Remote Access Control

aes.h File Reference

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Detailed Description

Header file for AES public functions.

This file contains the function prototypes for the AES algorithm. The implementation is found in the aes.c file. It only support encryption, since decryption is not required for the transmitter in this application. All transmissions except when sending a Teach Message use the same cryptographic key. This motivates precalculation of the AES key schedule, which speeds up the encryption process considerably. As this application is designed for ATtiny45, there is not enough SRAM to hold the whole key schedule if the larger AES key sizes are used (192 and 256 bits). Parts of the key schedule is therefore stored in on-chip EEPROM memory. Refer to the config.h file for configuration options.

• File: aes.h
• Compiler: IAR EWAVR 4.11B
• Supported devices: ATtiny45/85
• AppNote: AVR411 - Secure Rolling Code Algorithm for Wireless Link

Author:

Atmel Corporation: http://www.atmel.com
Support email: avr@atmel.com

Name

Revision

1193

Date

2006-10-31 14:21:08 +0100 (ti, 31 okt 2006)

Copyright (c) 2006, Atmel Corporation All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. The name of ATMEL may not be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY ATMEL ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE EXPRESSLY AND SPECIFICALLY DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Definition in file aes.h.

#include "common.h"
#include "config.h"

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Functions
void	calcKeySchedule (const byte __eeprom *key)
void	cipherLookup (byte *block)

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Function Documentation

void calcKeySchedule

(

const byte __eeprom *

key

)

Precalculate AES key schedule from given cipher key.

Definition at line 332 of file aes.c.

References addConstant(), addConstantFromEEPROM(), BLOCK_SIZE, BPOLY, copyBytes(), copyBytesFromEEPROM(), copyBytesToEEPROM(), cycleLeft(), KEY_SIZE, keyScheduleInEEPROM, keyScheduleInSRAM, SCHEDULE_SIZE, SCHEDULE_SPLIT, and subBytes().

Referenced by main(), and transmitTeachMessage().

{
        byte schedulePos; // Current position inside schedule.
        byte temp[4]; // Temporary word when expanding the key.
        byte roundConstant[4] = { 0x01, 0x00, 0x00, 0x00 };
        byte * keySchedule1 = keyScheduleInSRAM;
#if SCHEDULE_EXTRA > 0
        byte __eeprom * keySchedule2 = keyScheduleInEEPROM;
#endif

        // Copy entire key to start of schedule.
        copyBytesFromEEPROM( keySchedule1, key, KEY_SIZE );
        keySchedule1 += KEY_SIZE;
        // Copy last 4 bytes of key to temp word.
        copyBytes( temp, keySchedule1-4, 4 );

        // Expand key into schedule buffer 1 first.
        schedulePos = KEY_SIZE;
        while( schedulePos < SCHEDULE_SIZE ) {
                // Multiple of key size?
                if( (schedulePos % KEY_SIZE) == 0 ) {
                        cycleLeft( temp ); // Cycle left one byte.
                        subBytes( temp, 4 ); // Substitute each byte.
                        addConstant( temp, roundConstant, 4 ); // Add to temp.
                
                        // Modular doubling of round constant's first byte.
                        // This operation is done the following way to ensure cycle count
                        // independent from data contents. Take care when changing this code.
                        xor = 0;
                        if (roundConstant[0] & 0x80) {
                                xor = BPOLY;
                        }
                        roundConstant[0] <<= 1;
                        roundConstant[0]  ^= xor;
                }

#if KEY_SIZE > 24
                // Multiple of key size + block size, ie. block size into key.
                else if( (schedulePos % KEY_SIZE) == BLOCK_SIZE ) {
                        subBytes( temp, 4 ); // Substitute each byte.
                }
#endif

#if SCHEDULE_EXTRA > 0
                // Select correct source buffer for addition temp word.
                if( schedulePos <= SCHEDULE_SPLIT+KEY_SIZE-4 ) {
#endif
                        // Add with data KEY_SIZE backwards in schedule.
                        addConstant( temp, keySchedule1 - KEY_SIZE, 4 );

#if SCHEDULE_EXTRA > 0
                        // Copy temp word to currect destination buffer.
                        if( schedulePos <= SCHEDULE_SPLIT-4 ) {
#endif
                                copyBytes( keySchedule1, temp, 4 );
                                keySchedule1 += 4;
#if SCHEDULE_EXTRA > 0
                        } else {
                                copyBytesToEEPROM( keySchedule2, temp, 4 );
                                keySchedule1 += 4; // We need buffer 1 also.
                                keySchedule2 += 4;
                        }
                } else {
                        // Add with data KEY_SIZE backwards in schedule.
                        addConstantFromEEPROM( temp,
                                        keySchedule2 - KEY_SIZE, 4 );
                        copyBytesToEEPROM( keySchedule2, temp, 4 );
                        keySchedule2 += 4;
                }
#endif
                schedulePos += 4;
        }
}

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void cipherLookup

(

byte *

block

)

Use precalculated key schedule to encrypt the given data block.

Definition at line 288 of file aes.c.

References addConstant(), addConstantAndSubstitute(), addConstantFromEEPROM(), addConstantFromEEPROMAndSubstitute(), BLOCK_SIZE, keyScheduleInEEPROM, keyScheduleInSRAM, mixColumns(), ROUNDS, SCHEDULE_SPLIT_BLOCKS, and shiftRows().

Referenced by calcCMAC(), calcCMACSubkey(), and transmitTeachMessage().

{
        byte * keySchedule1 = keyScheduleInSRAM;
        byte round = 0;

#if SCHEDULE_EXTRA > 0
        byte __eeprom * keySchedule2 = keyScheduleInEEPROM;

        for( ; round < SCHEDULE_SPLIT_BLOCKS; ++round ) {
                addConstantAndSubstitute( block, keySchedule1, BLOCK_SIZE );
                shiftRows( block );
                mixColumns( block );
                keySchedule1 += BLOCK_SIZE;
        }

        for( ; round < ROUNDS-1; ++round ) {
                addConstantFromEEPROMAndSubstitute( block,
                                keySchedule2, BLOCK_SIZE );
                shiftRows( block );
                mixColumns( block );
                keySchedule2 += BLOCK_SIZE;
        }

        addConstantFromEEPROMAndSubstitute( block, keySchedule2, BLOCK_SIZE );
        shiftRows( block );
        keySchedule2 += BLOCK_SIZE;
        addConstantFromEEPROM( block, keySchedule2, BLOCK_SIZE );
#else
        for( ; round < ROUNDS-1; ++round ) {
                addConstantAndSubstitute( block, keySchedule1, BLOCK_SIZE );
                shiftRows( block );
                mixColumns( block );
                keySchedule1 += BLOCK_SIZE;
        }

        addConstantAndSubstitute( block, keySchedule1, BLOCK_SIZE );
        shiftRows( block );
        keySchedule1 += BLOCK_SIZE;
        addConstant( block, keySchedule1, BLOCK_SIZE );
#endif
}

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