Caesar

A Caesar cipher lets you add an arbitrary value, shifting each letter forwards or backwards. Traditionally, the offset is 3, making A into D, B into E, etc.

This is a standard Caesarian Shift cipher encoder, also known as a rot-N encoder. It's a style of substitution cipher where you can add one, two, or any number up to 25 to each letter and see how the string changes. This is an offshoot of the rot13 encoder. To perform this shift by hand, you could just write the alphabet on two strips of paper. Line them up so the top strip's A matches the bottom strip's D (for example) and then you can encode. A simple test to see how this works would be to use the alphabet as the input and see the output while changing the value of N.

This sort of cipher is also known as a wheel cipher. An inner wheel has the alphabet around its edge, and that is placed upon an outer wheel, also with the alphabet going around it. You can rotate the wheels so that ABC lines up with any series of letters, such as BCD or even one much further along, like QRS. Caesar originally shifted by 3 letters, so A -> D, B -> E, and so on, which was good enough for that time.

To encode something, just pick an N and type in your message. To decode something, set N to 26 minus the encryption N and it should be decoded for you. Alternatively, the cryptogram solver can help you manually solve ciphers that use this method.

You can make the cipher more complicated by shuffling the alphabet with a key. Performing this by hand would be nearly the same as with a plain Caesar alphabet, but you would write the scrambled alphabet instead of the sorted alphabet on both the outer and inner wheel rings (or top and bottom pieces of paper).

Examples:

That last one is the first step of the Cicada 3301 puzzle of January 2012. The image posted to 4chan had this text stuck on the end of the file, under the heading TIBERIVS CLAVDIVS CAESAR says, which is about as broad a hint as a puzzle ever gives.

The letters come out as a URL and the punctuation does not, because 3301 rotated an alphabet that included the digits and the symbols. A Caesar shift works on whatever alphabet you agree on beforehand; this page rotates the letters and leaves everything else alone, which is the usual convention and is why :// is still >33 in the output.