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36 Slides--Intro to Cryptography

# 36 Slides--Intro to Cryptography - CS103A HO 36 Intro to...

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CS103A HO# 36 Intro to Cryptography 2/15/08 1 Cryptography: Some References David Kahn. The Codebreakers (1967). Simon Singh. The Code Book (1999). Niels Ferguson and Bruce Schneier. Practical Cryptography (2003). Bruce Schneier. Applied Cryptography, 2 nd Edition (1996). Steve Burnett and Stephen Paine. RSA Security's Official Guide to Cryptography (2001). Brian A. LaMacchia et al. .NET Framework Security (2002). Michael Howard and David LeBlanc. Writing Secure Code, 2 nd Edition (2002). Bruce Schneier. Secrets and Lies (2000). Charles Pfleeger and Shari Pfleeger. Security in Computing, 3 rd Edition (2003). Cryptography The Basic Problem Alice m Bob m m Eve m Cryptography The Basic Solution Alice m, c := E(m) Bob c c, m := D(c) Eve c m : plaintext c : ciphertext Cryptography The Basic Solution Alice m, c := E(m) Bob c c, m := D(c) Eve c Eve's attacks: Break the code (reverse engineering) Replay the message Modify the message Block the message Fabricate a new message Alice m, c := E(m) Bob c c, m := D(c) Eve c Restricted Algorithm : security is based on keeping the algorithm secret. Difficult to use in a large or changing group. --When someone leaves the group, everyone must change algorithms --If someone reveals the secret, everyone must change algorithms No quality control on the algorithm, so it's difficult to know how secure it is. It is better to use a known, thoroughly studied algorithm whose security is based on a key . Alice m, c := E(m, K) Bob c c, m := D(c, K) Eve c Kerckhoffs' Principle : security is based only on keeping the key secret. Note that in the situation shown, the key is the same for both encryption and decryption. What Bob computes is D(E(m, K), K) Algorithms of this type are called symmetric , or secret key algorithms. The key is a shared secret between Alice and Bob.

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CS103A HO# 36 Intro to Cryptography 2/15/08 2 Symmetric Algorithms: One-Time Pad a t t a c k a t d a w n 13 2 21 18 14 1 7 12 25 17 16 3 n v o s q l h f c r m q + = (mod 26) Encode using a random key as long as the plaintext. Then throw away the key. Symmetric Algorithms: One-Time Pad a t t a c k a t d a w n 13 2 21 18 14 1 7 12 25 17 16 3 n v o s q l h f c r m q Decode using the same pad and subtracting.
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36 Slides--Intro to Cryptography - CS103A HO 36 Intro to...

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