One-to-one integer mapping function

Question

We are using MySQL and developing an application where we\'d like the ID sequence not to be publicly visible... the IDs are hardly top secret and there is no significant iss

Answer 1

If you want to ensure a 1:1 mapping then use an encryption (i.e. a permutation), not a hash. Encryption has to be 1:1 because it can be decrypted.

If you want 32 bit numbers then use Hasty Pudding Cypher or just write a simple four round Feistel cypher.

Here's one I prepared earlier:

import java.util.Random;

/**
 * IntegerPerm is a reversible keyed permutation of the integers.
 * This class is not cryptographically secure as the F function
 * is too simple and there are not enough rounds.
 *
 * @author Martin Ross
 */
public final class IntegerPerm {
    //////////////////
    // Private Data //
    //////////////////

    /** Non-zero default key, from www.random.org */
    private final static int DEFAULT_KEY = 0x6CFB18E2;

    private final static int LOW_16_MASK = 0xFFFF;
    private final static int HALF_SHIFT = 16;
    private final static int NUM_ROUNDS = 4;

    /** Permutation key */
    private int mKey;

    /** Round key schedule */
    private int[] mRoundKeys = new int[NUM_ROUNDS];

    //////////////////
    // Constructors //
    //////////////////

    public IntegerPerm() { this(DEFAULT_KEY); }

    public IntegerPerm(int key) { setKey(key); }

    ////////////////////
    // Public Methods //
    ////////////////////

    /** Sets a new value for the key and key schedule. */
    public void setKey(int newKey) {
        assert (NUM_ROUNDS == 4) : "NUM_ROUNDS is not 4";
        mKey = newKey;

        mRoundKeys[0] = mKey & LOW_16_MASK;
        mRoundKeys[1] = ~(mKey & LOW_16_MASK);
        mRoundKeys[2] = mKey >>> HALF_SHIFT;
        mRoundKeys[3] = ~(mKey >>> HALF_SHIFT);
    } // end setKey()

    /** Returns the current value of the key. */
    public int getKey() { return mKey; }

    /**
     * Calculates the enciphered (i.e. permuted) value of the given integer
     * under the current key.
     *
     * @param plain the integer to encipher.
     *
     * @return the enciphered (permuted) value.
     */
    public int encipher(int plain) {
        // 1 Split into two halves.
        int rhs = plain & LOW_16_MASK;
        int lhs = plain >>> HALF_SHIFT;

        // 2 Do NUM_ROUNDS simple Feistel rounds.
        for (int i = 0; i < NUM_ROUNDS; ++i) {
            if (i > 0) {
                // Swap lhs <-> rhs
                final int temp = lhs;
                lhs = rhs;
                rhs = temp;
            } // end if
            // Apply Feistel round function F().
            rhs ^= F(lhs, i);
        } // end for

        // 3 Recombine the two halves and return.
        return (lhs << HALF_SHIFT) + (rhs & LOW_16_MASK);
    } // end encipher()

    /**
     * Calculates the deciphered (i.e. inverse permuted) value of the given
     * integer under the current key.
     *
     * @param cypher the integer to decipher.
     *
     * @return the deciphered (inverse permuted) value.
     */
    public int decipher(int cypher) {
        // 1 Split into two halves.
        int rhs = cypher & LOW_16_MASK;
        int lhs = cypher >>> HALF_SHIFT;

        // 2 Do NUM_ROUNDS simple Feistel rounds.
        for (int i = 0; i < NUM_ROUNDS; ++i) {
            if (i > 0) {
                // Swap lhs <-> rhs
                final int temp = lhs;
                lhs = rhs;
                rhs = temp;
            } // end if
            // Apply Feistel round function F().
            rhs ^= F(lhs, NUM_ROUNDS - 1 - i);
        } // end for

        // 4 Recombine the two halves and return.
        return (lhs << HALF_SHIFT) + (rhs & LOW_16_MASK);
    } // end decipher()

    /////////////////////
    // Private Methods //
    /////////////////////

    // The F function for the Feistel rounds.
    private int F(int num, int round) {
        // XOR with round key.
        num ^= mRoundKeys[round];
        // Square, then XOR the high and low parts.
        num *= num;
        return (num >>> HALF_SHIFT) ^ (num & LOW_16_MASK);
    } // end F()

} // end class IntegerPerm