What typically defines the length of ciphertext in a running key encryption method?

In a running key encryption method, the length of the ciphertext is defined as being equal to the length of the key. This means that for each character of plaintext, a corresponding character from the key is used to perform the encryption operation, often through a mechanism such as a modular addition operation. This is crucial because if the key is shorter than the plaintext, it would need to be repeated or expanded, which can introduce vulnerabilities in the encryption process. Therefore, having the ciphertext length equal to the key length maintains the integrity and security of the encryption, as it ensures that each character of the plaintext is uniquely tied to a specific character of the key for the duration of the encryption, making it more resilient against analysis or attacks.

In this case, options implying the length of ciphertext would be shorter or longer than the plaintext, or fixed by the algorithm, do not accurately represent the operational principle of running key encryption. The critical point is that the ciphertext's length aligns with the key's length, ensuring that every segment of text has a corresponding secure mechanism for transformation without repetition or loss of information.