What is a running key in the context of encryption?

In the context of encryption, a running key refers to a method of generating a key used for encryption that is derived from a predetermined source, often a book or a text. This running key system typically involves using a sequence of text characters to create a numerical key that can be applied to the plaintext using various cipher techniques, such as Vigenère cipher.

Using a book as a source for the key allows for an extensive range of characters and increases the complexity of encrypted messages, making it more difficult for an unauthorized party to decipher them without knowledge of the specific text used. This method fundamentally relies on the uniqueness and variation of the book's content, giving rise to a key that changes depending on the passage chosen.

The other options describe different aspects of encryption or cryptographic concepts. For example, a fixed-size key pertains to symmetric algorithms that use a consistent key length for encryption, while hash functions are one-way functions used for integrity checks rather than encryption. A transposition method is a type of encryption technique that rearranges the characters of the plaintext but does not relate directly to the idea of a running key.