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22 Cards in this Set
- Front
- Back
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Outline a method that might be used to attack Monoalphabetic Substitution |
Brute force but very difficult since the number of possible keys is 26! = 4*10^26 |
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What is transposition cipher? |
A method of encryption by which the positions held by units of plaintext are shifted according to a system, so that the ciphertext constitutes a permutation of the text |
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Give an example of a transposition cipher |
Plaintext: "attack at midnight" Rail fence technique: A t c a m d i h t a k t i n g t Ciphertext: Atcamdihtaktingt |
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Modern encryption methods involve transformations using eXclusive OR. What is the main benefit of this? |
Speed is the main benefit of using XOR |
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Describe the operation of a single round of a Feistel Cipher |
1) Partitions block into two halves, L0 and R0 2) Processes through n rounds 3) Divide key into n subkeys, one per round 4) At each round i: 4.1) Perform substitution on current left half Li-1 by XORing it with F(Ki, Ri-1) 4.2) F is known as the round function 4.3) Swap halves |
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Give 3 examples of Feistel Ciphers |
1) DES 2) Triple DES 3) IDEA |
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List the 5 elements that define a Feistel Cipher |
1) Block size 2) Key size 3) Subkey generation algorithm 4) Number of rounds 5) The round function, F |
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The AES was selected in 2001 from several algorithms that met the minimum requirements laid down. List 5 of the criteria that were used to select from these qualified algorithms? |
1) Speed 2) Small memory requirement 3) Security 4) Simplicity 5) Suitability for both hardware and software |
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Give an overview of the operation of the DES algorithm |
1) A Data Encryption Standard made in 1976 2) Block size is 64 bits 3) Key size is 56 bits 4) There are 16 number of rounds 5) Feistel cipher with additional Initial and Final Permutations |
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Give an overview of the operation of the AES algorithm |
1) Advanced Encryption Standard made in 2001 2) Chosen by design competition 3) Requirements are 3.1) Algorithm in public domain 3.2) Royalty-free 3.3) Block size is 128 bits 3.4) Key sizes are 128, 192, 256 bits |
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Explain what is Triple DES |
Similar to DES but key length is 168 bits unlike DES which has 56 bits |
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How does Triple DES differ from DES? Use a diagram |
Uses three keys and three executions of DES. |
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Compare the 3DES encryption algorithm to the AES encryption algorithm |
AES is a much faster encryption algorithm than 3DES and also 3DES runs into problems when encrypting 32 gb of data or more with a single key whereas AES has the limit much higher |
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Explain precisely how a stream cipher such as RC4 can be broken by a known plaintext attack if the same stream key is used more than once |
If plaintext-ciphertext pairs can be gathered, then it is easy to record the keystream
For example: Say we have a known plaintext-ciphertext pair (Ci, Pi) We can easily find the stream key used by Ci = Pi + Ki Pi (+) Ci = Pi (+) Pi (+) Ki Pi (+) = Ki
Thus the cipher is broken if there's any way to predict key stream for the next cipher |
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What are the drawbacks of using a block cipher in Electronic Codebook (ECB) mode? |
Identical plaintext blocks are encrypted into identical ciphertext blocks therefore it does not hide data patterns aswell |
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How does Cipher Block Chaining (CBC) work? Use a diagram |
Each plaintext block is XORed with the previous ciphertext block before being encrypted |
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Explain how a Counter mode (CTR) cipher works. |
Each plaintext block is XORed with an encrypted counter |
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What are the advantages of Counter Mode (CTR) cipher over chaining modes such as Cipher Block Chaining? |
1) Calculations can be done in parallel
2) Can do pre-processing
3) Allows random access |
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Explain how Cipher Feedback (CFB) mode allows a block cipher that can be used as a stream cipher |
Ciphertext resulting from previous encryption is used in encryption and s bits of the result are XORed with s bits of plaintext |
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How does link encryption differ from end-to-end encryption? |
Link encryption has each pair of connected nodes sharing a key while an end-to-end encryption has only the source and destination assigned have the key |
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Give one scenario where you might recommend link encryption over end-to-end encryption |
WPA uses |
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Give one scenario where you might recommend end-to-end encryption over link encryption |
TLS/SSL uses |
