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AVR230: DES BootloaderFeatures• Fits All AVR Microcontrollers with Bootloader Capabilities• Enables Secure Transfer of Compiled Software or Sensitive

10AVR230 App Note2541D–AVR–04/05PC Application – GenTempThis application note includes a small PC application, which will generate a template forthe c

11AVR230 App Note2541D–AVR–04/05Command Line Arguments The application accepts the following command line arguments.First Run In the first run, typica

12AVR230 App Note2541D–AVR–04/05Figure 6. Program Flow of PC ApplicationCommandLine Args?OTHERNONECreateRead Configuration FileGive InstructionsHow M

13AVR230 App Note2541D–AVR–04/05The Encrypted File The Flash and EEPROM file are encrypted and stored in one target file. Before encryp-tion, however,

14AVR230 App Note2541D–AVR–04/05Figure 8. Creating the Encrypted FileThe steps are described below (numbers refer to Figure 8).1. Data is formatted i

15AVR230 App Note2541D–AVR–04/05Figure 9. Program Flow of AVR BootloaderSwitch SW7Pressed?YESNOBootldrLoad Initial Vector for CBCCalculate CRC of App

16AVR230 App Note2541D–AVR–04/05Key and Header Files Before the bootloader can be compiled, there are some parameters that need to be setup. To start

17AVR230 App Note2541D–AVR–04/05The following table summarizes some of the compiler options for currently supportedAVR devices. Please note that bootl

18AVR230 App Note2541D–AVR–04/05• Oscillator options. The oscillator fuse bits are device dependent. They may require configuration (affects USART).No

19AVR230 App Note2541D–AVR–04/05Figure 10. Program Flow of Update ApplicationThe Update application reads in files generated with the Create applicat

2AVR230 App Note2541D–AVR–04/05This application note shows how data to be transferred to Flash and EEPROM memo-ries can be secured at all times by usi

20AVR230 App Note2541D–AVR–04/05The update application is run from the command prompt. The command prompt argu-ments are listed in Table 11.It should

21AVR230 App Note2541D–AVR–04/05For single-key DES, the overall time can be estimated using the following equation.…where FS is the filesize in bytes,

22AVR230 App Note2541D–AVR–04/05Summary This application note has presented a method for transferring data securely to an AVRmicrocontroller with boot

23AVR230 App Note2541D–AVR–04/05References [1] Douglas Stinson:Cryptography: Theory and Practice,CRC Press, second edition, 1996[2] Electronic Frontie

Printed on recycled paper.2541D–AVR–04/05Disclaimer: The information in this document is provided in connection with Atmel products. No license, expr

3AVR230 App Note2541D–AVR–04/05application and programming it into Flash memory, while keys are required fordecrypting the incoming data. Lock bits ar

4AVR230 App Note2541D–AVR–04/05The DES algorithm uses a 56-bit encryption key, meaning that the number of possiblekey combinations is:DES is a block c

5AVR230 App Note2541D–AVR–04/05using a different subset (Kn) of the encryption key (only steps 1, 2 and 16 are illustratedin Figure 2). Finally, the b

6AVR230 App Note2541D–AVR–04/05Figure 4. The Decryption Flow According to the 3DES AlgorithmThe following keying options are defined in the standard.

7AVR230 App Note2541D–AVR–04/05attacks, the computing power required to break the key increases exponentially with thelength of the key. Since the ave

8AVR230 App Note2541D–AVR–04/05Are there any assumptions that can be made on what cleartext data will look like? Inorder for DES to be broken by brute

9AVR230 App Note2541D–AVR–04/05The procedure is discussed in more details below.Configuration File The configuration file contains a list of parameter