Caesar encryption is one of the earliest and simplest of encryption methods. The writer of the message and the recipient together devise a numeric "key" value between 1 and 25, which is then used to encrypt through shifting forward that many letters in the alphabet.

For example, If the key value is 4 and the message is "A," the encrypted version of the message would be "E."

When the recipient receives the message, s/he decrypts it by shifting the same number of letters backwards in the alphabet. "E" shifted backwards 4 spaces would then return the decrypted message of "A."

As another example, if the plaintext was "HELLO" and the key value was 3, the resulting encrypted message would be: "KHOOR"; to decrypt, you'd move back 3 spaces and recover the message "HELLO."

Pigpen encryption is a bit more complicated than the previous two. It involves a system of grids, dots, and in some cases, x's. In order to decypher the message, one must find the letter corresponding to the letter in the given grid, using the dots to determine which grid to look at, and the shape of the symbol to figure out which letter to choose. For example, if we were given the symbols:

using the key below:

We can see that the encrypted message says "HELLO"

One time pads work like Caesar ciphers, except that you generate a key for each letter in the message.

For example, if your message is HELLO and the keys are 1-6-3-4-2, the resulting encryption would be IKOPQ. Decryption works analogously, by shifting backwards in the alphabet instead of forwards.

Under perfect use, one-time pads are information-theoretically secure; that is, completely uncrackable, because the encrypted text reveals nothing about the message.

For example, an encrypted message "AA" could have been the message
AT (with keys 0-7) or IN (keys 18-13) or BY (keys 25-2) ... or, in fact, any two-letter word at all! Note that double letters (as the Ls in HELLO) are encrypted using different key values and so offer no extra information and that there is no simple way to try out all possible values to determine a likely one.

The downside is that one-time pads are hard to use perfectly. Perfect use requires the creation of a new set of key values for each letter in the message; the key values can never be reused; the key values are entirely randomly generated; and the key values remain a complete secret. Breaches in any of these conditions weakens the security of the one-time pad.