private static BigInteger decodeBigInteger(ByteString data) {
    return BigIntegerEncoding.fromUnsignedBigEndianBytes(data.toByteArray());
  }
Encodes a BigInteger using a big-endian encoding.
java_src/src/main/java/com/google/crypto/tink/signature/internal/RsaSsaPkcs1ProtoSerialization.java