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// Copyright 2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
///////////////////////////////////////////////////////////////////////////////
package com.google.crypto.tink.hybrid.internal;
import com.google.crypto.tink.subtle.Bytes;
import com.google.crypto.tink.subtle.X25519;
import com.google.errorprone.annotations.Immutable;
import java.security.GeneralSecurityException;
import java.util.Arrays;
/** Diffie-Hellman-based X25519-HKDF HPKE KEM variant. */
@Immutable
final class X25519HpkeKem implements HpkeKem {
private final HkdfHpkeKdf hkdf;
/** Construct X25519-HKDF HPKE KEM using {@code hkdf}. */
X25519HpkeKem(HkdfHpkeKdf hkdf) {
this.hkdf = hkdf;
}
private byte[] deriveKemSharedSecret(
byte[] dhSharedSecret, byte[] senderPublicKey, byte[] recipientPublicKey)
throws GeneralSecurityException {
byte[] kemContext = Bytes.concat(senderPublicKey, recipientPublicKey);
byte[] kemSuiteId = HpkeUtil.kemSuiteId(HpkeUtil.X25519_HKDF_SHA256_KEM_ID);
return hkdf.extractAndExpand(
/*salt=*/ null,
dhSharedSecret,
"eae_prk",
kemContext,
"shared_secret",
kemSuiteId,
hkdf.getMacLength());
}
/** Helper function factored out to facilitate unit testing. */
HpkeKemEncapOutput encapsulate(byte[] recipientPublicKey, byte[] senderPrivateKey)
throws GeneralSecurityException {
byte[] dhSharedSecret = X25519.computeSharedSecret(senderPrivateKey, recipientPublicKey);
byte[] senderPublicKey = X25519.publicFromPrivate(senderPrivateKey);
byte[] kemSharedSecret =
deriveKemSharedSecret(dhSharedSecret, senderPublicKey, recipientPublicKey);
return new HpkeKemEncapOutput(kemSharedSecret, senderPublicKey);
}
@Override
public HpkeKemEncapOutput encapsulate(byte[] recipientPublicKey) throws GeneralSecurityException {
return encapsulate(recipientPublicKey, X25519.generatePrivateKey());
}
@Override
public byte[] decapsulate(byte[] encapsulatedKey, HpkeKemPrivateKey recipientPrivateKey)
throws GeneralSecurityException {
byte[] dhSharedSecret =
X25519.computeSharedSecret(
recipientPrivateKey.getSerializedPrivate().toByteArray(), encapsulatedKey);
return deriveKemSharedSecret(
dhSharedSecret, encapsulatedKey, recipientPrivateKey.getSerializedPublic().toByteArray());
}
@Override
public byte[] getKemId() throws GeneralSecurityException {
if (Arrays.equals(hkdf.getKdfId(), HpkeUtil.HKDF_SHA256_KDF_ID)) {
return HpkeUtil.X25519_HKDF_SHA256_KEM_ID;
}
throw new GeneralSecurityException("Could not determine HPKE KEM ID");
}
}
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