Network Security (Encryption & Decryption)

1. Network Security(Encryption & Decryption) Dr. R. K. Rao

2. Model of Encryption & Decryption Process • Desire to communicate privately is a human trait • Study of ways to disguise messages so as to avert unauthorized interception is called CRYPTOGRAPHY • Encipher & Encrypt: Message Transformation done at the transmitter • Decipher & Decrypt: Inverse transformation done at the receiver

3. Reasons for using Cryptosystems • Privacy: To prevent unauthorized persons from extracting information from the channel (eavesdropping) • Authentication: To prevent unauthorized persons from injecting information into the channel (spoofing)

4. Model of a Cryptographic Channel

5. Model of a Cryptographic Channel • The message or plaintext is, M, is encrypted by an invertible transformation using a key K • The cipher text C is transmitted over an insecure public channel • When an authorized receiver obtains C, it decrypts C using inverse transformation using the key K

6. Model of a Cryptographic Channel • The parameter K refers to a set of symbols or characters called a Key, which dictates a specific encryption transformation • Originally, the security of cryptosystems depended on the secrecy of the entire encryption process. • In recent years, systems are developed for which the general nature of the encryption transformation or algorithm could be publicly revealed, since security of such systems depend on specific key used

7. Model of a Cryptographic Channel • In most cryptosystems, anyone with access to the key can both encrypt and decrypt messages • The key is transmitted to the community of authorized users over a secure channel • The key usually remains unchanged for a considerable number of transmissions • The goal of the Cryptanalyst is to produce an estimate of the plaintext by analyzing the ciphertext C obtained from the public channel

8. Types of Encryption Schemes • Block Encryption: The plain text is segmented into blocks of fixed size; each block is encrypted independently from the others • Stream Encryption: There is no fixed block size. Each plaintext bit is encrypted as it enters the sytem

9. System Goals • To provide an easy and inexpensive means of encryption and decryption to all authorized users in possession of the key • To ensure that the cryptanalyst’s task of producing an estimate of the plaintext without the benefit of the key is made difficult and expensive

10. System Goals • Successful cryptosystems are classified as: Unconditionally Secure: When the amout of information available to the cryptanalyst is insufficient to determine the encryption & decryption process Computationally Secure: Under the circumstances favorable to the cryptanalyst, the system security could be broken in x number of years

11. Classic Threats • Ciphertext only Attack: Cryptanalyst might have some knowledge of the general system and the language used in the message (only significant data available is the encrypted message intercepted from the public channel) • Known Plaintext Attack: Assumes knowledge of plaintext and knowledge of its ciphertext counterpart-rigid structure of most business forms

12. Classic Threats • Chosen plaintext Attack: When the cryptanalyst is in the position of selecting the plaintext- used in WWII to intercept Japanese cryptosystems

13. Classic Ciphers • Caesar Cipher- Used by Julius Caesar during Gallic Wars Each plaintext letter is replaced with a new letter obtained by an alphabetical shift Example: Plaintext: N O W I S T H E T I M E Ciphertext: Q R Z L V W K H W L P H The decryption key is simply the number of alphabetic shifts

14. Classic Ciphers • Polybius Square

15. Example of Polybius Square Plaintext: N O W I S Ciphertext: 33 43 25 42 34 • Trithemius Progressive Key: The row labeled shift 0 is identical to the usual arrangement of the alphabet. The letters in the next row are shifted one character to the left with an end-around shift for the leftmost position

16. Trithemius Progressive Key

17. Trithemius Progressive Key • Each successive row follows the same pattern of shifting the alphabet one character to the left as compared to the prior row. • This continues until the alphabet has been depicted in all possible arrangements of end-around shifts.

18. Using Trithemius Progressive Key • One method of using such an alphabet is to select the first character from the shift 1 row, the second cipher character from shift 2 row, and so on Plaintext: N O W I S T H E T I M E Ciphertext:O Q Z M X Z O M C S X Q

19. Using Trithemius Progressive Key • Another way of using the key is to employ a keyword Key: T Y P E T Y P E Plaintext: N O W I S T H E Ciphertext: G M L M L R W I

20. Using Trithemius Progressive Key • Yet another method starts with a single letter or word used as priming key. This key dictates the starting row or rows for encrypting the first or first few plaintext characters. Next, the plaintext characters themselves are used as the key. Key: F N O W I S T H Plaintext: N O W I S T H E Ciphertext: S B K E A L A L

21. Perfect Secrecy • Consider a cipher system with finite message, [M], and ciphertext, [C], spaces. That is: • For any , the a priori probability that is transmitted is:

22. Perfect Secrecy • Given that is received, the a posteriori probability that was transmitted is: • A cipher system is said to have perfect secrecy if for every message and every ciphertext , the a posteriori probability is equal to the a priori probability:

23. Perfect Secrecy • Thus, for a system, with perfect secrecy, a cryptanalyst who interprets obtains no further information to enable him or her to determine which message was transmitted

24. Example of Perfect Secrecy System • N=4 and U=4, K=4 • The transformation from message to ciphertext is obtained by:

25. Example of Perfect Secrecy System

26. Example of Perfect Secrecy System • A cryptanalyst intercepting one of the ciphertext messages would have no way of determining which of the 4 keys was used, and therefore whether or not the correct message is one of the 4. • A cipher system in which the number of messages, number of ciphertext transformations, and number of keys are all equal is said to have perfect secrecy, if and only if the following conditions are met:

27. Example of Perfect Secrecy System • There is only one key transforming each message to each ciphertext • All keys are equally likely