Cryptography and Symmetric Cryptography detail notes with its types

1. Lecture 5. Symmetric
Cryptography
Block cypher /stream sypher

2. Cryptography (1)
• Cryptography is the science of secret writing.
• Cryptanalysis is the science of analyzing and breaking
ciphers.
• Cryptology encompasses both subjects.
• Historically domain of spies and secret agents.
• Now considered to be a mathematical discipline.
• In modern data communications, Cryptography gives
the means to construct a secure logical channel over an
insecure physical connection.

3. Cryptography (2)

4. Cryptography (3)
• Services offered by Cryptography
• Data Confidentiality
• Data Integrity
• Data Origin Authentication
• Confidentiality
• Symmetric Encryption
• Asymmetric Encryption
• Integrity
• Hash Function
• Data Origin Authentication
• Digital Signatures

5. Attacks on Cryptography
• Cryptanalysis: Attack on cryptography based on knowledge of
implementation of cryptographic algorithm or some plaintext-
ciphertext pairs.
• Brute-force Attack: is to try every possible key on a piece of
ciphertext until an intelligible translation into plaintext is obtained.

6. Symmetric Encryption (1)
• Single-key encryption

7. Symmetric Encryption (2)
Block Cipher

8. Symmetric Encryption (3)
Stream Cipher

9. Symmetric Block Encryption Algorithms
• Data Encryption Standard (DES)
• Triple DES
• Advanced Encryption Standard (AES)

10. Exhaustive Key Search Analysis

11. Practical Security Issues
• Every Encryption Algorithm is prone to Brute Force Attack –
Computationally Secure Algorithm is required.
• Plaintext sources must be broken up into a series of fixed-length block
- Electronic Codebook (ECB) mode.
• If the last plaintext block has fewer bits than the blocklength, it is
padded with “filler” characters.

12. Stream Cipher
• Processes the input elements continuously, producing output one element at a time.
• May be designed to operate on one bit at a time or on units larger than a byte at a
time.
• A key is input to a pseudorandom bit generator that produces a stream of 8-bit
numbers that are apparently random.
• A pseudorandom stream is one that is unpredictable without knowledge of the input
key and which has an apparently random character.
• The output of the generator, called a keystream, is combined one byte at a time with
the plaintext stream using the bitwise exclusive OR (XOR) operation.
• A stream cipher can be as secure as a block cipher of comparable key length.
• Advantage of a stream cipher is that stream ciphers are almost always faster and use
far less code than do block ciphers.

13. Review
• How is cryptanalysis different from brute-force attack?
• What is the difference between block cipher encryption and stream
encryption?
• What are the two principal requirements for the secure use of
symmetric encryption?