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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
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HW#1 due by midnight today
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Solution will be posted soon
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HW#2 will be posted by the weekend
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Would be due in a week
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TA for the course: Yasemin Avcular
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Email: [email protected]
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Office hours: 23pm Tuesdays, most probably in
LC 257
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Lecture 2  Private Key
Cryptography
3
Outline of today’s lecture
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Cryptography Overview
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Private Key Cryptography: Encryption
9/17/2009
Lecture 2  Private Key
Cryptography
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Cryptography
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Etymology: Secret (Crypt) Writing (Graphy)
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Study of mathematical techniques to achieve
various goals in information security, such as
confidentiality, authentication, integrity, non
repudiation, etc.
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Not
the only means of providing information
security, rather a subset of techniques.
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Quite an old field!
9/17/2009
Lecture 2  Private Key
Cryptography
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Private Key/Public Key Cryptography
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Private Key
: Sender and receiver share a
common (private) key
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Encryption and Decryption is done using the
private key
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Also called conventional/sharedkey/singlekey/
symmetrickey cryptography
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Public Key
: Every user has a private key and
a public key
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Encryption is done using the public key and
Decryption using private key
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Also called twokey/asymmetrickey cryptography
9/17/2009
Lecture 2  Private Key
Cryptography
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Cryptography: Cast of Characters
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Alice (A) and Bob (B): communicating parties
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Eve (E): Eavesdropping (or passive)
adversary
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Mallory (M): ManintheMiddle (or active
adversary
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Trent (T): a trusted third party (TTP)
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Lecture 2  Private Key
Cryptography
7
Common Terminologies
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Plaintext
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Key
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Encrypt (encipher)
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Ciphertext
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Decrypt (decipher)
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Cipher
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Cryptosystem
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Cryptanalysis (codebreaking)
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Cryptology: Cryptography + Cryptanalysis
9/17/2009
Lecture 2  Private Key
Cryptography
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Private key model
9/17/2009
Lecture 2  Private Key
Cryptography
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Open vs Closed Design
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Closed Design (as was followed in military
communication during the World War I/II)
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Keep the cipher secret
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Also sometimes referred to as the “proprietary design”
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Bad practice! (why?)
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Open Design (
Kerckhoffs' principle
)
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Keep everything public, except the key
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Good practice – this is what we focus upon!
9/17/2009
Lecture 2  Private Key
Cryptography
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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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Lecture 2  Private Key
Cryptography
11
Security Model and Adversaries
1.
Ciphertext only
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Adversary knows only the ciphertext(s)
2.
Known plaintext
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Adversary knows a set of plaintextciphertext pairs
3.
Chosen (and adaptively chosen) plaintext
(CPA
attack)
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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.
 Fall '09
 Saxena
 Computer Security

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