The One-time pad (OTP) is a stream cipher defined by its unique properties. It requires a pre-shared secret key that is truly random and at least as long as the plaintext message. The encryption process involves a modular addition of the plaintext with the key to produce the ciphertext. For binary data, this operation is equivalent to a bitwise XOR. When implemented correctly—meaning the key is random, used only once, and kept secret—the OTP provides perfect secrecy and is theoretically unbreakable. The question's description precisely matches the operational mechanism of the one-time pad.

A. Running key cipher: This cipher uses a long, non-random key, such as text from a book, making it susceptible to cryptanalysis, unlike a true one-time pad.
C. Steganography: This is the practice of concealing the existence of a message within another medium, not a method of encrypting its content with a key.
D. Cipher block chaining: This is a mode of operation for block ciphers, not a cipher itself. It uses a fixed-length key, which is not equal to the message length.

1. Katz, J., & Lindell, Y. (2021). Introduction to Modern Cryptography (3rd ed.). CRC Press. In Chapter 2, Section 2.1, "The One-Time Pad," it is stated: "Let the plaintext be a bit string of length ℓ. The key is a uniformly chosen bit string of the same length ℓ... Encryption is done by XORing the plaintext and the key." (p. 26).

2. Stallings, W. (2017). Cryptography and Network Security: Principles and Practice (7th ed.). Pearson. In Chapter 2, Section 2.3, "One-Time Pad," the author describes the cipher: "The one-time pad... uses a random key that is as long as the message... The encryption operation is the exclusive-OR (XOR)." (p. 41).

3. University of California, Berkeley. (2020). CS 161: Computer Security, Lecture 6: "Stream Ciphers & One-Time Pad". The lecture notes specify the requirements for a one-time pad: "Key is a truly random sequence of bits of the same length as the message... C = P ⊕ K". (Slide 18).