Notes - Secure Communications

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

• 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 2⁵⁶ ~ 7.2x10¹⁶ keys
• if you have a computer that can do 10⁹ calculations a second would take about 3x10¹³ 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

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