Chapter Seven & Eight Flashcards
Cryptography
The study of securing information through the use of codes, ciphers, encryption, and other security techniques
Cryptanalysis
The study of how ciphers, codes, and cryptosystems work
and why
Cryptology
The combined study of cryptography and cryptanalysis
Kerckoff’s Principle
he security of a cryptosystem should not rely on the secrecy of the system
We want to assume attackers have access to knowledge before deploying
attacks
Caesar Cipher
– Replace each letter in a text by the 3rd letter
following it in the alphabet:
– ABCD becomes DEFG; CAT becomes FDW
– Variations rotate by different amounts
– Monoalphabetic substitution cipher
Vigenere Cipher
– Named after 16th century author Blaise
Vigenère
– Uses a series of different rotations
– The “key” may be a word – each letter
indicates a rotation
– Polyalphabetic substitution cipher
he Data Encryption Standard (DES) (1977)
– Developed by IBM and US government
– Adopted by banks to protect the earliest
electronic bank transactions
* Supported 56-bit keys: 256 different keys
– 72,057,594,037,927,900
* Over 72 quadrillion keys (7 x 1016)
- AES
128-bit, 192-bit, and 256-bit keys
– Smallest key is still too large for DES Cracker
* 128-bit key would take 1019 years
Key stream…
It is a stream of bits with these properties:
– Attackers can’t predict its contents in practice
The RC4 Stream Cipher (1987)
Rivest Cipher 4 (RSA)
* Prioritized speed of encryption without loss of
data in limited systems
* Byte cipher, doesn’t work at the bit-level
* Used in Microsoft End-to-End Encryption, PDF,
SSL, etc.
* Broken due to biases in the secret key
Pseudo-Random Numbers And Key
Streams (PRGNs)
Statistically random numbers: good for
simulations, bad for cryptography
Known Ciphertext
or ciphertext only
– All we know is the ciphertext
– Most difficult situation, most common
Known Plaintext
– We know some plaintext to match some of the ciphertext encrypted with a particular key
Chosen Plaintext
We can choose some plaintext to encipher
with our victim’s cipher, and retrieve the
ciphertext
Computational foundations
All strong encryption algorithms are built on top of “mathematically
intractable” algorithms (prime factorization – finding the factors of
extremely large prime numbers, discrete logarithms, etc.)
Cryptonet
set of people or devices that all
share the same secret key – transitive trust
Public and private keys…
– Public keys can be shared with attackers
– Private keys are kept secret by the owner
Elliptic Curve Cryptography
Similar to Diffie-Hellman
– Can calculate a shared secret
– Uses elliptic curve computations:
(y2 = x2 + ax + b)(mod p)
Elliptic curve key is 2–3x larger than a secret
key yielding a comparable search space
Quantum key distribution
-Applies Heisenberg’s Uncertainty Principle to detect eavesdropping
– Demonstrated using satellite communications
Quantum cryptanalysis
– Schor’s algorithm factors very large numbers
– A large quantum computer could attack
current public-key crypto techniques
Birthday attack:
comes from the birthday
paradox, where in
n group of people, two of
which are bound to have the same birthday
Randomly create two of the same file until the
same hash value is generated
MAC
cryptographic checksum applied to a
message
Keyed hash
– A way of verifying that some of our data has
not been modified by an attacker
– Keyed Hash – a value created from both the
message and the secret key to create a MAC
(Message Authentication Code)
Digital signature
A hash of the message
encrypted with the private key of the sender
– Includes…
* Hashing algorithm used
* Contents of the message
* Key generation algorithm
* Any other information provided by a CA
Man-In-The-Middle Attack (MITM)
attack
where any communication of the keys is
intercepted and spoofed by the attacker
– Forge messages from one party as legitimate
without knowing the keys themselves; simply
use them to create false messages
What is bit-flipping
It refers to a type of attack where an attacker modifies individual bits within a block of encrypted data in order to change its meaning or functionality. This attack is particularly effective against encryption schemes that use stream ciphers or block ciphers in certain modes of operation.
Volume
a logical space containing its own filesystem
a logical space containing its own filesystem
Designed using loops or rounds
Implements key schedules for different keys
Feistel structure (AKA Feistel network)
Skeleton of most block ciphers
Drawbacks of block ciphers
Can’t account for information outside of where information is
encrypted
Social engineering
Deceptive or manipulative practices to gain information from someone
(passwords, system configuration information, etc.)
RC4 was used in early wireless encryption (T/F)
True
Mode
is a technique for applying the cipher by
mixing its outputs to hide ciphertext patterns
Cipher stream modes: (T/F) Both OFB and CTR can generate the keystream
before the data is available to encrypt
True
Mode uses both XOR and the block cipher to
construct the ciphertext stream (T/F)
True
Most modes are mixing modes that hide patterns… Typical modes?
Typical modes:
– Electronic codebook (ECB) – no mixing at all
– Output feedback (OFB) – creates a keystream
– Counter (CTR) – creates a keystream
– Cipher feedback (CFB) – blocks and streams
– Cipher block chaining (CBC) – block oriented
Drawbacks of block ciphers
Can’t account for information outside of where information is
encrypted
AESGCM, XTS modes
Modern solution incorporating intermediate authentication values
(tag), advanced counter mode operations
AES GCM ( Advanced Encryption Standard in Galois Counter Mode)
Requires the use of a “tag” to authenticate
certain vulnerable features about the
message
* Length of message, patterns in
message/ciphertext/encryption algorithm
vulnerabilities, etc
AEAD –
Authenticated Encryption with
Additional Data
MS Windows – BitLocker feature
True
Apple OS X – FileVault feature
True
PGPDisk – part of commercial PGP crypto
True
Veracrypt – open-source FDE package
True
XTS is length-preserving
Does not incorporate padding or
concatenation or reduction of encrypted data
