CODEWITHSUNDEEP
javaencryptioncryptography
Sumit Singh
Sumit Singh

Reputation: 15896

How implements ISO9797 M2 Padding in java

I want to use AES encryption with ISO9797 M2 Padding . But in not under java by standard padding modes Cipher Algorithm Padding
How to implement ..?

Thank's

Upvotes: 3

Views: 3661

Answers (2)

rossum
rossum

Reputation: 15693

If it is not available in standard Java, then you could implement it yourself, and add the appropriate bytes to the end of your plaintext. Then encrypt using NoPadding so you don't get extra padding added. Decrypt using NoPadding again, and then remove the padding yourself.

The padding scheme is very simple to implement:

add an 0x80 byte
while not at block boundary
  add an 0x00 byte
endwhile

I had a quick look at the Bouncy Castle library, and that does not appear to have ISO 9797 either.

Upvotes: 4

Maarten Bodewes
Maarten Bodewes

Reputation: 94038

Please see the following code copied from Bouncy Castle. It is mainly used within the Bouncy Castle provider for DES MAC algorithms, but you can simply use it within the lightweight crypto library of Bouncy Castle or use it as a base for your own implementation:

package org.bouncycastle.crypto.paddings;

import java.security.SecureRandom;

import org.bouncycastle.crypto.InvalidCipherTextException;

/**
 * A padder that adds the padding according to the scheme referenced in
 * ISO 7814-4 - scheme 2 from ISO 9797-1. The first byte is 0x80, rest is 0x00
 */
public class ISO7816d4Padding
    implements BlockCipherPadding
{
    /**
     * Initialise the padder.
     *
     * @param random - a SecureRandom if available.
     */
    public void init(SecureRandom random)
        throws IllegalArgumentException
    {
        // nothing to do.
    }

    /**
     * Return the name of the algorithm the padder implements.
     *
     * @return the name of the algorithm the padder implements.
     */
    public String getPaddingName()
    {
        return "ISO7816-4";
    }

    /**
     * add the pad bytes to the passed in block, returning the
     * number of bytes added.
     */
    public int addPadding(
        byte[]  in,
        int     inOff)
    {
        int added = (in.length - inOff);

        in [inOff]= (byte) 0x80;
        inOff ++;

        while (inOff < in.length)
        {
            in[inOff] = (byte) 0;
            inOff++;
        }

        return added;
    }

    /**
     * return the number of pad bytes present in the block.
     */
    public int padCount(byte[] in)
        throws InvalidCipherTextException
    {
        int count = in.length - 1;

        while (count > 0 && in[count] == 0)
        {
            count--;
        }

        if (in[count] != (byte)0x80)
        {
            throw new InvalidCipherTextException("pad block corrupted");
        }

        return in.length - count;
    }
}

Upvotes: 1

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