The Caesar Cipher
Historically, most books explain that Caesar used a shift of 3. It is also likely that Caesar used only upper case letters in his messages though I've never seen any notes on actual messages. This is perhaps the weakest of the substitution ciphers because it only has 26 different keys.
The program is pretty straight forward and it retains both upper and lower case as well as punctuation. Unlike the core/Rot13 program, this one has a variable shift key and it allows encrypt and decrypt modes.
Below is a ciphertext example. You'll have to forgive my sense of humor.
Vjku ku c vguv qh vjg uwrgt ugetgv ekrjgt.
Vjku ku qpna c vguv. Kh vjku jcf dggp cp cevwcn
gxgpv, uqogqpg yqwnf dg ncwijkpi jauvgtkecnna
cv vjg rqqt ejqkeg qh gpetarvkqp.
There are a few modifications that are possible. Perhaps the most normal one would be conversion to upper case only. Following that would be to strip the punctuation and word divisions to create typical 5 letter groups. To be somewhat historic, the ciphertext should probably retain the word divisions at a minimum.
Note that if you use only upper case letters or only lower case letters, the program will retain that. If you omit punctuation or make 5 letter blocks then that will also be retained. Maybe the program doesn't do all the work, but it is plenty good enough to have some fun with.
Look for the letter frequency counter program to help crack this cipher. You could also try every letter of the alphabet and see which one worked.
Follow the github link to download files