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Lecture2 - Lecture 2 Private Key Cryptography CS 392/6813 Computer Security Fall 2009 Nitesh Saxena*Adopted from Previous Lectures by Nasir Memon

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Lecture 2: Private Key Cryptography CS 392/6813: Computer Security Fall 2009 Nitesh Saxena * Adopted from Previous Lectures by Nasir Memon 9/17/2009 Lecture 2 - Private Key Cryptography 2 Course Administration ± HW#1 due by midnight today ± Solution will be posted soon ± HW#2 will be posted by the weekend ± Would be due in a week ± TA for the course: Yasemin Avcular ± Email: [email protected] ± Office hours: 2-3pm Tuesdays, most probably in LC 257
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9/17/2009 Lecture 2 - Private Key Cryptography 3 Outline of today’s lecture ± Cryptography Overview ± Private Key Cryptography: Encryption 9/17/2009 Lecture 2 - Private Key Cryptography 4 Cryptography ± Etymology: Secret (Crypt) Writing (Graphy) ± Study of mathematical techniques to achieve various goals in information security, such as confidentiality, authentication, integrity, non- repudiation, etc. ± Not the only means of providing information security, rather a subset of techniques. ± Quite an old field!
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9/17/2009 Lecture 2 - Private Key Cryptography 5 Private Key/Public Key Cryptography ± Private Key : Sender and receiver share a common (private) key ± Encryption and Decryption is done using the private key ± Also called conventional/shared-key/single-key/ symmetric-key cryptography ± Public Key : Every user has a private key and a public key ± Encryption is done using the public key and Decryption using private key ± Also called two-key/asymmetric-key cryptography 9/17/2009 Lecture 2 - Private Key Cryptography 6 Cryptography: Cast of Characters ± Alice (A) and Bob (B): communicating parties ± Eve (E): Eavesdropping (or passive) adversary ± Mallory (M): Man-in-the-Middle (or active adversary ± Trent (T): a trusted third party (TTP)
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9/17/2009 Lecture 2 - Private Key Cryptography 7 Common Terminologies ± Plaintext ± Key ± Encrypt (encipher) ± Ciphertext ± Decrypt (decipher) ± Cipher ± Cryptosystem ± Cryptanalysis (codebreaking) ± Cryptology: Cryptography + Cryptanalysis 9/17/2009 Lecture 2 - Private Key Cryptography 8 Private key model
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9/17/2009 Lecture 2 - Private Key Cryptography 9 Open vs Closed Design ± Closed Design (as was followed in military communication during the World War I/II) ± Keep the cipher secret ± Also sometimes referred to as the “proprietary design” ± Bad practice! (why?) ± Open Design ( Kerckhoffs' principle ) ± Keep everything public, except the key ± Good practice – this is what we focus upon! 9/17/2009 Lecture 2 - Private Key Cryptography 10 Private Key Encryption: main functions 1. KeyGen: K = KeyGen(l) (l is a security parameter) 2. Enc: C = Enc(K,M) 3. Dec: M = Dec(K,M)
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9/17/2009 Lecture 2 - Private Key Cryptography 11 Security Model and Adversaries 1. Ciphertext only ± Adversary knows only the ciphertext(s) 2. Known plaintext ± Adversary knows a set of plaintext-ciphertext pairs 3. Chosen (and adaptively chosen) plaintext (CPA attack) ± Adversary chooses a number of plaintexts and obtains the corresponding ciphertexts 4.
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This note was uploaded on 11/18/2009 for the course CS 6813 taught by Professor Saxena during the Fall '09 term at NYU Poly.

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Lecture2 - Lecture 2 Private Key Cryptography CS 392/6813 Computer Security Fall 2009 Nitesh Saxena*Adopted from Previous Lectures by Nasir Memon

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