Cryptography Encryption Systems Flashcards
Running Key Cipher (aka key ciphers)
Compare to Concealment Cipher
Uses physical component (ie book or table) to provide polyalphabetick characters
Indicator block gives location in book where originator started
Parties must agree on book to use and where indicator block will be included in the cipher message
Concealment Cipher
Plaintext is interspersed within other written material
Parties must agree on the key value which defines which letters are part of actual message
Belongs in steganography realm
Substitution Cipher
Uses a key to substitute characters or character blocks with different characters or character blocks
Caesar and Vigenere cipher are two early types of these
Modulo 26 substitution cipher
Transposition Ciphers
Scrambles letters of original message in different order
The key determines the positions to which letters are moved
Symmetric Algorithms
aka
single-key, secret-key, private-key or shared-key cryptography
Use same key to decrypt as encrypt
Secrecy of key is critical
Calculate number of keys needed: # Users * (# Users -1) / 2
Secure out of band method required to provide secret key
Session key encrypts messages between users
Provide confidentiality but not authentication or nonrepudiation
Includes:
DES, AES, IDEA, Skipjack, Blowfish, Twofish, RC4/RC5/RC6, CAST
Stream based symmetric algorithms
perform encryption on bit-by-bit basis
Use key stream generators to create bit stream that is XOR’d with plaintext bit.
Result is XOR is the ciphertext
ie RC4
Advantages include:
lower error propagation because encryption performed on each bit
used more in hardware implementation
Use same key for encryption and decryption
cheaper to implement than block ciphers
employ only confusion (not diffusion)
Block symmetric algorithms
perform encryption by breaking message into fixed-length units
ie IDEA, Blowfish, RC5, RC6
Advantages:
easier implementation than stream-based
less susceptible to security issues
used more in software implementations
Employ both confusion and diffusion
Initialization vectors used by modes to ensure patterns not produced during encryption by using random values in algorithms
Asymmetric Algorithms
Use both public key and private key
public key known by all parties, private key only known by its owner
One key encrypts and other decrypts
deriving private key from public key is virtually impossible despite being mathematically related
Provide: confidentiality, integrity, authentication, nonrepudiation
For confidentiality, message should be encrypted with receiver’s public key (secure message format)
For authentication, message should be encrypted with sender’s private key (open message format)
Examples:
Diffie-Helman, RSA, El Gamal, ECC, Knapsack, DSA, Zero Knowledge Proof
Hybrid Ciphers
Use both types of algorithms (symmetric and asymmetric)
Provides confidentiality, authentication, nonrepudiation
Process:
- symmetric algorithm provides keys for encryption
- symmetric keys are passed to asymmetric algorithm which encrypts and distributes them
- message encrypted with symmetric key
- message and key are sent to receiver
- receiver decrypts symmetric key and uses it to decrypt message
Use hybrid encryption of parties don’t have shared secret key and large quantities of data must be transmitted
One-time pad
most secure encryption scheme
Key value is added to value of letters using a key that is same length as the plaintext message
To ensure OTP is secure, following conditions must be met for the key:
- be used only once
- be as long or longer than the message
- consist of random values
- be securely distributed
- be protected at source and destination
Steganography
message is hidden inside other object like picture or document
examples:
concealment cipher
digital watermarking - logo embedded in documents
deter using the materials in unauthorized manner
