Cryptography: Applications and Uses

Chapter 5: Cryptography
Objectives:
- Applications and uses of cryptography
- Encryption methodologies
- Cryptanalysis
- Management of Cryptography
- Key Management
Application and uses of Cryptography

What is Cryptography? Cryptography is the science of hiding information in plain sight, in order to conceal it from unauthorized parties.
- Substitution cipher first used by Caesar for battlefield communications

Encryption Terms and Operations
• Plaintext – an original message
• Ciphertext – an encrypted message
• Encryption – the process of transforming plaintext into ciphertext (also encipher)
• Decryption – the process of transforming ciphertext into plaintext (also decipher)
• Encryption key – the text value required to encrypt and decrypt data
Encryption Methodologies

• Substitution Cipher
– Plaintext characters are substituted to form ciphertext
• Transposition Cipher
– Plaintext messages are transposed into ciphertext
– Subject to frequency analysis attack • Monoalphabetic Cipher
– One alphabetic character is substituted or another
– Subject to frequency analysis attack • Polyalphabetic Cipher
– Two or more substitution alphabets
– Not subject to frequency attack
• Running-key Cipher
– Plaintext letters converted to numeric (A=0, B=1, etc.)
– Plaintext values “added” to key values giving ciphertext
– Modulo arithmetic is used to keep results in range 0-26

• One-time Pad
– Works like running key cipher, except that key is length of plaintext, and is used only once
– Highly resistant to cryptanalysis

Types of Encryption
• Block cipher
– Encrypts blocks of data, often 128 bits
• Stream cipher
– Operates on a continuous stream of data

Block Ciphers
• Encrypt and decrypt a block of data at a time
– Typically 128 bits
• Typical uses for block ciphers
– Files, e-mail messages, text communications, web
• Well known encryption algorithms
– DES, 3DES, AES, CAST, Twofish, Blowfish, Serpent