Transcript CMSC 414 Computer (and Network) Security Lecture 3 Jonathan Katz

CMSC 414
Computer (and Network) Security
Lecture 3
Jonathan Katz
Confidentiality
In more detail…
 Alice and Bob share a key K
– Must be shared securely
– Must be completely random
– Must be kept completely secret from attacker
– We don’t discuss (for now) how they do this
 Plaintext - encryption - ciphertext -
decryption
 Decryption must recover the message!
Notes…
 Can also be used to encrypt files (i.e.,
secure storage)
 We have not said anything yet about
security…
Some examples
 (Shift cipher)
 (Substitution cipher)
 (Vigenere cipher)
Attacks?
 Shift cipher
– Key space is too small!
– Insecure against ciphertext-only attack
• Frequency analysis
• Index of coincidence
– If an attacker can recover they key, a scheme is
clearly insecure
• What about the converse?
– Multiple other attacks and problems
Attacks?
 Substitution cipher
– Much larger key space
– Still not secure against ciphertext-only attack
(frequency analysis, digrams, trial and error)
– Having a large key space is necessary, but not
sufficient, to guarantee security…
• (Note that adversary can still recover the key)
Attacks?
 Vigenere cipher
– Index of coincidence
• Random text has index:  (26)-2 = 0.038
• English text has index:  (pi)2 = 0.065
– Can distinguish substitution and Vigenere
ciphers; if the latter, can determine key length
– Can further use this to determine key
Moral of the story?
 Don’t use “simple” schemes
 Thoroughly analyze schemes before using
– Better yet, use schemes that other, smarter
people have already analyzed…