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Exploiting Cache-Timing in AES: Attacks and Countermeasures

Exploiting Cache-Timing in AES: Attacks and Countermeasures. Ivo Pooters i.pooters@student.tue.nl March 17, 2008. Seminar Information Security Technology. Outline. Introduction About Cache AES Primer Cache-timing attacks Countermeasures Conclusion.

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Exploiting Cache-Timing in AES: Attacks and Countermeasures

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  1. Exploiting Cache-Timing in AES:Attacks and Countermeasures Ivo Pooters i.pooters@student.tue.nl March 17, 2008 Seminar Information Security Technology

  2. Outline • Introduction • About Cache • AES Primer • Cache-timing attacks • Countermeasures • Conclusion → Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion

  3. Side Channel Attacks  Cache-Timing Attacks • Timing Attack Based on the time taken by the device to execute particular operation. • Power Analysis Attack Based on analyzing the power consumptions of the device to execute particular operations. • Fault Attack Abnormal environmental conditions to generate malfunctions in the processor which provide additional access. → Side Channel Attacks → Cache-Timing Attacks → Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion comes your footer  Page 3

  4. Cache-Timing Attacks • Goal: Extract key information • The difference in access time for cache and main memory can reveal memory access patterns • Idea: Analyze time used for encrypting certain plaintexts to retrieve information of the secret key • No special equipment required! →Side Channel Attacks → Cache-Timing Attacks → Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 4

  5. What is Cache? Slow! Fast! Figure from [1] → What is cache? →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 5

  6. Advanced Encryption Standard • Symmetric cipher to replace DES • Three modes: AES-128, AES-192, AES-256 • 16-byte block size, 16-byte key, 16-byte intermediary states • Key expanded to 10 Round Keys → Advanced Encryption Standard → AES Algorithm → AES Memory Access →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 6

  7. AES Algorithm Figure from [3] →Advanced Encryption Standard → AES Algorithm → AES Memory Access →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 7

  8. AES Memory Access • Implementated as series of table lookups • 8 Tables precalculated; T0 , … , T3 and T0(10) , …, T3(10) • Each round r calculates intermediary state x(r+1) • State X(0) is simply p  k • Ki(r) is the i-th 4-byte word of the expanded round key →Advanced Encryption Standard → AES Algorithm → AES Memory Access →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 8

  9. Known Attacks • D.J. Bernstein describes a synchronous attack in [4] • Osvik et al describe a more general approach for synchronous attacks ([2]) • Applicable to existing systems, e.g. dm-crypt • Manipulate the cache to influence delays • Asynchronous attacks ([2]) • No interaction required with the encryption algorithm • Use own program to manipulate cache and analyze the timings → Known Attacks → The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 9

  10. The Bernstein Attack • Described by D.J. Bernstein in [4] on OpenSSL AES Implementation • Synchronous attack: attacker can trigger encryption with known plaintext. • Simple server setup: • Server started with secret key • Server Reads a UDP packet from network. UDP packet have variable length but start with 16-byte nonce • Server copies high precision timestamp and nonce to response • Server encrypts the packet content • Server sends the response: 2 x timestamp, scrambled zero and nonce → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 10

  11. Attack Summary • Special case for r=0 • Consider T0[x0(0)] = T0[k0 p0] • Timing for lookup depends on value of k0 p0 → AES Timing leaks information on k0 • This is true for any ki pi , for i = 0,…,15 → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 11

  12. Attack Summary cont’d • Assume the attacker • Watches the total time taken by victim to handle many p’s • Totals the AES times for each possible p13 • Observes the total time is maximum for p13 = 147 • Assume the attacker can experiment in the same environment with known k’s and finds that overall AES maximum when k13 p13 = 8. • Now, k13 = 8  147 → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 12

  13. The actual Attack, step 1 • Attacker runs server with known key: all zeroes • About 222 random 400-byte packets encrypted • Study the resulting timings for e.g. p13 : • Timing max at p13 = 8 • Since k13 = 0, Timing max when x13 (=k13 p13) = 8 • See next slide for results → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 13

  14. Results for p13 → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 14

  15. The actual Aattack, step 1 cont’d • For some key bytes, not all the bits are leaked from this attack run. • E.g. p5 results show stronger correlation between values of p5 • Timings for p5  {0,1,2,3,4,5,6,7} statistically indistinguishable. • This means timing analysis would leak k5  {0,1,2,3,4,5,6,7}, i.e. top 5 bits of k5 → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 15

  16. Results for p5 → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 16

  17. The actual Attack, step 2 • Now send packets to the victims server which uses a secret key • Step 1 gives values for xi = ki pi with max timing. • Step 2 gives values for pi with max timing. • Combining the results from step 1 with step 2 yields the leaked key-bits. → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 17

  18. The actual Attack, step 2 cont’d • The attacker repeats attack with various packet sizes to pinpoint the keys • Most likely not all key-bits are leaked, but enough for brute-force search • For the attack described by Bernstein, the brute force < 1 minute! → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 18

  19. Evaluation • Time in order of hours for AES-128 • More noise in measurement can be solved with more samples • Attacker should be able to trigger encryptions • To do experiments, attacker needs the exact same system as victim → Known Attacks→ The Bernstein Attack → Attack Summary → The actual Attack → Evaluation →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 19

  20. Countermeasures • Avoid memory access: use bit slice implementation or crude slow arithmetic and logical operations • Hide timing: worst-case constant time, slow. Every operation as slow as memory access • Static cache: disable cache-sharing and load all tables in cache → Countermeasures →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 20

  21. Conclusions • Input dependant table lookups make AES vulnerable to cache-timing attacks • Bernstein has found a feasible cache-timing attack. • Osvik et al describe describe even faster and more applicable attacks • Countermeasures exist, but hinder performance → Conclusions → References →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 21

  22. QUESTIONS ? →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 22

  23. References • [1] U. Drepper. Memory Part 2: CPU Caches. http://lwn.net/Articles/252125/ • [2] D. Osvik, A. Shamir, E. Tromer. Cache-attacks and Countermeasures: the Case of AES. November 2005 • [3] Specification for the Advanced Encryption Standard. November 2001 • [4] D.J. Bernstein. Cache-Timing Attacks on AES. April 2005 →Conclusions → References →Introduction → About Cache → AES Primer → Cache-Timing Attacks → Countermeasures → Conclusion Here comes your footer  Page 23

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