The Kamasutra cipher is one of the earliest known substitution methods. It is described in the Kamasutra around 400 BC. The purpose was to teach women how to hide secret messages from prying eyes.
The key is the permutation of the alphabet. The plaintext and the ciphertext alphabet are the same. The alphabet is divided in two halves to pair the letters:
F Y M Q G V O P D J R A K
The letter “F” becomes the letter “C” and “B” is replaced by “G”. The word "EXAMPLE" would be encoded by: "MVLESAM".
The Kamasutra cipher can easily be cracked with a frequency-analysis.