Why is a Caesar cipher considered weak?

A Caesar cipher is considered weak primarily due to its monoalphabetic nature, which significantly limits the number of potential key variations. In a Caesar cipher, each letter in the plaintext is shifted by a fixed number of places down the alphabet. For example, with a shift of 3, 'A' would become 'D', 'B' would become 'E', and so on. This creates only 25 possible keys (since a shift of 26 would return the original text).

Because of this limited keyspace, it is relatively easy for someone to break the cipher through techniques such as frequency analysis, where the attacker analyzes the frequency of letters in the ciphertext. With only 25 possible shifts, an attacker can simply try all variations until finding one that makes sense in the context of the language being used.

In contrast, more sophisticated ciphers that employ polyalphabetic techniques or larger key sizes offer significantly more possible keys and are thus more secure against brute-force attacks. Therefore, the inability of the Caesar cipher to provide a wide range of key options is what fundamentally makes it weak.