The best known encryption practiced in antiquity is called the "Ceasar cipher," after Julius Caesar, who used it in some of his private letters. To write the ciphertext, take each letter of the plain text and shift it forward three places in the alphabet. A becomes D, B becomes E, etc. The recipient simply reverses the process. The cipher is described by Suetonius in his Life of the Deified Julius 56.

The 4th century BCE Greek military writer Aeneas Tacticus lists a great variety of ways in which secret messages can be passed in chapter 31 of his book How to Survive a Siege. Many of these tricks involve hiding a plaintext message in some inconspicuous place, such as stitched inside a sandal, on thin sheets of lead which are then rolled up and worn as jewelry, or tattooed onto a bald messenger's head who then let his hair grow out to cover it. A couple of his suggestions could be considered encryption, though. These include:

• Take a book or other text, go through it and mark the letters that make up your message with a dot. The recipient reads the dotted letters in sequence to find your message. Alternatively, write an innocuous letter of your own and mark the letters in it that make up your real message.
• Take an astragal (the knuckle bone of a sheep or goat) and bore a number of holes in it equal to the number of letters in the alphabet (24 for Ancient Greek). Mark one hole as the first letter of the alphabet; the rest of the holes in sequence around the bone are the rest of the letters. Take a thread and thread it through the holes corresponding to the letters of your message one by one. When your message is complete, wrap the rest of the thread around the bone so it looks like an ordinary ball of thread. The recipient unwinds the thread and notes down each letter as they pull the thread out of each hole. Reversed, the letters spell out your message.
• Write only the consonants of your message and substitute dots for the vowels, the number of dots corresponding to the place of the vowel in the alphabet.