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Notation Reminder - Computer and Network Security - Lecture Slides, Slides of Computer Science

University of AllahabadComputer Science

These are the Lecture Slides of Computer and Network Security which includes Authorization, Social Security Number, Trouble with Passwords, Cryptographic Keys, Dictionary Attack, Bad Passwords, Password Experiment, Random Characters etc. Key important points are: Notation Reminder, Public Key Notation, Symmetric Key Notation, Decrypt Ciphertext, Uses for Public Key Crypto, Digital Signature, Revocation Problem, Sign and Encrypt, Hybrid Cryptosystem

Typology: Slides

2012/2013

Uploaded on 03/22/2013

dhimant
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Part 1 Cryptography 1
Notation Reminder
Public key notation
oSign message Mwith Alice’s private key
[M]Alice
oEncrypt message Mwith Alice’s public key
{M}Alice
Symmetric key notation
oEncrypt plaintext Pwith symmetric key K
C = E(P,K)
oDecrypt ciphertext Cwith symmetric key K
P = D(C,K)
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Part 1  Cryptography 1

Notation Reminder

 Public key notation

o Sign message M with Alice’s private key

 [M]Alice

o Encrypt message M with Alice’s public key

 {M}Alice

 Symmetric key notation

o Encrypt plaintext P with symmetric key K

 C = E(P,K)

o Decrypt ciphertext C with symmetric key K

 P = D(C,K)

Part 1  Cryptography 2

Uses for Public Key Crypto

Part 1  Cryptography 4

Non-non-repudiation

 Alice orders 100 shares of stock from Bob

 Alice computes MAC using symmetric key

 Stock drops, Alice claims she did not order

 Can Bob prove that Alice placed the order?

 No! Since Bob also knows symmetric key,

he could have forged message

 Problem: Bob knows Alice placed the order,

but he can’t prove it

Part 1  Cryptography 5

Non-repudiation

 Alice orders 100 shares of stock from Bob

 Alice signs order with her private key

 Stock drops, Alice claims she did not order

 Can Bob prove that Alice placed the order?

 Yes! Only someone with Alice’s private key

could have signed the order

 This assumes Alice’s private key is not

stolen (revocation problem)

Part 1  Cryptography 7

Confidentiality and

Non-repudiation

 Suppose that we want confidentiality

and non-repudiation

 Can public key crypto achieve both?

 Alice sends message to Bob

o Sign and encrypt {[M]

Alice

Bob

o Encrypt and sign [{M}

Bob

]

Alice

 Can the order possibly matter?

Part 1  Cryptography 8

Sign and Encrypt

Alice Bob

{[M]Alice}Bob

 Q: What is the problem?

 A: Charlie misunderstands crypto!

Charlie

{[M]Alice}Charlie

 M = “Activate your weapon”

Part 1  Cryptography 10

Confidentiality

in the Real World

Part 1  Cryptography 11

Symmetric Key vs Public Key

 Symmetric key +’s

o Speed

o No public key infrastructure (PKI) needed

 Public Key +’s

o Signatures (non-repudiation)

o No shared secret