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// Copyright 2022 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.EllipticCurves;
import com.google.crypto.tink.subtle.EllipticCurves.CurveType;
import com.google.crypto.tink.subtle.EllipticCurves.PointFormatType;
import com.google.errorprone.annotations.Immutable;
import java.security.GeneralSecurityException;
import java.security.KeyPair;
import java.security.interfaces.ECPrivateKey;
import java.security.interfaces.ECPublicKey;
/** Diffie-Hellman-based P-256, P-384 and P-521 HPKE KEM variant. */
@Immutable
final class NistCurvesHpkeKem implements HpkeKem {
private final CurveType curve;
private final HkdfHpkeKdf hkdf;
/** Construct HPKE KEM using {@code curve}. */
static NistCurvesHpkeKem fromCurve(CurveType curve) throws GeneralSecurityException {
switch (curve) {
case NIST_P256:
return new NistCurvesHpkeKem(new HkdfHpkeKdf("HmacSha256"), CurveType.NIST_P256);
case NIST_P384:
return new NistCurvesHpkeKem(new HkdfHpkeKdf("HmacSha384"), CurveType.NIST_P384);
case NIST_P521:
return new NistCurvesHpkeKem(new HkdfHpkeKdf("HmacSha512"), CurveType.NIST_P521);
}
throw new GeneralSecurityException("invalid curve type: " + curve);
}
private NistCurvesHpkeKem(HkdfHpkeKdf hkdf, CurveType curve) {
this.hkdf = hkdf;
this.curve = curve;
}
private byte[] deriveKemSharedSecret(
byte[] dhSharedSecret, byte[] senderPublicKey, byte[] recipientPublicKey)
throws GeneralSecurityException {
byte[] kemContext = Bytes.concat(senderPublicKey, recipientPublicKey);
byte[] kemSuiteID = HpkeUtil.kemSuiteId(getKemId());
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, KeyPair senderKeyPair)
throws GeneralSecurityException {
ECPublicKey recipientECPublicKey =
EllipticCurves.getEcPublicKey(curve, PointFormatType.UNCOMPRESSED, recipientPublicKey);
byte[] dhSharedSecret =
EllipticCurves.computeSharedSecret(
(ECPrivateKey) senderKeyPair.getPrivate(), recipientECPublicKey);
byte[] senderPublicKey =
EllipticCurves.pointEncode(
curve,
PointFormatType.UNCOMPRESSED,
((ECPublicKey) senderKeyPair.getPublic()).getW());
byte[] kemSharedSecret =
deriveKemSharedSecret(dhSharedSecret, senderPublicKey, recipientPublicKey);
return new HpkeKemEncapOutput(kemSharedSecret, senderPublicKey);
}
@Override
public HpkeKemEncapOutput encapsulate(byte[] recipientPublicKey) throws GeneralSecurityException {
KeyPair keyPair = EllipticCurves.generateKeyPair(curve);
return encapsulate(recipientPublicKey, keyPair);
}
@Override
public byte[] decapsulate(byte[] encapsulatedKey, HpkeKemPrivateKey recipientPrivateKey)
throws GeneralSecurityException {
ECPrivateKey privateKey =
EllipticCurves.getEcPrivateKey(
curve, recipientPrivateKey.getSerializedPrivate().toByteArray());
ECPublicKey publicKey =
EllipticCurves.getEcPublicKey(curve, PointFormatType.UNCOMPRESSED, encapsulatedKey);
byte[] dhSharedSecret = EllipticCurves.computeSharedSecret(privateKey, publicKey);
return deriveKemSharedSecret(
dhSharedSecret, encapsulatedKey, recipientPrivateKey.getSerializedPublic().toByteArray());
}
@Override
public byte[] getKemId() throws GeneralSecurityException {
switch (curve) {
case NIST_P256:
return HpkeUtil.P256_HKDF_SHA256_KEM_ID;
case NIST_P384:
return HpkeUtil.P384_HKDF_SHA384_KEM_ID;
case NIST_P521:
return HpkeUtil.P521_HKDF_SHA512_KEM_ID;
}
throw new GeneralSecurityException("Could not determine HPKE KEM ID");
}
}
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