Foundations & Basic Concepts
Questions 1 - 7
Cryptography can protect which TWO of the three CIA properties?
What is cryptanalysis? How is it different from
cryptography?
According to Kerckhoffs's Principle, the encryption
algorithm must be kept secret - only the key is known publicly.
Given the encryption notation, if Alice sends Bob a message using
symmetric encryption, which statement is correct?
What is the field that combines both cryptography and cryptanalysis
called?
Cryptosystems are classified along three independent dimensions. Which
of the following is NOT one of them?
Symmetric encryption is approximately 30,000 times
faster than asymmetric (public-key) encryption.
Classical Substitution Ciphers
Questions 8 - 15
In the Caesar Cipher with shift n=14, encrypting the letter 'a'
(position 0) gives which result?
En(x) = (x + n) mod 26 | Dn(x) = (x − n) mod 26
Scenario: Using Caesar Cipher with shift n=3, encrypt the word "CAT".
Use the formula En(x) = (x + 3) mod 26. A=0, B=1, C=2, D=3, E=4, F=5,
G=6...
ROT13 is a special case of the Caesar Cipher with a shift of 13, and
applying it twice to a message returns the original plaintext.
In a Keyword Cipher using keyword "PSU IS MY CHOICE", what is the
FIRST step before building the cipher alphabet?
What statistical technique is used to attack substitution ciphers by
exploiting the fact that letters in natural language appear with
different frequencies?
The Playfair Cipher is more secure than a monoalphabetic cipher
because it encrypts digrams (letter pairs). How many possible digrams
are there?
In the Playfair Cipher, what do you insert between two repeated
letters in a plaintext pair? (E.g., what happens to "LL" in HELLO?)
The One-Time Pad is the only type of substitution
cipher that is theoretically unbreakable, because
every ciphertext is equally likely for any plaintext.
Transposition & Modern Block Ciphers
Questions 16 - 22
In a transposition cipher, what happens to the plaintext characters?
What are the two specific numbers associated with DES - its block size
and key size?
Triple DES (3DES) uses the pattern Encrypt-Decrypt-Encrypt with three
keys. What is its effective key length?
AES (Advanced Encryption Standard) uses 128-bit blocks and supports
key lengths of 128, 192, or 256 bits.
When encrypting a block cipher, if the last block of plaintext is too
short, what must be done?
In CBC (Cipher Block Chaining) mode, what is the
first block XOR'd with before encryption begins?
Which block cipher mode can be performed in
parallel and can recover from dropped blocks?
Hill Cipher & Vigenère
Questions 23 - 26
The Hill Cipher's main strength over monoalphabetic ciphers is that
it:
In the Hill Cipher, what mathematical operation is used to
decrypt? (What key is used instead of the original
key matrix K?)
In the Vigenère Cipher, if the keyword is shorter than the plaintext,
what happens?
In the Hill Cipher example from the slides, encrypting "july" with key
matrix [11 8 / 3 7] gives the ciphertext "DELW".
Cryptanalysis Attacks & Brute Force
Questions 27 - 30
In a Chosen Plaintext Attack, what advantage does the
attacker have over a Known Plaintext Attack?
Scenario: An attacker intercepts several encrypted messages between
Alice and Bob. The attacker has ONLY the ciphertext - no plaintext, no
ability to encrypt or decrypt their own messages. What type of attack
is this?
Based on the brute force table from the slides, approximately how long
would it take to break a 56-bit DES key at 1 million decryptions per
microsecond?
In a Chosen Ciphertext Attack, what is the ultimate
goal of the attacker - what are they trying to find?
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