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Quiz 6 – statistics
The entropy or average information of an image is a measure of the degree of randomness
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in the image.
Watermark / Entropy Question class average: 0.61 / 2 [30.5%]
In order to exchange confidential messages with each other, Alice and Bob use an encryption system/software in which every new message is encrypted using a different symmetric key.
Trudy is able to capture/sniff these messages, and she also wants to be able to decrypt them.
If Trudy is in the position to conduct (only) one of the following types of attacks on the given system, which one should she choose to conduct in order to achieve her objective in the shortest period of time?
2) chosen plaintext
3) chosen ciphertext
4) chosen text
1) ciphertext only
Appropriate only in systems where the same key is used multiple times …
You have captured a ciphertext that was exchanged between Alice and Bob.
You have also learned that:
– The given ciphertext is produced using Vigenere cipher;
– The encryption key used is a 5-character sequence that starts and ends with letter ¡®A¡¯. – The plaintext corresponding to the captured ciphertext starts with word ¡®HELLO¡¯.
If the first word of the captured ciphertext is ¡®HFNMO¡¯, what was the encryption key used by Alice and Bob?
Plaintext: H E L L O Key: ABCBA Ciphertext: H F N M O
In which of the following situations the use of Frequency Analysis would be an appropriate approach to finding the plaintext?
1) a short plaintext was encrypted using a simple substitution cipher (e.g., Caesar)
3) a short plaintext was encrypted using a complex substitution cipher (e.g., Vigenere)
4) a long plaintext was encrypted using a complex substitution cipher (e.g., Vigenere)
2) a long plaintext was encrypted using a simple substitution cipher (e.g., Caesar)
Frequency Analysis is only appropriate for plaintexts/ciphertexts that contain a significant number of characters/letters, and their distribution/histogram matches the overall character/letter distribution of the respective language …
In class we have discussed the operation of a symmetric-encryption system with a Key Distribution Center (KDC).
Consider such a system with 6 users, as shown in the below figure. Currently, in this system, Alice is in the process of exchanging confidential messages with Bob, and Ted is in the process of exchanging confidential messages with Betsy.
What is the overall number of symmetric keys that the KDC of this system needs to know/store at this particular point in time? (I.e., what is the number of symmetric keys in use?)
1) 2 2) 4 3) 6
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