RC4 in (almost) a tweet

rc4.js

function*r(k){t=x=>(y=x%n)in s?s[y]:y;w=_=>t((s[i%n]=t(j+=f=t(i++)))+(s[j%n]=f));for(n=256,s=[i=j=0];i<n;)w(j+=k[i%k.length]);for(j=0,i=1;;)yield w()}

// A raw RC4 stream.
// Works in Firefox only (needs lots of ES6).
// 150 chars

// Usage:
for (var b of r([65,65,65])) {
  console.log(b);
}