public static EcdsaPrivateKey createEcdsaPrivKey(EcdsaPublicKey pubKey, byte[] privKey) {
    final int version = 0;
    return EcdsaPrivateKey.newBuilder()
        .setVersion(version)
        .setPublicKey(pubKey)
        .setKeyValue(ByteString.copyFrom(privKey))
        .build();
  }
@return a {@code EcdsaPrivateKey} constructed from {@code EcdsaPublicKey} and the byte array of
    private key.
java_src/src/main/java/com/google/crypto/tink/testing/TestUtil.java