Symmetric Key Cryptography

1. Symmetric Key Cryptography Secure communications Secure storage Computationally efficient Depends on a shared secret

2. Symmetric Key Cryptography Alice and Bob want to communicate securely. • Alice & Bob agree on a crypto algorithm • Alice & Bob agree on a key • Alice encrypts message with the key • Alice sends ciphertext to Bob • Bob decrypts with the key and reads the message

3. Symmetric Key Cryptography C Alice Bob Key: K Message: M Ciphertext: C = EK(M) Key: K Ciphertext: C Message: M = DK(C) = DK(EK(M))

4. Data Encryption StandardDES • Started life as Lucifer from IBM – Early 70’s • NBS (NIST) 5/15/73 RFP for an Encryption Algorithm • IBM Responded with a Modified Lucifer • NBS Published DES with IBM’s grant of a nonexclusive royalty free license to use

5. Caveats • NSA reviewed the algorithm • NSA changed the S-Boxes slightly • PARANOIA • NSA inserted a back door • NSA made it easier to crack

6. Realities • NIST made DES a FIPS standard 11/23/76 • NSA stated that DES was one of their biggest mistakes. • No one has found any back door for the algorithm • Status of DES was reviewed every 5 years • Supposed to change every 10 years

7. DES • 64 bit block symmetric key encryption • 56 bit key • Lots of weak keys • Modified up to a 112 bit key – Triple DES • Served most commerce applications for more than 30 years • All ATM transactions • Bank transfers • Credit card applications

8. DES Key • Originally 64 bits. • In the 70’s comms was noisy • Every 8th bit of the key was a checksum of the previous 7 bits • Effective key length is 56 bits • Key space is 256 • 64+ weak keys

9. AES • Advanced Encryption Standard replaced DES in 2003 • Significant improvement • 126, 198, 256 bit key • No weak keys

10. Data Encryption StandardOverview of one block of plaintext Starts with an initial permutation. DES consists of 16 rounds. Each round uses a different key derived from the original key. Ends with the inverse of the initial Permutation. Based On: Applied Cryptography, Bruce Schneier, Wiley, 1996

11. Initial Permutation Bit 58 goes to bit 1 Bit 50 goes to bit 2 Bit 42 to bit 3 etc.

12. One Round

13. Expansion Permutation32 bits to 48 bits Bit 32 goes to bit 1 Bit 1 goes to bit 2

14. We Need a Key

15. Key The DES key is 64 bits. However every eighth Bit is a parity bit. Hence there are only 56 relevant bits. From these 56 bits 16 48 bit keys are generated.

16. Key Generation Original key is split in half. Each half is shifted and passed On to the next round. The two halves are merged to 56 bits and then Compressed to 48 bits.

17. Disperse the Data over the Key Data Key 0 1 a Åa = 0 (a Åb)Å a = b 0 0 1 1 1 0

18. Back Down to 32 Bits

19. S – Box Substitution • The S-Boxes are magic • How they were developed is unknown • This is what NSA tweaked 48 bits are divided into 8 groups of 6 sequential bits. Each group of 6 bits generates 4 bits. Using the 8 tables in the following slide the resultant 4 bits are determined.

20. Each S-Box is a 4 x 16 matrix. The output of the substitution is the matrix entry. The input row is determined by bit 1 | bit 6. The input column is determined by bit 2 – bit 5. Example: Input (6 bits) Row address 102 = 210 Column address 01102 = 610 Output for S-Box 6 = 12 1 0 1 1 0 0

21. One Permutation

22. P-Box Permutation

23. Disperse The Left into The Right

24. Close to the End

25. Final Permutation=(Initial Permutation)-1

26. DES Cracks • 1999 DES 56 bit key cracked in 23 hours • 1998 “Cracking DES” EFF • 1K$ worth of FPGAs and crack DES in a week

27. $250,000 Custom Board56 Hours, 1998

28. $1,500 FPGADES Crack < 24 Hours