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Skipjack

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Designed by NSA 1985 1990. For use in Clipper/Capstone ... Escrow key split. Stored in two different locations. Algorithm Protection. Classified ... – PowerPoint PPT presentation

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Title: 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
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