Remote Access Control

cmac.c

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// This file has been prepared for Doxygen automatic documentation generation.
#include "cmac.h"
#include "aes.h"
#include "memory.h"
#include "common.h"
#include "config.h"



__no_init byte __eeprom CMACSubkey[ CMAC_SUBKEY_SIZE ];



void calcCMACSubkey( byte * cryptoBlock )
{
        byte blockSize;
        byte * blockPtr;
        byte overflow;

        // Fill crypto block with all zeros.
        blockSize = BLOCK_SIZE;
        blockPtr = cryptoBlock;
        do {
                *blockPtr++ = 0;
        } while( --blockSize );

        // Encrypt zeros with secret key (assuming
        // key schedule has been created with secret key)
        cipherLookup( cryptoBlock );

        // Multiply by 2 modulo (0b1^120 cat 0b10000111).
        blockSize = BLOCK_SIZE - 1;
        blockPtr = cryptoBlock;
        overflow = blockPtr[0] & 0x80;
        do {
                blockPtr[0] = (blockPtr[0] << 1) | (blockPtr[1] >> 7);
                ++blockPtr;
        } while( --blockSize );
        blockPtr[0] <<= 1;
        if( overflow ) {
                blockPtr[0] ^= 0x87;
        }

#if MESSAGE_SIZE_WO_MAC < BLOCK_SIZE
        // Multiply by 2 modulo (0b1^120 cat 0b10000111) again.
        blockSize = BLOCK_SIZE - 1;
        blockPtr = cryptoBlock;
        overflow = blockPtr[0] & 0x80;
        do {
                blockPtr[0] = (blockPtr[0] << 1) | (blockPtr[1] >> 7);
                ++blockPtr;
        } while( --blockSize );
        blockPtr[0] <<= 1;
        if( overflow ) {
                blockPtr[0] ^= 0x87;
        }
#endif

        copyBytesToEEPROM( CMACSubkey, cryptoBlock, BLOCK_SIZE );
}



void calcCMAC( const byte * message, byte * MACStorage )
{
        // Copy message data to MACStorage, which should be BLOCK_SIZE.
        copyBytes( MACStorage, message, MESSAGE_SIZE_WO_MAC );

#if MESSAGE_SIZE_WO_MAC < BLOCK_SIZE
        // Pad message before calculating CMAC.
  #if MESSAGE_SIZE_WO_MAC + 1 < BLOCK_SIZE
        byte pos = MESSAGE_SIZE_WO_MAC + 1;
        do {
                MACStorage[ pos ] = 0x00;
        } while( ++pos < BLOCK_SIZE );
  #endif
        MACStorage[ MESSAGE_SIZE_WO_MAC ] = 0x80;
#endif

        // Add (XOR) subkey.
        addConstantFromEEPROM( MACStorage, CMACSubkey, BLOCK_SIZE );

        // Encrypt, assuming secret key schedule is precalculated.
        cipherLookup( MACStorage );
}