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Refactor authenticated DHKEM paths to eliminate intermediate heap allocation #3

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Merged
nadimkobeissi merged 4 commits into from
May 15, 2026
Merged

Refactor authenticated DHKEM paths to eliminate intermediate heap allocation #3

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a90c5af
refactor(dh.rs): eliminate heap allocation in auth encap/decap paths
danieldia-dev May 15, 2026
b4bff45
bench: add auth_encap and auth_decap benchmarks for x25519
danieldia-dev May 15, 2026
8f2240c
refactor(dh.rs): apply extract_and_expand_pieces to all auth paths
danieldia-dev May 15, 2026
f738ec5
fix(bench): remove redundant AuthKem import
danieldia-dev May 15, 2026
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27 changes: 21 additions & 6 deletions benches/bench.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -38,15 +38,34 @@ fn bench_kem(c: &mut Criterion) {
})
});

let mut os = OsRng;
let (sk_r, pk_r) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
let mut os = OsRng;
let (_, enc) = DhKemX25519HkdfSha256::encap(&mut os.unwrap_mut(), &pk_r).unwrap();
g.bench_function("x25519/decap", |b| {
b.iter(|| DhKemX25519HkdfSha256::decap(black_box(&enc), &sk_r).unwrap())
});
}

{
let mut os = OsRng;
let (sk_s, _) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
let (_, pk_r) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
g.bench_function("x25519/auth_encap", |b| {
b.iter(|| {
let mut os = OsRng;
DhKemX25519HkdfSha256::auth_encap(&mut os.unwrap_mut(), black_box(&pk_r), &sk_s)
.unwrap()
})
});

let (sk_r, pk_r) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
let (sk_s, pk_s) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
let (_, enc) =
DhKemX25519HkdfSha256::auth_encap(&mut os.unwrap_mut(), &pk_r, &sk_s).unwrap();
g.bench_function("x25519/auth_decap", |b| {
b.iter(|| DhKemX25519HkdfSha256::auth_decap(black_box(&enc), &sk_r, &pk_s).unwrap())
});
}

g.bench_function("p256/generate", |b| {
b.iter(|| {
let mut os = OsRng;
Expand Down Expand Up @@ -81,7 +100,6 @@ fn bench_setup(c: &mut Criterion) {
});

{
let mut os = OsRng;
let (enc, _ctx) =
X25519Suite::setup_sender_base(&mut os.unwrap_mut(), &pk_r, b"info").unwrap();
g.bench_function("x25519_chacha/setup_receiver_base", |b| {
Expand Down Expand Up @@ -119,7 +137,6 @@ fn bench_seal_open(c: &mut Criterion) {
},
);

let mut os = OsRng;
let (enc, ct) =
X25519Suite::seal_base(&mut os.unwrap_mut(), &pk_r, b"info", b"aad", &pt).unwrap();
g.bench_with_input(
Expand All @@ -135,7 +152,6 @@ fn bench_seal_open(c: &mut Criterion) {
}

{
let mut os = OsRng;
let (_, mut sender) =
X25519Suite::setup_sender_base(&mut os.unwrap_mut(), &pk_r, b"info").unwrap();
for &size in &[64usize, 1024, 16384] {
Expand All @@ -156,7 +172,6 @@ fn bench_export(c: &mut Criterion) {
let mut g = c.benchmark_group("export");
let mut os = OsRng;
let (_sk_r, pk_r) = DhKemX25519HkdfSha256::generate(&mut os.unwrap_mut()).unwrap();
let mut os = OsRng;
let (_enc, ctx) = X25519Suite::setup_sender_base(&mut os.unwrap_mut(), &pk_r, b"info").unwrap();

for &len in &[32usize, 64, 128] {
Expand Down
48 changes: 31 additions & 17 deletions src/kem/dh.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,9 @@ use rand_core::{CryptoRng, RngCore};
use zeroize::{Zeroize, ZeroizeOnDrop, Zeroizing};

use crate::HpkeError;
use crate::kdf::{Kdf, labeled_expand, labeled_expand_pieces, labeled_extract};
use crate::kdf::{
Kdf, labeled_expand, labeled_expand_pieces, labeled_extract, labeled_extract_pieces,
};
use crate::kem::{AuthKem, Kem};
use crate::sealed::Sealed;

Expand Down Expand Up @@ -198,6 +200,26 @@ fn extract_and_expand<D: DiffieHellman, H: Kdf>(
)
}

fn extract_and_expand_pieces<D: DiffieHellman, H: Kdf>(
dh_pieces: &[&[u8]],
kem_context: &[&[u8]],
) -> Result<Vec<u8>, HpkeError> {
let suite = suite_id::<D>();
let eae_prk = Zeroizing::new(labeled_extract_pieces::<H>(
&[],
&suite,
b"eae_prk",
dh_pieces,
));
labeled_expand_pieces::<H>(
&eae_prk,
&suite,
b"shared_secret",
kem_context,
D::SHARED_SECRET_LEN,
)
}

/// RFC 9180 §7.1.3 `DeriveKeyPair` rejection-sampling loop for NIST P-curves
/// and secp256k1.
///
Expand Down Expand Up @@ -455,14 +477,9 @@ impl<D: DiffieHellman, H: Kdf> AuthKem for DhKem<D, H> {
let pk_e = D::pk_from_bytes(&enc.0)?;
let dh1 = Zeroizing::new(D::dh(&sk_r.sk, &pk_e)?);
let dh2 = Zeroizing::new(D::dh(&sk_r.sk, &pk_s.0)?);
let mut dh = Zeroizing::new(Vec::with_capacity(dh1.len() + dh2.len()));
dh.extend_from_slice(&dh1);
dh.extend_from_slice(&dh2);

// Cached `sk_r.pk_bytes` replaces a per-call base-point scalar mult.
let pk_sender = D::pk_to_bytes(&pk_s.0);
Ok(DhSharedSecret(extract_and_expand::<D, H>(
&dh,
Ok(DhSharedSecret(extract_and_expand_pieces::<D, H>(
&[&dh1, &dh2],
&[&enc.0, &sk_r.pk_bytes, &pk_sender],
)?))
}
Expand All @@ -482,11 +499,11 @@ fn encap_with<D: DiffieHellman, H: Kdf, R: CryptoRng + RngCore>(
None => extract_and_expand::<D, H>(&dh1, &[&enc, &pk_recipient])?,
Some(sk_s) => {
let dh2 = Zeroizing::new(D::dh(&sk_s.sk, &pk_r.0)?);
let mut dh = Zeroizing::new(Vec::with_capacity(dh1.len() + dh2.len()));
dh.extend_from_slice(&dh1);
dh.extend_from_slice(&dh2);
// Cached sender public-key bytes — no `sk_to_pk` round trip.
extract_and_expand::<D, H>(&dh, &[&enc, &pk_recipient, &sk_s.pk_bytes])?
extract_and_expand_pieces::<D, H>(
&[&dh1, &dh2],
&[&enc, &pk_recipient, &sk_s.pk_bytes],
)?
}
};

Expand Down Expand Up @@ -521,14 +538,11 @@ pub(crate) fn auth_encap_with_ikm<D: DiffieHellman, H: Kdf>(
let (sk_e, pk_e) = D::derive(ikm_e)?;
let dh1 = Zeroizing::new(D::dh(&sk_e, &pk_r.0)?);
let dh2 = Zeroizing::new(D::dh(&sk_s.sk, &pk_r.0)?);
let mut dh = Zeroizing::new(Vec::with_capacity(dh1.len() + dh2.len()));
dh.extend_from_slice(&dh1);
dh.extend_from_slice(&dh2);
let enc = D::pk_to_bytes(&pk_e);
let pk_recipient = D::pk_to_bytes(&pk_r.0);
Ok((
DhSharedSecret(extract_and_expand::<D, H>(
&dh,
DhSharedSecret(extract_and_expand_pieces::<D, H>(
&[&dh1, &dh2],
&[&enc, &pk_recipient, &sk_s.pk_bytes],
)?),
DhEncappedKey(enc),
Expand Down
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