gocrypto

README

GoCrypto Extensions

I'm using this as a repo for crypto extensions I need in Go.
Right now, it has two focuses: the NXP LRP (leakage resistant primitive) specification, and a custom CRC calculator.

Information about LRP can be found here and here. LRP is primarily used in the NXP 424 DNA NFC chip.

LRP Usage

LRP uses a single key to seed multiple keys, so the function to create the cryptosystem is called NewStandardMultiCipher(k). This returns an LrpMultiCipher struct. You then choose a specific LrpCipher by index by calling the Cipher(idx) method.

This struct implements the cipher.Block interface, but also has the EncryptAll(msg, padding) function and the DecryptAll(msg, padding) functions for ease of use. I suggest that you pass in true for the padding unless there is a specific reason not to (some NXP functions specifically operate without padding).

LRP CMAC

NXP uses this cipher for as a drop-in for CMACs, but not in a way that you might expect. Instead of using the encryption mode for the CMAC, it uses a separate internal primitive to do this. Therefore, to get at the encryption mode that is used for generating MACs, use CipherForMAC(idx) instead of Cipher(idx). For generating a CMAC (which is often done with this protocol), there is a helper function CipherForMAC(idx).MAC() and CipherForMAC(idx).ShortMAC().

For NXP, Key 0 (CipherForMAC(0)) is usually used for generating CMACs, and Key 1 (Cipher(1)) is used for encryption/decryption.

LRP Example

package main

import (
	"encoding/hex"
	"fmt"
	"github.com/johnnyb/gocrypto/lrp"
)

func main() {
	message := "My Message"

	// Setup the system
	key := []byte{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f }
	mc := lrp.NewStandardMultiCipher(key)

	// Grab a cipher using Key 1
	c := mc.Cipher(1)

	// Encrypt/Decrypt
	encryptedMessage := c.EncryptAll([]byte(message), true)
	result := string(c.DecryptAll(encryptedMessage, true))

	fmt.Printf("Here is our original message: %s\n", result)

	// Grab a cipher for doing MACs using Key 0
	macCipher := mc.CipherForMAC(0)

	// Generate a MAC
	h, _ := cmac.NewWithTagSize(macCipher, 16)
	h.Write([]byte(message))
	messageMac := h.Sum(nil)

	fmt.Printf("MAC for message: %s\n", hex.EncodeToString(messageMac))

	// Simpler version
	messageMac = macCipher.CMAC([]byte(message))

	// 8-byte MAC specified by NXP
	shortMac := macCipher.ShortCMAC([]byte(message))
}

Crc Example

package main

import ( "fmt" "github.com/johnnyb/gocrypto/crc" )

func main() { message := "My Message" messageBytes := []byte(message)

// Setup the CRC
mycrc := crc.NewCustomCrc(32, 0x104C11DB7, 0, 0, false, false)

// The long way
crcCalc := mycrc.NewCalculator()
crcCalc.Update(messageBytes, 0, len(messageBytes))
fmt.Printf("CRC (long way) is: %d\n", crcCalc.GetValue())

// The easy way
result := mycrc.PerformCrc(messageBytes)
fmt.Printf("CRC (easy way) is: %d\n", result)

}

## Limitations

This library is currently not very optimized.  Most of the test cases, however, 
were taken directly from the documentation, so it should work correctly.

## Additional Note

In the implementation of LRP, the counter has a specific size: 4 bytes.  
The LRP standard seems to indicate that the size of the counter for LRP 
is just based on how many bits you need.  In any case, if you set
CounterSize on the LrpCipher to 4, you will get the behavior in the chip,
and if you leave it you will get the behavior in the document.