[tlunch]

What is RC4?

[pcwalton]

A formerly-popular cipher that works better than you'd expect given its extreme simplicity--simple enough that you can encrypt and decrypt it by hand with a deck of cards.

Nowadays it's insecure though, so you should never use it.

[andy81]

Old encryption algorithm, don't use it.

Infamous for "kerberoasting", among other exploits.

[beardyw]

Is that - don't use it ... with playing cards?

[Tomte]

A stream cipher.