001package com.nimbusds.jose.util;
002
003
004import java.math.BigInteger;
005
006
007/**
008 * Big integer utilities.
009 *
010 * @author Vladimir Dzhuvinov
011 * @version $version$ (2013-03-21)
012 */
013public class BigIntegerUtils {
014
015
016        /**
017         * Returns a byte array representation of the specified big integer 
018         * without the sign bit.
019         * 
020         * @param bigInt The big integer to be converted. Must not be
021         *               {@code null}.
022         *
023         * @return A byte array representation of the big integer, without the
024         *         sign bit.
025         */
026        public static byte[] toBytesUnsigned(final BigInteger bigInt) {
027
028                // Copied from Apache Commons Codec 1.8
029
030                int bitlen = bigInt.bitLength();
031
032                // round bitlen
033                bitlen = ((bitlen + 7) >> 3) << 3;
034                final byte[] bigBytes = bigInt.toByteArray();
035
036                if (((bigInt.bitLength() % 8) != 0) && (((bigInt.bitLength() / 8) + 1) == (bitlen / 8))) {
037                
038                        return bigBytes;
039                
040                }
041
042                // set up params for copying everything but sign bit
043                int startSrc = 0;
044                int len = bigBytes.length;
045
046                // if bigInt is exactly byte-aligned, just skip signbit in copy
047                if ((bigInt.bitLength() % 8) == 0) {
048                        
049                        startSrc = 1;
050                        len--;
051                }
052                
053                final int startDst = bitlen / 8 - len; // to pad w/ nulls as per spec
054                final byte[] resizedBytes = new byte[bitlen / 8];
055                System.arraycopy(bigBytes, startSrc, resizedBytes, startDst, len);
056                return resizedBytes;
057        }
058
059
060        /**
061         * Prevents public instantiation.
062         */
063        private BigIntegerUtils() {
064
065        }
066}