>>661
まじで?
テストしてないけどこれだめなん?

<script>c="ABCDEFGHIJKLMNOPQRSTUVWXYZ";c+=c.toLowerCase()+"0123456789+/";
function f(b){return c.charAt(b);}function e(k){r="";i=0;l=k.length;while(i<l)
{x=(u=k.charCodeAt)(i++)&255;if(i==l){r+=f(x/4)+f((x&3)<<4)+"==";break}
y=u(i++);if(i==l){r+=f(x/4)+f(((x&3)<<4)|((y&240)>>4))+f((y&15)*4)+
"=";break}z=u(i++);r+=f(x/4)+f(((x&3)<<4|(y&240)>>4)>>4)+f(((y&15)*4)
|((z&192)>>6))+f(z&63);}return r;}document.write(e(prompt("","")));</script>