If you have a key K[0 plaintextli]] L-1], then cyphertext[i] = XOR(K[i %L], a. H
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Question
If you have a key K[0 plaintextli]] L-1], then cyphertext[i] = XOR(K[i %L], a. How would you decrypt the ciphertext? b. Describe how you would attack the cipher (for a key length of 5, namely, 123XQ) using the following three attacks: i. known plaintext ii. known ciphertext iii. character frequency means. c. For some unknown key de-cipher the ciphertext below and determine the key. It is English text with punctuation. TgoVYgoVApqiEpqjAgoVVXXVCtUsJrXnHr i. Say which attack you used ii. describe the steps you took to determine the key.Explanation / Answer
A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream. Since encryption of each digit is dependent on the current state of the cipher, it is also known as state cipher. In practice, a digit is typically a bit and the combining operation an exclusive-or(XOR).
The pseudorandom keystream is typically generated serially from a random seed value using digital shift registers. The seed value serves as the cryptographic key for decrypting the ciphertext stream. Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: in some modes of operation, a block cipher primitive is used in such a way that it acts effectively as a stream cipher. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity. However, stream ciphers can be susceptible to serious security problems if used incorrectly (see stream cipher attacks); in particular, the same starting state (seed) must never be used twice.
The known-plaintext attack (KPA) is an attack model for cryptanalysis where the attacker has access to both the plaintext (called a crib), and its encrypted version (ciphertext). These can be used to reveal further secret information such as secret keys and code books. The term "crib" originated at Bletchley Park, the British World War IIdecryption operation
a ciphertext-only attack (COA) or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. While the attacker has no channel providing access to the plaintext prior to encryption, in all practical ciphertext-only attacks, the attacker still has some knowledge of the plaintext. For instance, the attacker might know the language in which the plaintext is written in or the expected statistical distribution of characters in the plaintext. Standard protocol data and messages are commonly part of the plaintext in many deployed systems and can usually be guessed or known efficiently as part of a ciphertext-only attack on these systems.
A substitution, or permutation, replaces a character of the input stream by a character from the alphabet set of the target ciphertext.
A transposition replaces a character from the original plaintext by another character of that same plaintext. This results in shuffling yet still preserving the characters of the original plaintext.
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