The main limitation of using a Caesar cipher is the small key size, which size is equal to that of the plaintext alphabet set. When using Caesar cipher on English letters, the key size is limited to 26, the key size is 26. An attacker can easily brute force such cipher by trying all 26 possible options for the key. And it will not take much time or computational effort to do so. With only 26 possible keys, Caesar Cipher is far from secure. A dramatic increase in the key space can be achieved by allowing an arbitrary substitution, where the alphabet mapping can be any permutation of the 26 alphabets. A permutation of a finite set of elements is an ordered sequence of all the elements of the set with each element appearing exactly once. In general, there are n! permutations of a set of n elements. Let's look at an example. Let's look at an example for a monoalphabetic cipher. And the key is given as it's shown here. Using this key, monoalphabetic cipher maps A to D, the alphabet B to the alphabet K, C to V, D to Q and so on for encryption. Using the same logic for the plaintext, MEETMELATER, monoalphabetic cipher looks at the key to determine the mapping for each letter. The letter M gets mapped to the letter C. The letter E gets mapped to the letter F. T to U and so on. And as a result, the encryption for the plain text MEETMELATER produces the ciphertext CFFUCFSDUFY. The decryption process reverses the encryption to take the ciphertext and generate the plain text. From the key, the letter C gets mapped to the letter M, the letter F to the letter E, U to T, S to L, and so on. As discussed previously, when given the plaintext alphabet size of n, the possible number of keys is n! Using English alphabets with n=26, this number becomes greater than 4 times 10 to the 26th power. This is a huge number for a brute force attacker to search through. To put it into perspective, such a key space size is ten orders of magnitude greater than the key space for data encryption standard, or DES, which is widely used nowadays While the key space may be big enough to prevent brute force attack, the monoalphabetic substitution cipher has a different vulnerability and is not secure. Monoalphabetic cipher does not sufficiently obscure the underlying language characteristics of the plain text. In the following lesson, we will discuss how the natural redundancy and bias in the plain text can be used for a cryptoanalyst breaking monoalphabetic cipher.