Changes

{{Warningbox|This tutorial is an add-on to [[Tutorial A5 Breaking AES-256 Bootloader]]has been updated for ChipWhisperer 5 release. It continues working on If you are using 4.x.x or 3.x.x see the same firmware, showing how to obtain the hidden IV and signature "V4" or "V3" link in the bootloader. '''It is not possible to do this bonus tutorial without first completing the regular tutorial''', so please finish Tutorial A5 firstsidebar.}}

''This {{Infobox tutorial is under construction! Check back in a few days.''|name = A5: Breaking AES-256 Bootloader|image = |caption = |software versions =|capture hardware = CW-Lite, CW-Lite 2-Part, CW-Pro|Target Device = |Target Architecture = XMEGA/Arm|Hardware Crypto = No|Purchase Hardware = }}

== Bootloader Source Code ==In this tutorial, we have the luxury of seeing the source code of the bootloader. This is generally not something we would have access to in the real world, so we* Jupyter file: 'll try not to use it to cheat''PA_Multi_1-Breaking_AES-256_Bootloader. (Peeking at <code>supersecret.h</code> counts as cheating.)ipynb'''

<pre>// Continue with decryptiontrigger_high(); aes256_decrypt_ecb(&ctx, tmp32);trigger_low(); // Apply IV (first 16 bytes)for (i = 0; i < 16; i++){ tmp32[i] ^= iv[i];}XMEGA Target ==

//Save IV See the following for next time from original ciphertext using:for * ChipWhisperer-Lite Classic (i = 0; i < 16; i+XMEGA)* ChipWhisperer-Lite Capture +XMEGA Target on UFO Board (including NAE-SCAPACK-L1/L2 users){ iv[i] = tmp32[i* ChipWhisperer-Pro +16];}XMEGA Target on UFO Board

https:// Tell the user that the CRC check was okayputch(COMM_OK);putch(COMM_OK);chipwhisperer.readthedocs.io/en/latest/tutorials/pa_multi_1-openadc-cwlitexmega.html#tutorial-pa-multi-1-openadc-cwlitexmega

//Check the signatureif ((tmp32[0] == SIGNATURE1) && (tmp32[1] ChipWhisperer-Lite ARM / STM32F3 Target == SIGNATURE2) && (tmp32[2] == SIGNATURE3) && (tmp32[3] == SIGNATURE4)){ // Delay to emulate a write to flash memory _delay_ms(1);} </pre>

This gives us a pretty good idea of how See the microcontroller is going to do its job. However, we can go one step further and find the exact assembly code that the target will execute. If you have Atmel Studio and its toolchain on your computer, you can get the assembly file from the command line withfollowing for using:<pre>* ChipWhisperer-Lite 32-bit (STM32F3 Target)avr* ChipWhisperer-objdump Lite Capture + STM32F3 Target on UFO Board (including NAE-m avr SCAPACK-D bootloader.hex > disassembly.txt<L1/pre>L2 users)This will convert the hex file into assembly code, making it more human* ChipWhisperer-readable. The important part of this assembly code is:<pre> 344: d3 01 movw r26, r6 346: 93 01 movw r18, r6 348: f6 01 movw r30, r12 34a: 80 81 ld r24, Z 34c: f9 01 movw r30, r18 34e: 91 91 ld r25, ZPro + 350: 9f 01 movw r18, r30 352: 89 27 eor r24, r25 354: f6 01 movw r30, r12 356: 81 93 st Z+, r24 358: 6f 01 movw r12, r30 35a: ee 15 cp r30, r14 35c: ff 05 cpc r31, r15 35e: a1 f7 brne .-24 ; 0x348 360: fe 01 movw r30, r28 362: b1 96 adiw r30, 0x21 ; 33 364: 81 91 ld r24, Z+ 366: 8d 93 st X+, r24 368: e4 15 cp r30, r4 36a: f5 05 cpc r31, r5 36c: d9 f7 brne .-10 ; 0x364STM32F3 Target on UFO Board

36ehttps: 84 ea ldi r24, 0xA4 ; 164 370: 0e 94 16 02 call 0x42c ; 0x42c 374: 84 ea ldi r24, 0xA4 ; 164 376: 0e 94 16 02 call 0x42c ; 0x42c//chipwhisperer.readthedocs.io/en/latest/tutorials/pa_multi_1-openadc-cwlitearm.html#tutorial-pa-multi-1-openadc-cwlitearm

382: 8a 89 ldd r24, Y+18 ; 0x12 384: 8b 3e cpi r24, 0xEB ; 235 386: 09 f0 breq .+2 ; 0x38a 388: 94 cf rjmp .-216 ; 0x2b2  38a: 8b 89 ldd r24, Y+19 ; 0x13 38c: 82 30 cpi r24, 0x02 ; 2 38e: 09 f0 breq .+2 ; 0x392 390: 90 cf rjmp .-224 ; 0x2b2  392: 8c 89 ldd r24, Y+20 ; 0x14 394: 8d 31 cpi r24, 0x1D ; 29 396: 09 f0 breq .+2 ; 0x39a 398: 8c cf rjmp .-232 ; 0x2b2  39a: 83 e3 ldi r24, 0x33 ; 51 39c: 97 e0 ldi r25, 0x07 ; 7 39e: 01 97 sbiw r24, 0x01 ; 1 3a0: f1 f7 brne .-4 ; 0x39e 3a2: 87 cf rjmp .-242 ; 0x2b2</pre>Let's examine this code in more detail. == The IV ==* Suggest some ideas== The Signature ==* Timing attack* Show firmware = Attacking the IV =Steps:* Investigation** Look at bootloader code** Move trigger** Record 1** Show different instructions in trace* Making the attack feasible** Capture a bunch (500?)** Apply decryption** Look at one bit** Find means + plot** Find differences + plot* Automating the attack** Finding the attack points** Getting a single bit** Building the IV bytes* Full script in appendix Example: <pre>#Imports for IV Attackfrom Crypto.Cipher import AES def initPreprocessing(self): self.preProcessingResyncSAD0 = preprocessing.ResyncSAD.ResyncSAD(self.parent) self.preProcessingResyncSAD0.setEnabled(True) self.preProcessingResyncSAD0.setReference(rtraceno=0, refpoints=(6300,6800), inputwindow=(6000,7200)) self.preProcessingResyncSAD1 = preprocessing.ResyncSAD.ResyncSAD(self.parent) self.preProcessingResyncSAD1.setEnabled(True) self.preProcessingResyncSAD1.setReference(rtraceno=0, refpoints=(4800,5100), inputwindow=(4700,5200)) self.preProcessingList = [self.preProcessingResyncSAD0,self.preProcessingResyncSAD1,] return self.preProcessingList class AESIVAttack(object): numSubKeys = 16  @staticmethod def leakage(textin, textout, guess, bnum, setting, state): knownkey = [0x94, 0x28, 0x5D, 0x4D, 0x6D, 0xCF, 0xEC, 0x08, 0xD8, 0xAC, 0xDD, 0xF6, 0xBE, 0x25, 0xA4, 0x99, 0xC4, 0xD9, 0xD0, 0x1E, 0xC3, 0x40, 0x7E, 0xD7, 0xD5, 0x28, 0xD4, 0x09, 0xE9, 0xF0, 0x88, 0xA1] knownkey = str(bytearray(knownkey)) ct = str(bytearray(textin))  aes = AES.new(knownkey, AES.MODE_ECB) pt = aes.decrypt(ct) return getHW(bytearray(pt)[bnum] ^ guess)</pre> = Appendix D AES-256 IV Attack Script = '''NB: This script works tutorial is not available for 0the ChipWhisperer Nano.10 release or later, see local copy in doc/html directory of chipwhisperer release if you need earlier versions''' Full attack script, copy/paste into a file then add as active attack script: <pre>#IV Attack Scriptfrom chipwhisperer.common.autoscript import AutoScriptBase#Imports from Preprocessingimport chipwhisperer.analyzer.preprocessing as preprocessing#Imports from Capturefrom chipwhisperer.analyzer.attacks.CPA import CPAfrom chipwhisperer.analyzer.attacks.CPAProgressive import CPAProgressiveimport chipwhisperer.analyzer.attacks.models.AES128_8bit# Imports from utilList # Imports for AES256 Attackfrom chipwhisperer.analyzer.attacks.models.AES128_8bit import getHW #Imports for IV Attackfrom Crypto.Cipher import AES class AESIVAttack(object): numSubKeys = 16  @staticmethod def leakage(textin, textout, guess, bnum, setting, state): knownkey = [0x94, 0x28, 0x5D, 0x4D, 0x6D, 0xCF, 0xEC, 0x08, 0xD8, 0xAC, 0xDD, 0xF6, 0xBE, 0x25, 0xA4, 0x99, 0xC4, 0xD9, 0xD0, 0x1E, 0xC3, 0x40, 0x7E, 0xD7, 0xD5, 0x28, 0xD4, 0x09, 0xE9, 0xF0, 0x88, 0xA1] knownkey = str(bytearray(knownkey)) ct = str(bytearray(textin))  aes = AES.new(knownkey, AES.MODE_ECB) pt = aes.decrypt(ct) return getHW(bytearray(pt)[bnum] ^ guess) class userScript(AutoScriptBase): preProcessingList = [] def initProject(self): pass  def initPreprocessing(self): self.preProcessingResyncSAD0 = preprocessing.ResyncSAD.ResyncSAD(self.parent) self.preProcessingResyncSAD0.setEnabled(True) self.preProcessingResyncSAD0.setReference(rtraceno=0, refpoints=(6300,6800), inputwindow=(6000,7200)) self.preProcessingResyncSAD1 = preprocessing.ResyncSAD.ResyncSAD(self.parent) self.preProcessingResyncSAD1.setEnabled(True) self.preProcessingResyncSAD1.setReference(rtraceno=0, refpoints=(4800,5100), inputwindow=(4700,5200)) self.preProcessingList = [self.preProcessingResyncSAD0,self.preProcessingResyncSAD1,] return self.preProcessingList  def initAnalysis(self): self.attack = CPA(self.parent, console=self.console, showScriptParameter=self.showScriptParameter) self.attack.setAnalysisAlgorithm(CPAProgressive, AESIVAttack, None) self.attack.setTraceStart(0) self.attack.setTracesPerAttack(100) self.attack.setIterations(1) self.attack.setReportingInterval(25) self.attack.setTargetBytes([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]) self.attack.setTraceManager(self.traceManager()) self.attack.setProject(self.project()) self.attack.setPointRange((4800,6500)) return self.attack  def initReporting(self, results): results.setAttack(self.attack) results.setTraceManager(self.traceManager()) self.results = results  def doAnalysis(self): self.attack.doAttack()</pre> = Attacking the Signature =