What does the Nihilist Cipher crib rule entail?

The Nihilist Cipher crib rule states that the crib, which is a piece of known plaintext used to help decrypt the cipher, must be longer than the keyword length. The rationale behind this rule is that for effective decryption, the crib should provide enough context and letters to offset potential ambiguities that arise from the keyword’s repetition and the inherent structure of the cipher.

In the Nihilist Cipher, the keyword is used to shift the letters of the plaintext based on their corresponding positions in the cipher alphabet. If the crib is shorter than or equal to the keyword length, it may not provide sufficient information to securely deduce the parts of the message, thus limiting the effectiveness of the decryption process. By ensuring the crib is longer than the keyword, it increases the chances of successfully aligning the known text with the correct shifts determined by the keyword, leading to a more effective decryption effort.

The option addressing the length of the crib relative to the keyword length correctly identifies the necessary relationship for effective application of the Nihilist Cipher crib rule.