Econ 325 exams > infs midterm > Flashcards

infs midterm Flashcards

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1
Q

availability

A

accessible to authorized users at appropriate times

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2
Q

aspects of security

A

security attack, security service, security mechanism

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3
Q

threat

A

something that can damage or destroy an asset

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4
Q

vulnerability

A

a weakness or gap in your protection

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5
Q

risk

A

where assets, threats, and vulnerabilities intersect

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6
Q

authentication

A

assurance that communicating entity is the one claimed

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7
Q

access control

A

prevention of unauthorized use

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8
Q

data confidentiality

A

protection of data from unauthorized disclosure

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9
Q

data integrity

A

data received is as sent by an authorized entity

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10
Q

non-repudiation

A

protection against denial by one of the parties in a communication

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11
Q

security mechanism

A

feature designed to detect, prevent, or recover from a security attack
no single mechanism that will support all services required
however one particular element underlies many of the security mechanisms in use:
cryptographic techniques

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12
Q

symmetric key encryption

A

plaintext-> encryption algorith with secret key shasred by sender and recepient, transmitted cipher text-> decryption using shared key

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13
Q

Kerckhoff’s principle

A

System security should depend only on the secrecy of the key, not the algorithm.

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14
Q

Caesar Cipher

A

A substitution cipher that shifts each letter by a fixed number
𝑘
E(k,p)=(p+k)mod26,

D(k,c)=(c−k)mod26.
can be broken By brute-force testing all 25 possible shifts.

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15
Q

keyspace for cryptosystem

A

For the Caesar cipher, any value from the set {1, 2, …, 25} can be a key
The set of usable keys is referred to as a cryptosystem’s keyspace
Cryptosystems with a small keyspace are vulnerable to a brute-force search for the proper key

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16
Q

What are passive vs. active attacks?

A

Passive = eavesdropping or monitoring; Active = modifying, disrupting, or injecting data.

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17
Q

What is a monoalphabetic cipher?

A

A substitution cipher where any permutation of 26 letters forms the key

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18
Q

What is the size of the monoalphabetic cipher keyspace?

A

26! ≈ 4×10²⁶ keys

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19
Q

What is frequency analysis?

A

Cryptanalysis method using letter frequency patterns in the ciphertext

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20
Q

What is a transposition cipher

A

A: A cipher that rearranges (permutes) letters of plaintext without altering them.

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21
Q

How does the Rail Fence cipher work?

A

Write letters diagonally across multiple rows, then read row by row.
meet me after the toga party = MEMATRHTGPRYETEFETEOAAT

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22
Q

What is the Pigpen cipher?

A

A symbol-substitution cipher mapping letters to grid symbols—easy to memorize, vulnerable to pattern matching.

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23
Q

What are the three pillars of information security?

A

Confidentiality, Integrity, Availability.

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24
Q

What is confidentiality?

A

Ensures computer assets are accessed only by authorized parties.

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25
What is integrity?
Ensures assets are modified only by authorized parties or in authorized ways.
26
What is availability?
Ensures assets are accessible to authorized parties at appropriate times.
27
What is Kerckhoff’s Principle?
Security should depend only on the secrecy of the key, not the algorithm.
28
What is cryptography?
The practice of securing information using encryption and decryption with a secret key.
29
What is a Caesar cipher?
Shifts each letter by k positions: E(k,p)=(p+k) mod 26. Example: k=3 → A→D.
30
What is cryptanalysis?
Science of breaking cryptosystems by deducing keys or plaintexts.
31
What is a monoalphabetic cipher?
Each letter maps to another letter using a substitution alphabet. Example: A→Q, B→W.
32
What is frequency analysis?
Breaking ciphers by matching ciphertext letter frequencies to normal language patterns.
33
What is a transposition cipher?
Rearranges plaintext letters. Example: Rail Fence cipher writes diagonally and reads rows.
34
What is a product cipher?
Combines substitution and transposition for stronger encryption.
35
What is a symmetric cipher?
Uses one shared secret key for both encryption and decryption.
36
What is a block cipher?
Encrypts fixed-size blocks (e.g., 64 or 128 bits). Example: DES.
37
What is a stream cipher?
Encrypts data bit-by-bit or byte-by-byte. Example: RC4.
38
What is a Feistel cipher?
Splits plaintext into halves processed in rounds: L_i=R_{i−1}, R_i=L_{i−1} XOR F(R_{i−1},K_i).
39
What are confusion and diffusion?
Confusion hides key–ciphertext relation; diffusion spreads plaintext statistics across ciphertext.
40
What is DES?
Data Encryption Standard; 64-bit block cipher using 56-bit key, Feistel-based.
41
What is AES?
Advanced Encryption Standard; 128-bit block cipher with 128/192/256-bit keys.
42
Who developed AES?
Rijndael algorithm by Daemen and Rijmen, adopted as AES in 2001.
43
What are AES transformations?
SubBytes, ShiftRows, MixColumns, AddRoundKey.
44
What is AES key expansion?
Generates round keys using Rijndael key schedule.
45
What is Triple DES?
Encrypt–Decrypt–Encrypt: C=E(K3,D(K2,E(K1,P))).
46
What is ECB mode?
ECB encrypts each plaintext block independently using the same key: C_i=E_K(P_i). Each block is treated as a separate message.
47
What is CBC mode?
CBC chains blocks together by XORing each plaintext block with the previous ciphertext block before encryption The first block uses an Initialization Vector (IV): C_i=E_K(P_i XOR C_{i−1}).
48
What is CFB mode?
CFB turns a block cipher into a self-synchronizing stream cipher. It encrypts the previous ciphertext block, then XORs it with the plaintext to produce ciphertext. The first step uses an IV instead of C -1 : C_i=P_i XOR E_K(C_{i−1}).
49
What is OFB mode?
OFB generates a keystream by repeatedly encrypting the output of the cipher, independent of the plaintext, with an initial O−1=IV: O_i=E_K(O_{i−1}), C_i=P_i XOR O_i.
50
What is CTR mode?
CTR mode encrypts a counter value for each block and XORs the result with plaintext, Each block uses an incremented counter (Counter_i = Nonce + i).: O_i=E_K(i), C_i=P_i XOR O_i.
51
What is public-key cryptography?
Uses key pair: public for encryption, private for decryption. Example: RSA.
52
What problems does public-key crypto solve?
Key distribution and digital signature verification.
53
What is a divisor?
b divides a if a=mb. Example: 3|9.
54
What is the GCD?
Largest integer dividing both numbers. Example: GCD(8,12)=4.
55
What is the Euclidean algorithm?
Finds GCD using gcd(a,b)=gcd(b,a mod b).
56
What is modular arithmetic?
Arithmetic under mod n. Example: 17 mod 5=2.
57
When are two numbers congruent mod n?
If a mod n=b mod n. Example: 17≡5 (mod 12).
58
What is the Extended Euclidean Algorithm?
Finds x,y such that ax+by=gcd(a,b); used for modular inverses.
59
How to find modular inverse?
If gcd(a,n)=1, then x in ax≡1(mod n) is the inverse of a mod n.
60
What is RSA encryption?
C=M^e mod n; decryption: M=C^d mod n.
61
What is RSA key generation?
Pick primes p,q; n=pq; φ(n)=(p−1)(q−1); choose e; find d where e·d≡1(mod φ(n)).
62
RSA example?
p=17,q=11,e=7,d=23 → C=88^7 mod187=11, M=11^23 mod187=88.
63
What is fast modular exponentiation?
Computes a^b mod n efficiently using repeated squaring.
64
What are the advantages of ECB?
Simple to implement. Allows parallel encryption/decryption of blocks. Efficient for short or random data (like encryption keys).
65
What are the disadvantages of ECB?
Identical plaintext blocks produce identical ciphertext blocks, revealing patterns. Leaks structural information — especially visible in images or repeated data. Not suitable for long or patterned data.
66
Example of ECB weakness?
When encrypting an image, the outline of the original is still visible since identical colors (plaintext blocks) produce the same ciphertext pattern.
67
Why is the IV important in CBC?
It ensures identical plaintexts produce different ciphertexts when re-encrypted. Must be unique and unpredictable for each encryption session.
68
What are the advantages of CBC?
Conceals patterns in the plaintext. Good for large files or data streams. Each ciphertext block depends on all previous blocks.
69
Q: What are the disadvantages of CBC?
Encryption can’t be parallelized since each block depends on the previous one. A single bit error in a ciphertext block corrupts the corresponding plaintext block and the next one. Requires secure handling of the IV.
70
What are the advantages of CFB?
Allows encryption of data smaller than block size (e.g., bytes). Self-synchronizing: a few bits of error are tolerated before recovery. Can process data as it arrives (real-time streams).
71
What are the disadvantages of CFB?
Encryption and decryption cannot be parallelized. A bit error in ciphertext affects several bits of decrypted plaintext. Slower than pure stream ciphers.
72
What are the advantages of OFB?
Bit errors in ciphertext do not propagate beyond the corrupted bit. Allows precomputation of the keystream (useful for high-speed or intermittent connections). Ideal for noisy channels (e.g., satellite links).
73
What are the disadvantages of OFB
Requires unique IV for every session (reusing IVs leads to key reuse attacks). Sender and receiver must remain synchronized — lost data breaks alignment. Not self-synchronizing like CFB.
74
What are the advantages of CTR?
Allows parallel encryption and decryption. Precomputation possible (encrypt counters ahead of time). Simple implementation — no chaining needed. No error propagation.
75
What are the disadvantages of CTR
Counter (Nonce + i) must never repeat with the same key, or security is broken. Requires careful synchronization between sender and receiver.
76
Why are product ciphers stronger?
Substitution ⊕ transposition in sequence achieves Shannon’s confusion + diffusion.
77
Block vs Stream cipher (plain words)?
Block: encrypt fixed-size chunks (e.g., 128 bits). Stream: encrypt one bit/byte at a time with keystream.
78
Shannon’s S-P network idea in one line?
Layer S-boxes (substitution) and P-boxes (permutation) to realize confusion & diffusion.
79
Feistel structure decrypts with same circuit—how
Swap halves each round; apply subkeys in reverse order to invert.
80
DES quick facts
64-bit blocks, 56-bit key, 16 Feistel rounds; adopted 1977; derived from IBM Lucifer
81
Why was DES controversial?
Short key (56 bits) and classified design criteria; still useful historically/legacy
82
Who is Évariste Galois (why mentioned)?
Founder of group theory/Galois fields; AES arithmetic uses GF(2^8)
83
AES in one line (structure)?
SP-network with rounds of SubBytes, ShiftRows, MixColumns, AddRoundKey over bytes.
84
Why not Double-DES?
A: Vulnerable to meet-in-the-middle attack; effective strength ≈ 2⁵⁶.
85
Q: What is Triple-DES (EDE 2-key)?
A: C = Eₖ₁(Dₖ₂(Eₖ₁(P))); if K₁ = K₂ → single DES.
86
Q: What is 3-key Triple-DES?
A: C = Eₖ₃(Dₖ₂(Eₖ₁(P))); avoids 2-key weaknesses.
87
Q: How is message padding handled in ECB?
A: Pad final block with known value or pad length byte.
88
Q: What does the IV do in CBC?
A: Starts chaining; should be unique and sent securely (e.g. ECB first).
89
Q: What does “CFB-s” mean?
A: Feedback of s bits (1/8/64/128); useful for byte streams.
90
Q: OFB key stream property?
A: Precomputable stream Oᵢ = Eₖ(Oᵢ₋₁); bit errors don’t propagate.
91
Q: CTR mode rule and warning?
A: Cᵢ = Pᵢ ⊕ Eₖ(counterᵢ); never reuse (key, nonce+counter) pair.
92
Q: What problem does public-key cryptography solve?
A: Key distribution and digital signature verification.
93
Q: Public-key applications?
A: Encryption/decryption, signatures, key exchange.
94
Q: Public-key requirements?
A: Hard to derive private key from public; easy to use each key with algorithm.
95
Q: Why is public-key slow?
A: Uses very large numbers and hard math problems (e.g. factoring).
96
Q: Division algorithm form?
A: a = q n + r with 0 ≤ r < n.
97
Q: Modular addition/multiplication rule?
A: [(a mod n)+(b mod n)] mod n = (a+b) mod n.
98
Q: RSA encryption and decryption equations.
A: C = Mᵉ mod n; M = Cᵈ mod n.
99
Q: Fast modular exponentiation idea.
A: Square each step and multiply when bit=1; reduces time to O(log e).
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