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AES Side Channel Attacks

AES Side Channel Attacks. Biru Cui Sam Skalicky. Outline. AES algorithm Side channel attacks Side channel attack against AES Cache-collision timing attack against AES Countermeasures. AES Algorithm. Key Expansion Initial Round Add Round Key – bitwise xor Rounds Sub Bytes - Sbox

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AES Side Channel Attacks

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  1. AES Side Channel Attacks Biru Cui Sam Skalicky

  2. Outline • AES algorithm • Side channel attacks • Side channel attack against AES • Cache-collision timing attack against AES • Countermeasures

  3. AES Algorithm • Key Expansion • Initial Round • Add Round Key – bitwise xor • Rounds • Sub Bytes - Sbox • Shift Rows – rows shifted cyclically • Mix Columns – mixing operation on the columns • AddRoundKey • Final Round (no Mix Columns) • Sub Bytes • Shift Rows • Add Round Key

  4. Rijndel Starting Data

  5. Rijndel AES Steps

  6. Rijndel Sub Bytes

  7. Rijndel Shift Rows

  8. Rijndel Mix Columns

  9. Rijndel Add Round Key

  10. AES Algorithm • AES Lookup Table Optimizations • Transposed State by Bertoni • Speedup in decryption • CAM based by Li • Combined Sbox& inv Sbox into single table • FPGA implementations • Pre-computed GF ops in LUTs

  11. Attacks on AES • Brute force • Related Key • Side Channel

  12. Side Channel Attacks • Attacks through some implementation deficiency • Timing of computations • Power Analysis • Fault Injection • Electromagnetic Radiation • Acoustic Cryptanalysis • Cache

  13. Cache-collision timing attack against AES • Cache collision • Hit • Miss • Time

  14. Process Operation • Cache observation CFS - Scheduler Victim Process Spy Process Cache

  15. AES Cache Side Channel Attack • AES-128 • Key recovery after observing ~100 encryptions • Implementation in Linux against OpenSSL 0.9.8n • Program does not require special privileges on the host machine • Linux kernel task scheduler compromised • Observe every memory access • (CFG) Completely Fair Scheduler

  16. AES Cache Attack Features • No heuristic info about plain/cyphertexts • Works against compressed tables • 2 phase operation: • Observation • ~100 encryptions • ~2-3 seconds • Analysis • ~3 minutes

  17. Process Operation • Cache observation CFS - Scheduler Victim Process Spy Process Cache

  18. Cache-collision timing attack against AES • AES: operations on each byte

  19. Cache-collision timing attack against AES • System information • Pentium III 1.0 GHz • L1 cache 32K (split data/instr.) • L2 cache 256K • “T” lookup table size 256x256=64k • Implication • If the table is fully loaded in the cache, then there is no cache miss. This is important for why we can do first round and final round attack.

  20. Cache-collision timing attack against AES • AES: the computation of every round

  21. Actual Results, Pentium III

  22. Cache-collision timing attack against AES Plaintext Key xor Table Key xor Table … Table Key xor [6]

  23. Cache-collision timing attack against AES Plaintext If a plaintext byte is known, as well as a first-round table lookup, a key byte is learned Key xor Table Key xor Table … Table Key xor [6]

  24. Cache-collision timing attack against AES • First Round Attack • Spy process flush the cache • The lookup table is not in the cache. In other words, the cache collision is only due to same lookup table access index.

  25. Cache-collision timing attack against AES • First Round Attack

  26. Cache-collision timing attack against AES • First Round Attack • If cache hits ( access time less than average access time) • Counts the average encryption time for all and pair. If there is a low average time occurs for a pair and , there is high probability that .

  27. Cache-collision timing attack against AES • Final Round Attack • The final round lookup table is different from previous lookup table , so there is no in the cache. And if there is a collision, it’s due to same lookup table index.

  28. Cache-collision timing attack against AES • Final Round Attack • No MixColumns operations

  29. Cache-collision timing attack against AES • Final Round Attack

  30. Cache-collision timing attack against AES • Final Round Attack • If cache hits ( access time less than average access time) • Counts the average encryption time for all and pair. If there is a low average time occurs for a pair and , there is high probability that .

  31. Cache-collision timing attack against AES • Result

  32. Countermeasures • AES can be performed without using lookup tables • Give OS ability to partition cache between processes • Put AES table into ROM, add special instructions • Separate AES hardware on chip (new Intel CPUs)

  33. References • [1] Rijndel flash movie: http://www.cs.bc.edu/~straubin/cs381-05/blockciphers/rijndael_ingles2004.swf • [2] G. Bertoni, et al.,"Efficient Software Implementation of AES on 32-Bit Platforms” • [3] H. Li, "A New CAM Based S/S−1-Box Look-up Table in AES” • [4] M. McLoone et al. "Rijndael FPGA Implementations Utilising Look-Up Tables” • [5] D. Gullasch et al. "Cache Games – Bringing Access-Based Cache Attacks on AES to Practice“ • [6] J. Bonneau et al. “Cache-Collision Timing Attacks Against AES” • [7] Dag Arne Osvik et al. “Cache Attacks and Countermeasures: the Case of AES”

  34. Backup slides

  35. Original Mix Columns Equations

  36. Revised Mix Columns Equations

  37. FPGA LUT Implementation

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