java_src/src/main/java/com/google/crypto/tink/hybrid/internal/X25519HpkeKem.java

// 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");
  }
}