Cryptography Flashcards
Plaintext –
the original message
Ciphertext –
the encrypted message
Cryptographic system –
Specific method of encryption and decryption (also called a cipher or cryptosystem)
Transposition –
rearranging elements
Substitution –
replacing elements
Code –
Words are substituted for other words.
Encryption is NOT the same as a code
Block Cipher
Operate on blocks of plaintext and ciphertext – usually 64 bits.
Stream Ciphers
Convert plaintext to ciphertext 1 bit at a time.
Symmetric –
Same key is used for both encryption and decryption (also called private key encryption)
DES
Data Encryption Standard
AES -
Advanced Encryption Standard
IDEA –
International Data Encryption Algorithm
S-Boxes
Integral part of DES algorithm
Selects which 32 bits to use after the key and 32-bit data block have been shifted and combined
Skipjack
Uses 80-bit keys
Used in Clipper chip
- Meant to help FBI combat criminals using encryption
- Built in back door for law enforcement
Government has “master key”
- Key escrow – session key held for later release to law enforcement when needed
Problems with Symmetric Methods
Same key is used to encrypt and decrypt
Shared key is more likely to be compromised
Possible to brute force short keys
Certain keys are weak
Different keys can produce identical ciphertext
Distribution of keys
Diffie-Hellman Method
Each party creates their own private key
Each party computes a public key using a mathematical function of the private key
Public keys are exchanged
Message key is computed from other person’s public key and your own private key
If the math is right, the message key is the same on both sides
SET
Secure Electronic Transactions
The SET™ Specification, is an open technical standard for the commerce industry developed by Visa and MasterCard as a way to facilitate secure payment card transactions over the Internet.
Digital Certificates
Used to encode and verify messages
Requires a Certificate Authority that creates a digital certificate based on a private key and other authentication information
X.509
A popular standard for defining digital certificates
VPN
Virtual Private Network
Key Escrow and Key Recovery
Keys held in “reserve” in case the original keys are lost or damaged
Usually split between two parties, each with half of the key
Often requires two or more people to access and recover key
Differential Cryptanalysis
Look for differences in pairs of messages
Only works on certain ciphers
Linear Cryptanalysis
Looks for simple approximation of encryption function
Differential Power Analysis
Measures power consumption of hardware encryption devices
Covert Channels –
“communication paths that were neither designed nor intended to transfer information at all.”
Anonymity –
“finding ways to hide the metacontent of messages (i.e. the sender and the recipients of a message).”
Steganography –
concerned with hiding the existence or presence of a message.
Watermarks –
used to indicate ownership
Unobtrusive communications –
Important on the modern battlefield.
Detection of a signal may lead quickly to the sender being attacked.
Tamper proofing –
Information hidden may be a signed “summary” or a hash value which can be used to detect unauthorized modifications.
Data augmentation –
Information can be added for the benefit of the public (e.g. details about the work, annotations, purchasing info, …)
Substitution systems –
substitute redundant parts of a cover with a secret message.
Transform domain techniques –
Embed secret information in a transform space of the signal (e.g., in the frequency domain).
Spread spectrum techniques –
adopting ideas from spread spectrum communication
Statistical methods –
Encode info by changing several statistical properties of a cover and use hypothesis testing in the extraction process
Distortion techniques –
Store information by signal distortion and measure the deviation from the original cover in the decoding step.
Cover generation methods –
Encode information in the way a cover for secret communication is created.
Stego-only attack:
only the stego-object is available for analysis
Known cover attack:
the original cover-object and stego-object are both available.
Known message attack:
the hidden message and the stego-object are available.
Chosen stego attack:
The stego tool (algorithm) and stego-object are known.
Chosen message attack:
goal is to determine the specific stego tool or algorithm
Known stego attack:
the tool (algorithm) is known and both the original and steg-objects are available.
