Different CRC's, same file?

Question

Hello Sun Forums,
I have a question for all you Sun experts. The nature of my company's product is such that when we compile our code, the same set of files always needs to have the same CRC. One of our developers here mentioned that he thinks that every time we build the jars, even on the same set of files, a time stamp, or the manifest, or something, is causing the same jar files to have a different CRC with each new compile.
Can someone tell me how to avoid having each new compile of the same code generate different CRC's for the jars? Thanks!!!
Mike

Answer 1

Here's a CRC-32 brute forcer. The first version used reflection, which was fairly slow (I figured worst case on my computer was going to be around half an hour). The CRC-32 algorithm and table was taken from the JDK's sources at j2se/src/share/native/java/util/zip/zlib-1.1.3/zcrc32.c
Worst case on my machine (Core2/2.4 running XP) is around 75 seconds, and will try all possible byte sequences of 1 to 4 bytes. I don't know if this necessarily means every possible value is covered, but the random CRCs it picks have worked:
import java.util.Random;
public class ChangeCRC2
  // Table copied from j2se/src/share/native/java/util/zip/zlib-1.1.3/zcrc32.c
  public static int[] CRC_TABLE = {
    0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
    0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
    0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
    0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
    0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
    0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
    0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
    0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
    0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
    0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
    0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
    0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
    0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
    0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
    0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
    0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
    0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
    0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
    0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
    0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
    0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
    0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
    0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
    0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
    0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
    0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
    0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
    0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
    0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
    0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
    0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
    0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
    0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
    0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
    0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
    0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
    0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
    0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
    0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
    0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
    0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
    0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
    0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
    0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
    0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
    0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
    0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
    0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
    0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
    0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
    0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
    0x2d02ef8d
  // Unused, from first version
  public static int computeCRC(int crc, byte[] b, int offset, int len)
    crc = ~crc;
    while (--len >= 0)
      crc = CRC_TABLE[(crc ^ b[offset++]) & 0xff] ^ (crc >>> 8);
    return ~crc;
  public static int checkMatch(int crc, int targetCrc, int n)
    crc = ~crc;
    targetCrc = ~targetCrc;
    int count = 0;
    while (++count <= 4)
      if ((crc = CRC_TABLE[(crc ^ n) & 0xff] ^ (crc >>> 8)) == targetCrc)
        return count;
      n >>>= 8;
    return -1;
  public static void main(String[] argv) throws Exception
    int startingCRC = 0;
    int endingCRC = 0;
    if (argv.length == 2)
      startingCRC = (int)Long.parseLong(argv[0], 16);
      endingCRC = (int)Long.parseLong(argv[1], 16);
    else
      Random r = new Random();
      startingCRC = r.nextInt();
      endingCRC = r.nextInt();
    System.out.println("Looking for byte sequence to change " + Long.toHexString(startingCRC & 0xFFFFFFFFL) + " to " + Long.toHexString(endingCRC & 0xFFFFFFFFL));
    long startTime = System.currentTimeMillis();
    int n = 0;
    do
      int numBytes = checkMatch(startingCRC, endingCRC, n);
      if (numBytes > 0)
        long elapsedTime = System.currentTimeMillis() - startTime;
        System.out.print("Found " + numBytes + " byte sequence after " + elapsedTime + " ms:");
        for (int i = 0; i < numBytes; i++)
          System.out.print(" " + Integer.toHexString((n >>> (i << 3)) & 0xFF));
        System.out.println();
        return;
    } while (++n != 0);
    System.out.println("Did not find match!");
Sample run:
java ChangeCRC2
Looking for byte sequence to change 156d85dc to bb7e7925
Found 4 byte sequence after 60017 ms: fe 64 f1 d8EDIT: I also have one more version that has a worst case of about 9-10 seconds, but it's a little uglier (four nested loops).
Edited by: paul.miner on Jun 25, 2008 12:11 PM