Random Subsititution Cipher
Last week we learned Caesar cipher, and we saw that because the key only has 26 possibilities, it is quite easy to guess the key by brute force. This week we will look at another cipher in which the key has many possibilities and is harder to brute force. This cipher is called Random Substitution Cipher.
The key in random substitution cipher is a whole new alphabet in which each letter in the English alphabet is mapped to a randomly chosen letter in the new alphabet. The below table is an example.
Encryption using the above key should map "MATH IS NOT HARD" to the ciphertext "HFCR JY GZC RFLK". Decryption is done reversely: we just map the letters in the key back to the English alphabet.
Since the key is an alphabet instead of a number, there can be a large number of possibilities for the key. For instance, when mapping A in the English alphabet to a new letter, we can choose any of the letters in the English alphabet, so there are 26 possibilities. Next, when mapping B, we can choose from the remaining 25 letters. When mapping C, we can choose the remaining 24 letters, and so on. The total number of possible keys is 26 * 25 * 24 *... * 1, which is a very large number! This makes guessing the key by brute force impossible.
