C839 Intro to Cyrpto Practice Questions with
Verified Correct Answers
Key Clustering
2 different keys generate the same cipher text
Pseudo-Random Number Generators (PRNGs)
repeats the random numbers after a given time (periodic); fast, but deterministic
True Random Number Generators (TRNGs)
generates a true random number, and uses some form of random process; usually used by
crypto, lotteries, games etc.
symmetric encryption advantages
Less computationally intensive • Produces a smaller file size • Allows for faster transmissions
symmetric encryption disadvantages
Exchanging of the shared secret key • Trust between parties sharing the key • Management of
keys: n(n − 1)/2 • No authentication or non-repudiation
binary XOR
If they are the same, the result is "0" • If they are different the result is "1"
binary AND
will always produce a 1 output if both of its inputs are 1 and • will produce a 0 output if one
or both of its inputs are 0
binary OR
, will always produce a 1 output if either of its inputs are 1 and will produce a 0 output if both
of its inputs are 0
keys
tells the algorithm how to work; should be longer than the msg
DES
64 bit block; 56 bit key; 16 rounds; Feistel structure
3DES
64 bit block; 168 bit key; 48 rounds
Electronic Code Book (ECB)
weakest DES mode; no IV or error propagation; encrypts each group with the same key,
leaving each group independent of the others; ciphertext won't change for the same plaintext
used in diff blocks
cipher block chaining (CBC)
DES mode that depends on cipher text from prior block to encrypt next block; contains IV
and error propagation
cipher feedback (CFB)
DES mode that is a stream, similar to CBC; real-time encryption; self-synching
output feedback mode (OFB)
DES mode that's just like CFB, but no error, propagation because the seed value is used
before it becomes ciphertext or the XOR is applied
counter mode (CTR)
Verified Correct Answers
Key Clustering
2 different keys generate the same cipher text
Pseudo-Random Number Generators (PRNGs)
repeats the random numbers after a given time (periodic); fast, but deterministic
True Random Number Generators (TRNGs)
generates a true random number, and uses some form of random process; usually used by
crypto, lotteries, games etc.
symmetric encryption advantages
Less computationally intensive • Produces a smaller file size • Allows for faster transmissions
symmetric encryption disadvantages
Exchanging of the shared secret key • Trust between parties sharing the key • Management of
keys: n(n − 1)/2 • No authentication or non-repudiation
binary XOR
If they are the same, the result is "0" • If they are different the result is "1"
binary AND
will always produce a 1 output if both of its inputs are 1 and • will produce a 0 output if one
or both of its inputs are 0
binary OR
, will always produce a 1 output if either of its inputs are 1 and will produce a 0 output if both
of its inputs are 0
keys
tells the algorithm how to work; should be longer than the msg
DES
64 bit block; 56 bit key; 16 rounds; Feistel structure
3DES
64 bit block; 168 bit key; 48 rounds
Electronic Code Book (ECB)
weakest DES mode; no IV or error propagation; encrypts each group with the same key,
leaving each group independent of the others; ciphertext won't change for the same plaintext
used in diff blocks
cipher block chaining (CBC)
DES mode that depends on cipher text from prior block to encrypt next block; contains IV
and error propagation
cipher feedback (CFB)
DES mode that is a stream, similar to CBC; real-time encryption; self-synching
output feedback mode (OFB)
DES mode that's just like CFB, but no error, propagation because the seed value is used
before it becomes ciphertext or the XOR is applied
counter mode (CTR)
