SHORT Cmac calculation

[xam8re]

HI.
I set all keys as default as this:
`
public KeySet getKeySet() {
KeySet keySet = new KeySet();
keySet.setUsesLrp(false);

    // This is the "master" key
    KeyInfo key0 = new KeyInfo();
    key0.diversifyKeys = false;
    key0.key = Ntag424.FACTORY_KEY;
    keySet.setKey(Permissions.ACCESS_KEY0, key0);

    // No standard usage
    KeyInfo key1 = new KeyInfo();
    key1.diversifyKeys = false;
    key1.key = Ntag424.FACTORY_KEY;
    keySet.setKey(Permissions.ACCESS_KEY1, key1);

    // Usually used as a meta read key for encrypted PICC data
    KeyInfo key2 = new KeyInfo();
    key2.diversifyKeys = false;
    key2.key = Ntag424.FACTORY_KEY;
    keySet.setKey(Permissions.ACCESS_KEY2, key2);

    // Usually used as the MAC and encryption key.
    // The MAC key usually has the diversification information setup.
    // KeyInfo key3 = new KeyInfo();
    // key3.diversifyKeys = false;
    // key3.systemIdentifier = "testing".getBytes(StandardCharsets.UTF_8); // systemIdentifier is usually a hex-encoded string based on the name of your intended use.
    // key3.version = 1; // Since it is not a factory key (it is *based* on a factory key, but underwent diversification), need to set to a version number other than 0.
    // key3.key = Ntag424.FACTORY_KEY;

    KeyInfo key3 = new KeyInfo();
    key3.diversifyKeys = false;
    key3.key = Ntag424.FACTORY_KEY;
    keySet.setKey(Permissions.ACCESS_KEY3, key3);

    // No standard usage
    // keySet.setKey(Permissions.ACCESS_KEY3, key3);
    KeyInfo key4 = new KeyInfo();
    key4.diversifyKeys = false;
    key4.key = Ntag424.FACTORY_KEY;
    keySet.setKey(Permissions.ACCESS_KEY4, key4);

    // This is used for decoding, but documenting that key2/key3 are standard for meta and mac
    keySet.setMetaKey(Permissions.ACCESS_KEY2);
    keySet.setMacFileKey(Permissions.ACCESS_KEY3);

    return keySet;
}

`

Link created is :
https://sdm.nfcdeveloper.com/tagpt?uid=04176AC20A1090&ctr=00006B&cmac=788E828B11B3627E

testing the cmac:
byte[] key = Ntag424.FACTORY_KEY; byte[] uid = ByteUtil.hexToByte("04176AC20A1090"); ctr = 0x6B; PiccData picc = new PiccData(uid, ctr, false); picc.setMacFileKey(key); byte[] calc = picc.performShortCMAC(key); // MAC on PICC-only data

give me a different cmac. Where I'm wrong?

[AndroidCrypto]

Hi Massimiliano, it looks like from your created link that you are working with Plaintext SDM including tag UID and read counter. I created a tutorial and setup a complete example app that is exactly working with this library (ntag424-java). You find the complete tutorial series on medium.com:

Part 1: https://medium.com/@androidcrypto/demystify-the-secure-dynamic-message-with-ntag-424-dna-nfc-tags-android-java-part-1-b947c482913c

Part 2: https://medium.com/@androidcrypto/demystify-the-secure-dynamic-message-with-ntag-424-dna-nfc-tags-android-java-part-2-1f8878faa928

The tutorial is here on GitHub: https://github.com/AndroidCrypto/Ntag424SdmFeature

The Plaintext example is located in this file: https://github.com/AndroidCrypto/Ntag424SdmFeature/blob/master/app/src/main/java/de/androidcrypto/ntag424sdmfeature/PlaintextSunActivity.java

There you find exactly what data needs to be included to get the CMAC matching. If you prefer to run the "server" part (=reader) you should have a close look to https://github.com/AndroidCrypto/Ntag424SdmFeature/blob/master/app/src/main/java/de/androidcrypto/ntag424sdmfeature/NdefReaderActivity.java - here the complete reading, decryption and CMAC verification is running.

Good luck,
Michael