Chapter Seven & Eight Flashcards
(44 cards)
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
