I need a simple cryptographic code to put on a t-shirt

I never worked with cryptography but I wanted to add an easter egg on the design I'm making for a t-shirt. I need it to have a key that will be embroided on the front design wich is:

15122017

that can be used to decrypt the text that is in the back (still have to think what to put). I tried testing the encryption on Cryptii.com with the rc4 encryption and it worked well both ways but when I tried decoding that same result of the previous site on dcode.fr it didn't worked. I liked the way hexadecimal result looked but to be honest I'm really lost on how to make this work for every decoding website.

The problem is that dcode.fr expects the key to be ASCII ("SECRET"), while Cryptii.com expect hexadecimal (53 45 43 52 45 54). So in effect when you put the hex to both sites, dcode.fr interprets it as the ASCII string, not hex. If you convert the key from ASCII to/from hex, you can encrypt on one site and decrypt on the other. Also, some other sites display the encrypted message in base64 instead of hex.

As an example, the following sequence works:

1. In dcode.fr, type the message "This is a test!" and key "SECRET". After encrypting, you will get the hex output "C8 06 05 0C 2F 49 50 E8 83 46 C8 D6 12 76 C4".
2. Going to Cryptii.com, switch to a hex message and paste in "C8 06 05 0C 2F 49 50 E8 83 46 C8 D6 12 76 C4" and key "53 45 43 52 45 54" and it will decode to "This is a test!"

Unfortunately this means that unless your target audience understands how and when to do this sort of conversion, "work for every decoding website" isn't going to be feasible. I suspect you will have to choose one site, create your shirt appropriately, and somehow let your audience know the URL of the chosen site.