Scan Based Attack on Dedicated Hardware Implementation of Data Encryption Standard Bo Yang ECE Dept Polytechnic Univ Kaijie Wu ECE Dept Univ of Illinois Chicago Ramesh Karri ECE Dept Polytechnic Univ cad.poly.edu/encryption Why is Scan a Bad Design For Test Methodology?

Scan DFT is extremely popular Scan DFT is extensively deployed Scan DFT is extensively deployed 82% of all ICs use Scan DFT for testing 82% of all ICs use Scan DFT for testing Scan DFT is widely supported Scan DFT is widely supported Fast Scan and TestKcompress: Mentor Graphics Fast Scan and TestKcompress: Mentor Graphics DFT compiler and TetraMAX ATPG: Synopsys DFT compiler and TetraMAX ATPG: Synopsys Encounter Test: Cadence Encounter Test: Cadence

Objective Show how secrets on a crypto chip can be compromised Show how secrets on a crypto chip can be compromised Demonstrate that scan is a terrible design-for-test methodology Demonstrate that scan is a terrible design-for-test methodology

Data Encryption Standard DES is a symmetric encryption algorithm DES is a symmetric encryption algorithm encryption key = decryption key encryption key = decryption key Decryption = Encryption -1 Decryption = Encryption -1 ENCRYPT (plaintext, bit key) = ciphertext ENCRYPT (plaintext, bit key) = ciphertext DECRYPT (ciphertext, bit key) = plaintext DECRYPT (ciphertext, bit key) = plaintext 64-bit plaintext, 64-bit ciphertext, 56-bit secret key 64-bit plaintext, 64-bit ciphertext, 56-bit secret key

DES Encryption Initial Permutation Plaintext Round Function R L 48-bit Round Key Inverse Permutation Ciphertext 16 identical rounds 16 identical rounds one 48-bit round key per round one 48-bit round key per round bit round keys are generated from 56-bit secret bit round keys are generated from 56-bit secret 32 64

One DES Round LiLi RiRi Round Key K i + L i+1 R i+1 r Expansion S-box 1S-box Permutation 32 a b c d

DES Hardware Architecture Cipher Block Chaining mode  Iterative arch Cipher Block Chaining mode  Iterative arch Input, L, R, Output Regs ( FFs) Input, L, R, Output Regs ( FFs)

Mounting a scan attack Calculate X from W Calculate X from W Calculate Y from Z Calculate Y from Z Solve Key mixing Solve Key mixing

Two-step scan attack Step 1: Determine L and R registers in the scan chain Step 1: Determine L and R registers in the scan chain Step 2: Discover round key 1 from L 0, R 0, L 1 and R 1 Step 2: Discover round key 1 from L 0, R 0, L 1 and R 1

Scan Attack step 1 … IC Flip-flops of input register TDO Apply Plaintext 1:000000…  run in normal mode for 1 clock cycle  scan out bitstream 1: 01101… Apply Plaintext 2:100000…  run in normal mode for 1 clock cycle  scan out bitstream 2: 01101… Input, L, R and output registers can be determined Input, L, R and output registers can be determined 199 cycles to locate 1 FF  cycles to locate 1 FF  192× cycles to locate all FFs clock reset

How can we get K i ?  Round Key K i = a xor b  Expansion is a bijection  r  a is easy  Permutation is a bijection  d  c is easy  s-box is not a bijection  c  b is not easy RiRi Expansion Round Key, K i S-box 1S-box Permutation 32 r a b c d 48

Scan attack step 2 Address  s-box is not a bijection  c  b is not easy Every value appears 4 times in an s-box Every value appears 4 times in an s-box Every value appears only once in each row No s-box column has two or more identical values No s-box column has two or more identical values

Scan attack step 2 3 chosen plaintexts are enough to get a round key 3 chosen plaintexts are enough to get a round key apply a1=( ) 16 and observe c1 apply a2=( ) 16 and observe c2 apply a3=(4A1C ) 16 and observe c3 Derive round key K1 Derive round key K1 Several such 3-tuples exist !!! Several such 3-tuples exist !!! Round Key, K i S-box 1S-box a b c 48

Scan attack step 2 Apply three plaintexts Apply three plaintexts Apply PT1 = ( ) 16 Apply PT1 = ( ) 16 Scan-out CT1 from round register Scan-out CT1 from round register Apply PT2 = ( ) 16 Apply PT2 = ( ) 16 Scan-out CT2 from round register Scan-out CT2 from round register Apply PT3 = ( ) 16 Apply PT3 = ( ) 16 Scan-out CT3 from round register Scan-out CT3 from round register Derive round key K1 Derive round key K1 LiLi RiRi Round Key K i + L i+1 R i+1 r Expansion S-box 1S-box Permutation 32 a b c d

Discover round key Discover round key Discover round key K1  399×3=1197 clock cycles 2 clock cycles in normal mode for plaintext to reach R0, L0 198 clock cycles in scan mode to scan out R0, L0 1 clock cycle in normal mode for plaintext to reach R1, L1 198 clock cycles in scan mode to scan out R1, L1

Discover user secret Discover user secret as follows: 48-out-of-56 secret bits from round key K1 7-out-of-remaining 8 secret bits from round key K2 Secret bits 17, 20, 23, 40, 41, 49, 50 Secret bit 46 from round key K3 1197×2 clock cycles to discover round keys K2 and K3

Summary of the attack Determine the positions of flip flops in the round register in the scan chain Determine the positions of flip flops in the round register in the scan chain Scan round 1 and round 2 results Scan round 1 and round 2 results Discover round keys K1, K2 and K3 Discover round keys K1, K2 and K3 Discover user secret from round keys Discover user secret from round keys

Concluding remarks Do not use Scan DFT in crypto chips! Do not use Scan DFT in crypto chips! FIPS “A cryptographic module shall employ physical security mechanisms in order to restrict unauthorized physical access to the contents of the module and to deter unauthorized use or modification of the module... (In 1994 at the peak of Scan DFT research) FIPS “A cryptographic module shall employ physical security mechanisms in order to restrict unauthorized physical access to the contents of the module and to deter unauthorized use or modification of the module... (In 1994 at the peak of Scan DFT research) Translation: “Do not use scan DFT” Translation: “Do not use scan DFT” Why should you ? Why should you ?

Beware of Scan DFT Crypto chips are an excellent case study to show how bad scan DFT is. Crypto chips are an excellent case study to show how bad scan DFT is. Your IC may be used in secure applications in the future. Beware of the security issues when you design ICs. Your IC may be used in secure applications in the future. Beware of the security issues when you design ICs.

Scan Attack: Assumptions The attacker can access scan chains The attacker can access scan chains Round key registers are not in the scan chain Round key registers are not in the scan chain The attacker knows the algorithm The attacker knows the algorithm The attacker need not have access to high level timing diagrams The attacker need not have access to high level timing diagrams Avalanche effect (when does encryption begin and how long does it take?) Avalanche effect (when does encryption begin and how long does it take?) Modes of operation (CBC) Modes of operation (CBC)