Upload
herbert-williams
View
213
Download
0
Embed Size (px)
Citation preview
CSC 386 – Computer Security
Scott Heggen
Agenda
• Cryptography (i.e., Confusion and Diffusion)
The Fundamental Flaw of Ciphers
Original Alphabet
Caesar Offset
(shift +2)
RandomSubstitution
…
A C L
B E K
C F M
D G Y
E H A
F I J
G J F
… … …
k =
Message index
Pseudo-Random
Offset
0 Shift +4
1 Shift -4
2 Shift +6
3 Shift +22
4 Shift +1
5 Shift -14
6 Shift -8
… …
Frequency Analysis
• The English language has trends:• “e” appears in English about 12% of the time• “t” = ~9%• “a” = ~8%•
• Common digrams:• “he”• “an”• “in”• “th”
Data Encryption Standard (DES)
• Developed by IBM in the 70s as the standard for encryption by the U.S. government• Based on Feistel networks• Remained the standard for 20+ years• Has since been broken by exhaustive search and replaced with a new
standard
Feistel Network
Reversible function! (meaning the decryption method is the same as encryption)
The Key
• A single key is permutated into 16 subkeys• A series of left-shifts on the original key• Each subkey is then permuted (diffused)
The F-function
• First, the block is expanded• Second, the expanded block is X-
OR’ed with the subkey• Next, fed into “S boxes” which
map 6 bits to 4 bits• Based on a table
• Finally, output is permuted• bits are diffused
DES
Assignment
• For Wednesday:• Read up on DES (Chapter 14, Page 265 – 266). I highly suggest reading up on
additional sources on the web, such as: • http://en.wikipedia.org/wiki/Data_Encryption_Standard• http://www.youtube.com/watch?v=qHZKze24kVo• http://people.eku.edu/styere/Encrypt/JS-DES.html
• Quiz on DES:• 1 question on The Feistel Structure• 1 question on the f-function• 1 question on Sub-key Generation