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Explore key encryption methods like Shift, Substitution, and Vigenere ciphers. Learn about security goals, attacks, DES, AES, and more. Understand how to achieve and assess security in network communications.
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CMSC 414Computer (and Network) SecurityLecture 4 Jonathan Katz
Some examples • (Shift cipher) • (Substitution cipher) • (Vigenere cipher)
Moral of the story? • Key space should be large • Necessary, but not sufficient • Don’t use “simple” schemes • Thoroughly analyze schemes before using • Better yet, use schemes that other, smarter people have already analyzed…
Re-thinking the problem • What do we mean by security? • I.e., not being able to determine the key?? • Types of attacks • Perfect security • One-time pad • Computational security • Block ciphers and modes of encryption • DES and AES
Notions of Security • What constitutes a “break”? • What kind of attacks? • Note: always assume adversary knows full details of the scheme (except the key…) • Never aim for “security through obscurity”
Security goals? • Adversary unable to recover the key • Necessary, but meaningless on its own… • Adversary unable to recover entire plaintext • Good, but is it enough? • Adversary unable to determine any information at all about the plaintext • Sounds great! • Can we achieve it?
One-time pad • (One-time pad)
Properties of one-time pad? • Achieves perfect secrecy (proof) • No eavesdropper (no matter how powerful) can determine any information whatsoever about the plaintext • (Essentially) useless in practice… • Long key length • Can only be used once (hence the name!)
Weaken security guarantee? • Instead of requiring that no adversary can learn anything about the plaintext… • …require that no adversary running in any “reasonable amount of time” can learn anything about the plaintext except with “very small probability” • “Reasonable time” = 106 years • “Very small probability” = 2-64 • Computational security
Simpler characterization? • Equivalent to the following, simpler definition: • Given a ciphertext C which is known to be an encryption of either M0 or M1, an adversary cannot guess which one was actually encrypted • More precisely, no adversary running in reasonable amount of time can guess correctly with probability significantly better than ½.
The take-home message • Weakening the definition slightly allows us to construct much more efficient schemes! • Strictly speaking, no longer 100% absolutely guaranteed to be secure • Security of encryption now depends on security of building blocks (which are analyzed extensively, and are assumed to be secure) • Given enough time, the scheme can be broken
Security? • We now have a working definition of what it means for encryption to be secure • What sort of attacks should we consider?
Attacks • Ciphertext only • Known plaintext • Chosen plaintext • Chosen ciphertext (includes chosen plaintext attacks)
Attacks… • A typical standard is security against chosen-plaintext attacks • Security against chosen-ciphertext attacks is increasingly required • Note that the one-time pad is insecure against known-plaintext attack
Randomized encryption • To be secure against chosen-plaintext attack, encryption must be randomized • We will see later how this comes into play
Block ciphers • Keyed permutation; input/output length • Large key space • Modeled as a (family of) random permutations… • Example – “trivial” encryption: • C = FK(m) • This is not randomized…