1. CSCI 391: Practical Cryptology Introduction

2. Definitions Digital encryption techniques are used to protect data in two ways: to maintain privacy and to prove integrity [1] Cryptography comes from the Greek words Kryptos, meaning hidden, Graphen, meaning to write. Cryptography is the art of communicating confidentially through an insecure channel Cryptanalysis is the art of deciphering those communications when one is not the legitimate receiver. Cryptology is the union of these two domains. Cryptology is actually the study of codes and ciphers.

3. Definitions Plaintext the original message that is to be changed into a secret form Cipher, Cryptosystem, Encryption Method is a procedure for converting the plaintext message into encrypted message – ciphertext The word cipher originates from the Hebrew word "Saphar," meaning "to number." Decryption - the process that conducted by the intended receiver to recover original message (plaintext) from the encrypted message (ciphertext) Key - keeps the message secret

4. Encryption-Decryption PlaintextCiphertext Encryption Method Encryption key CiphertextPlaintext Decryption method Decryption Key

5. Basic Communication Scenario Alice: 1.Takes original text, plaintext and encrypts it using encryption method and encryption key. 2. Sends encrypted message, ciphertext, to Bob Bob: 1.Receives ciphertext from Alice 2.Uses decryption key to decrypt ciphertext and to obtain original message, plaintext Eve

6. Eve’s Goals Read the message Find the key and read ALL messages encrypted with this key Corrupt Alice’s message (integrity) Masquerade as Alice, and communicate with Bob even though he believes he is communicating with Alice (authentication)

7. Attack on a Cryptosystem Attempt to decrypt encrypted message without knowledge of the decryption key. Four basic sorts of possible attacks: Ciphertext only: Eve has only a fragment of the encrypted message: to decrypt the specific message to obtain the key and to be able to decrypt all subsequent messages Known plaintext: Eve has a copy of the ciphertext and the corresponding plaintext. The goal is to deduce the key Chosen plaintext: Eve gains the access to the encryption machine, she doesn’t have a key, but she can use the some of the plaintexts and corresponded ciphertexts, the goal is to deduce the key Chosen ciphertext: Eve gains the access to the decryption machine.

8. Symmetric and Asymmetric Ciphers Symmetric Cipher – knowledge of encryption key is equivalent to knowledge of the decryption key, or the same key is used for encryption and decryption. Asymmetric Cipher (Public-Key Cryptosystem) the knowledge of encryption key give little information about the decryption key and vice versa

9. Early History of Cryptology Polybius’ Checkboard 205 – 123 B.C. Plaintext POLYBIUS produces the ciphertext 3534315412244543 ZYXWV5 UTSRQ4 PONML3 KIJHGF2 EDCBA1 54321