I like TEA (or XTEA or XXTEA).  Not quite as small as RC4 but still pretty compact.<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tiny_Encryption_Algorithm" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Tiny_Encryption_Algorithm</a>