How to set MAC in bitmap but value in field

[Bjarne Henriksen]

Hi

I need to calculate the mac for a message.
But I'm struckling a bit as the bitmap has to include mac field but no value should be set on the field.

How do I do that?

Best regards
Bjarne

[Alejandro Revilla]

In the implementations I’ve been involved, I solved the problem by setting the MAC fields with a dummy place holder, and then adjusting it in an ISOFilter.

i.e. the filter would use code like this:

    public ISOMsg filter (ISOChannel channel, ISOMsg m, LogEvent evt)
        throws VetoException
    {
        try {
            m.set (128, sm.generateEDE_MAC (generateMockImage(channel, m), tak));
        } catch (ISOException e) {
            evt.addMessage (e);
            throw new VetoException (e);
        }
        return m;
    }

    private byte[] generateMockImage (ISOChannel channel, ISOMsg m)
        throws ISOException
    {
        byte[] msg = m.pack();

        int offset = 4 + (m.getMaxField() > 64 ? 16 : 8);

        ByteBuffer buf = ByteBuffer.allocate (msg.length - offset);
        buf.put (msg, offset, msg.length - offset);

        return buf.array();

    }

Hope this helps.

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@apr

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[Bjarne Henriksen]

Alejandro,

thanks for your quick reply:)

The way I read your code your mock-method returns method without mac and without bitmap.
I need the message without the mac but with the bitmap and the mac-bit set.

But your solution made me think a little, and I think I can do it by packing the message and copy the byte array excluding the last 4 (for this implementation) bytes:)


/Bjarne

[Alejandro Revilla]

But your solution made me think a little, and I think I can do it by packing the message and copy the byte array excluding the last 4 (for this implementation) bytes:)

Exactly. You set a dummy MAC field so that jPOS properly sets the bitmap, then you don’t take that last part into account when computing the MAC. Good luck!

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[chhil]

Just curious, why cant we set the mac field as an empty string, this would set the bitmap.

Calculate the MAC and then put it in that field.

-chhil

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[Alejandro Revilla]

Your approach could do if you trick the packager into using IF_NOP, but if we have a say 8-bytes binary field packager, I'm afraid the empty string approach wouldn't work.

But your idea is good Murtuza, perhaps we can change IFB_BINARY to accept a configuration parameter, or create a new binary field packager that could produce no output for a zero-length byte array.

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[chhil]

On Tue, Dec 10, 2013 at 9:49 AM, Alejandro Revilla <a...@jpos.org> wrote:

create a new binary field packager that could produce no output for a zero-length byte array.

I like this approach.

[luo...@peacesoft.net]

What about we change the BITMAP manually?

Ex: if the field is 64, which is in the last byte of bitmap, then change the 9th byte ( we count 2 bytes of MIT )

packedMsg[9] = (byte) (packedMsg[9] ^ 0x01); 

[Alejandro Revilla]

You can set a dummy value (i.e. 8 zeroes), compute MAC (excluding the last 8 bytes), and then properly set that field.

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