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Cryptography and Authentication

7 important questions on Cryptography and Authentication

What is the central problem for this chapter?

Problem: measures against many possible threats must be taken
- confidentiality
- authentication
- integrity

Contents
- cryptographic algorithms
- key distribution
- authentication protocols
- example systems and protocols
- firewalls

What cryptographic algorithms are there?

Symmetric-key ciphers
- the same key is used for encryption and decryption

Public-key ciphers
- different keys are used for encryption and decryption

Cryptographic hashes
- one way function to compute message digest

What are Cipher principles and requirements

Principle
- encryption of plaintext m results in ciphertext c
- decryption of ciphertext c recovers the original plaintext m
- cryptographic algorithms are public
- cryptographic algorithms are parameterized by keys
- it is sufficient to keep key secret

Requirements
- only holder of decryption key can recover plaintext m
- attacker cannot deduce key from plaintext m+ ciphertext c
- it must be computationally impractical to try all possible key values
- algorithms must be computationally efficient

Last two are a contradiction

How does RSA authentication work?

What if the keys are used in different order
- encrypt m with private key
- decrypt c with public key

No confidentiality
- everyone can decrypt c

Authentication
- proof that owner of key created c
- but no integrity (no proof that c isnt modified)

What are block vs stream ciphers?

Block: (DES, RSA, MD5)
- all operate on blocks of data. Could result in recognizable patterns. DES can be used for confidentiality and authenticity
DES:
- ECB encrypts block per block = patterns: short mssgs
- CBC: XOR with previous cipher text (is slow): conf & integrity
- CTR: Uses counter and goes parallel. Does no authentication but fast.
- GCM: Is authenticated encryption: integrity and confidentiality.

Stream: (RC4)
- consider plaintext as stream of bits. Fast and easy. Security problems (WEB and SSL/TLS). Cannot be used for authenticity. Good for pictures.

How can keys be distributed?

Participants A and B need to know what keys to use

Challenge-response protocol
- includes timestamps thus timeless
- B proces it knows the private key

Public key authentication
- based on timestamps from clocks. What if they dont have certificates?

Needham Schroeder
- Key Distribution Center shares key with both parties (through iniant A)
- susceptable for replay attacks

Kerberos
- key derived from password
- Authentication Server & Ticketgrant server
- Wireless uses RADIUS

What is Diffie Hellman?

A and B choose 2 parameters: Large priemgetal p and small integer g. With private numbers a and b a key is calculated.

Susceptible for man-in-the-middle-attack
Solution: certify public parameters
Thus back to square one

DH is used in IPsec and SSL/TLS