand_a(
rho.clone(),
K.clone(),
tr.clone(),
P::decode_s1(s1_enc),
P::decode_s2(s2_enc),
P::decode_t0(t0_enc),
None,
)
}

Expand Down Expand Up @@ -607,7 +618,7 @@ impl<P: MlDsaParams> signature::Keypair for SigningKey<P> {
/* Discard t0 */
let (t1, _) = t.power2round();

VerifyingKey::new(self.rho.clone(), t1, Some(self.A_hat.clone()), None)
VerifyingKey::new(self.rho.clone(), t1, self.A_hat.clone(), None)
}
}

Expand Down Expand Up @@ -676,10 +687,9 @@ impl<P: MlDsaParams> VerifyingKey<P> {
fn new(
rho: B32,
t1: Vector<P::K>,
A_hat: Option<NttMatrix<P::K, P::L>>,
A_hat: NttMatrix<P::K, P::L>,
enc: Option<EncodedVerifyingKey<P>>,
) -> Self {
let A_hat = A_hat.unwrap_or_else(|| expand_a(&rho));
let enc = enc.unwrap_or_else(|| Self::encode_internal(&rho, &t1));

let t1_2d_hat = (Elem::new(1 << 13) * &t1).ntt();
Expand All @@ -694,6 +704,12 @@ impl<P: MlDsaParams> VerifyingKey<P> {
}
}

#[inline]
fn new_expand_a(rho: B32, t1: Vector<P::K>, enc: Option<EncodedVerifyingKey<P>>) -> Self {
let A_hat = expand_a(&rho);
Self::new(rho, t1, A_hat, enc)
}

/// Computes µ according to FIPS 204 for use in ML-DSA.Sign and ML-DSA.Verify.
///
/// # Errors
Expand Down Expand Up @@ -782,7 +798,7 @@ impl<P: MlDsaParams> VerifyingKey<P> {
pub fn decode(enc: &EncodedVerifyingKey<P>) -> Self {
let (rho, t1_enc) = P::split_vk(enc);
let t1 = P::decode_t1(t1_enc);
Self::new(rho.clone(), t1, None, Some(enc.clone()))
Self::new_expand_a(rho.clone(), t1, Some(enc.clone()))
}
}

Expand Down Expand Up @@ -927,9 +943,8 @@ where
// Compress and encode
let (t1, t0) = t.power2round();

let verifying_key = VerifyingKey::new(rho, t1, Some(A_hat.clone()), None);
let signing_key =
SigningKey::new(rho, K, verifying_key.tr.clone(), s1, s2, t0, Some(A_hat));
let verifying_key = VerifyingKey::new(rho, t1, A_hat.clone(), None);
let signing_key = SigningKey::new(rho, K, verifying_key.tr.clone(), s1, s2, t0, A_hat);

KeyPair {
signing_key,
Expand Down
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