Just use the base64 program from the coreutils package:

echo QWxhZGRpbjpvcGVuIHNlc2FtZQ== | base64 --decode

Or, to include the newline character

echo `echo QWxhZGRpbjpvcGVuIHNlc2FtZQ== | base64 --decode`

output (includes newline):

Aladdin:open sesame