- Secure Communications
- Actual message is called plaintext
- Encryption methods are called keys
- Ciphertext is the encrypted message
- Goals of an attacker or eavesdropper
- Read the message
- Find the key and read all messages encrypted with that key
- Corrupt initial message
- Communicated with receiver of message masquerading as sender
- Possible Attacks
- Attacker has ciphertext only
- Attacker has copy of ciphertext and plaintext
- Attacker gains temporary access to encryption machine to create large quantities of ciphertext with known plaintext
- Attacker gains temporary access to decryption machine to decrypt several strings of symbols
- Kerckhoff's Principle
- In assessing the security of a cryptosystem one should always assume the enemy knows the method being used
- Symmetric and Public Key Algorithms
- symmetric key
- encryption and decryption keys are known to both sender and receiver
- many cases encryption and decryption key is same
- DES Data Encryption Standard AES Advanced Encryption Standard
- Two types of ciphers
- stream ciphers
- data is fed in small bits, output in small bits
- block ciphers
- data is fed in blocks and fed into algorithm at once outputed at once
- public key
- introduced in 1970s
- RSA encryption
- Non-mathematical way to do public key encryption Receiver is Alice Sender is Bob
- Bob sends Alice a box and an unlocked padlock
- Alice puts message in box locks Bob's lock on it then sends the box back to Bob
- authorization issues if first transmission is intercepted and lock is substituted
- Computation of public keys are several orders of magnitude higher than symmetric keys
- Codes - one to one usages Ciphers - encrypts every string of characters
- Key Length
- Brute Force attack -Try every single possible key to see which one yields meaningful decryptions
- DES Algorithm has 56 bit key and thus 2
^{56}~ 7.2x10^{16}keys - if you have a computer that can do 10
^{9}calculations a second would take about 3x10^{13}years to complete - Longer keys are advantageous but not necessarily more difficult to break
- Other methods of attack
- Frequency Analysis
- BirthdayAttacks
- One time pad is an unbreakable code
- however requires a key as long as plaintext and the key can only be used once so is not practical

## Tuesday, September 10, 2013

### Notes - Secure Communications

The following are notes from Introduction to Cryptography with Coding Theory.

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