CHAPTER 2: CRYPTOGRAPHIC TOOLS

CHAPTER 2: CRYPTOGRAPHIC TOOLS

1. 2.1 CONFIDENTIALITY W/ SYMMETRIC ENCRYPTION

   • Symmetric Encryption a.k.a. single-key encryption
     • the universal technique for providing confidentiality for transmitted @ stored data
     • 5 ingredients o symmetric encryption scheme
       • Plaintext
       • Encryption algorithm
       • Secret key
       • Ciphertext
       • Decryption algorithm
     • 2 requirements for secure use:
       • i) need a strong encryption algorithm
       • ii) sender & receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure
   • Attacking Symmetric Encryption
     • 1.Cryptanalytic Attacks
       • a) Rely on: - nature of the algorithm
       • b) Exploits the character of the algorithm to attempt to deduce a specific plaintext @ the key being used
     • 2.Brute-Force Attack - Try all possible keys on some ciphertext until an intelligible transation into plaintext is obtained
   • Symmetric Block Encryption Algorithms
     • Data Encryption Standard (DES)
       • 2.Strength concerns: - concerns about algorithm - use of 56-bit key
       • 1.The most widely used encryption scheme a.k.a. Data Encryption Algorithm (DEA) - FIPS PUB 46
     • Triple DES (3DES)
       • 1.Repeats basic DES algorithm 3 times using either 2 @ 3 unique keys
       • 2.1st standardized for use in financial apps. in ANSI standard X9.17 in 1985
       • 3.Attractions: - underlying encryption algorithm is the same as in DES
       • 4.Drawbacks: - uses a 64-bit block size
     • Advanced Encryption Standard (AES)
       • 1.Needed a replacement for 3DES
       • 2.NIST called for proposals for a new AES in 1997
       • 3.Selected Rijndael in Nov. 2001
     • Practical Security Issues
       • 1.Typically symmetric encryption is applied to a unit of data larger than a single 64-bit @ 128-bit block
       • 2.Electronic codebook (ECB) mode is the simplest approach to multiple-block encryption
       • 3.Modes of operation
   • Stream Cipher - produces output 1 element at a time - encrypts plaintext 1 byte at a time
   • Block Cipher - produces an output block for each input block - can reuse keys

2. 2.2 MSG. AUTHENTICATION & HASH FUNCTIONS

   • Msg. Authentication
     • 1. Protects against active attacks
     • 2. Verifies received msg is authentic
     • 3. Can use conventional encryption
   • Hash Function Requirements
     • 1.Can be applied to a block of data of any size
     • 2.Produces a fixed-length output
     • 3.H(x) is relatively easy to compute for any given x
     • 4.One-way @ pre-image resistant
     • 6.Collision resistant @ strong collision resistance
     • 5.Computationally infeasible to find y ≠ x such that H(y) = H(x)
   • Security of Hash Functions
     • 1.There are 2 approaches to attacking a secure hash functions: - cyptanalysis - brute-force attacks
     • 2.SHA most widely used hash algorithm
     • 3.Additional secure hash function apps. - passwords - intrusion detection

3. 2.3 PUBLIC KEY ENCRYPTION

   • Public-Key Encryption Structure
     • 1.Publicly proposed by Diffie & Hellman in 1976
     • 2.Based on mathematical functions
     • 3.Asymmetric
     • 4.Some form of protocol is needed for distribution
   • 5 ingredients of a public-key encryption scheme
     • Plaintext
     • Encryption algorithm
     • Public & private keys
     • Cipher text
     • Decryption key
   • Requirements for Public-Key Cryptosystems
     • 2.Computationally for sender knowing public key to encrypt messages
     • 3.Computationally easy for receiver knowing private key to decrypt cipher text
     • 4.Computationally infeasible for opponent to determine private key from public key
     • 5.Computationally infeasible for opponent to otherwise recover original message
     • 6.Useful if either key can be used for each role
     • 1.Computationally easy to create key pairs
   • Asymmetric Encryption Algorithms
     • 1.RSA (Rivest, Shamir, Adleman)
     • 2.Diffie-Hellman key exchange algorithm
     • 3.Digital Signature Standard (DSS)
     • 4.Elliptic Curve Cryptography (ECC)

4. 2.4 DIGITAL SIGNATURES & KEY MGT.

   • Digital Signatures
     • used for authenticating both source & data integrity
     • created by encrypting hash code with private key
     • does not provide confidentiality
   • Digital Envelopes
     • protects a msg. w/out needing to first arrange for sender & receiver to have the same secret key
     • equates to the same thing as a sealed envelope containing an unsigned letter

5. 2.5 RANDOM & PSEUDORANDOM NUMBERS

   • Random Numbers Uses include generation of:
     • keys for public-key algorithms
     • stream key for symmetric stream cipher
     • session key
   • Random Number Requirements
     • 1.Randomness Criteria - uniform distribution - independence
     • 2.Unpredictability - each number is statistically independent of others numbers in the sequence
   • Random vs. Pseudorandom
     • 1.Cryptographic apps. typically make use of algorithmic techniques for random no. generation
     • 2.Pseudorandom numbers are: - sequences produced that satisfy statistical randomness tests
     • 3.True random number generator (TRNG) - uses nondeterministic source to produce randomness

6. 2.6 PRACTICAL APPS. ENCRYPTION OF STORED DATA

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