Skipjack

1
Skipjack

• Skipjack and KEA Algorithm
• Presentation by Matthew S. Ford

2
Where did it come from?

• Designed by NSA 1985 1990
• For use in Clipper/Capstone
• Specification released by NIST in 1998

3
LEAF

• Law Enforcement Access Field
• Chip Identifier
• Escrow key split
• Stored in two different locations

4
Algorithm Protection

• Classified
• Secret patent application

5
Modes of Operation
6
Skipjack properties

• 64 bit blocks
• 80 bit key
• 32 rounds

7
Encrypting a Block (ECB)

• State represented by wi, 1 lt i lt 4
• wi 2 byte words
• wi starts off as plaintext block
• Apply RuleA 8 times
• Apply RuleB 8 times
• Apply RuleA 8 times
• Apply RuleB 8 times

8
Decrypting a Block (ECB)

• Wi starts off as cyphertext block
• Apply RuleBInv 8 times
• Apply RuleAInv 8 times
• Apply RuleBInv 8 times
• Apply RuleAInv 8 times

9
Stepping Rules
10
G-permutation
11
F table
12
G example
0x00
0x00
0x00
0x00
0xA3
0x00
0xA3
0x2C
0xA3
0x00
0x2C
0xE9
0x2C
0x00
0x4A
0x1A
0x4A
0x36
0x36
0x4A
13
Cryptanalysis

• 31 round variant can be broken better than brute
  force
• Boomerang attack
• Strength of the cipher seems to closely
  correspond with size of keyspace.
• Secure but not conservatively so
• No known weak keys including all zeros and all
  ones

14
Conclusion

• Appears to be secure enough for 80 bit key.
• 80 bit key size is enough to protect for today
  but is questionable for tomorrow.
• Design rational not known.
• Algorithm seems to match previous known details.
• No known weak keys