One of the earliest descriptions of encryption by substitution appears in the Kama-sutra, a text written in the 4th century AD by the Brahmin scholar Vatsyayana, but based on manuscripts dating back to the 4th century BC. The Kama-sutra recommends that women should study 64 arts, including cooking, dressing, massage and the preparation of perfumes. The list also includes some less obvious arts, including conjuring, chess, bookbinding and carpentry. Number 45 on the list is mlecchita-vikalpa, the art of secret writing, advocated in order to help women conceal the details of their liaisons. One of the recommended techniques involves randomly pairing letters of the alphabet, and then substituting each letter in the original message with its partner.