Kevin, a security team member, was instructed to share a policy document with the employees. As it was supposed to be shared within the network, he used a simple algorithm to encrypt the document that just rearranges the same characters to produce the ciphertext. Identify the type of cipher employed by Kevin in the above scenario.

A transposition cipher is a method of encryption where the positions of the characters in the plaintext are shifted or permuted according to a regular system to form the ciphertext. The characters themselves are not changed, only their order. The scenario explicitly states that Kevin's algorithm "just rearranges the same characters to produce the ciphertext," which is the defining characteristic of a transposition cipher. This method focuses on diffusion by rearranging the plaintext characters, contrasting with substitution ciphers that replace them.

A. Substitution cipher: This type of cipher replaces plaintext characters with different characters or symbols, rather than rearranging the original ones.
B. Stream cipher: This is a symmetric cipher that encrypts data one bit or byte at a time using a pseudorandom keystream, a fundamentally different mechanism than character rearrangement.
D. Block cipher: This cipher operates on fixed-size blocks of data, applying complex rounds of both substitution and permutation. It is not a simple rearrangement of characters.

1. Paar, C., & Pelzl, J. (2010). Understanding Cryptography: A Textbook for Students and Practitioners. Springer-Verlag Berlin Heidelberg. In Chapter 1, Section 1.3, "Classical Ciphers," it is stated: "In contrast to substitution ciphers, transposition ciphers do not replace symbols but change their position. That is, transposition ciphers perform a permutation on the letters of the plaintext." (p. 8).

2. Stallings, W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson. Chapter 2, Section 2.3, "Transposition Techniques," describes these ciphers as methods that "rearrange the letters of the plaintext to produce the ciphertext."

3. Katz, J., & Lindell, Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. Chapter 1, Section 1.3, "Historical Ciphers and Their Cryptanalysis," distinguishes between substitution ciphers and transposition ciphers, defining the latter as those that permute the letters of the plaintext. (p. 13).