You are given a string very likely that it's base64 of some data (its length is divisible by 4 and last character is '=' sign). So:

$ base64 --decode > file
QlpoOTFBWSZTWTxSmOAAAAsJAF/gOwAgADEAAAiZMNT0JbKzhCQcyQtA2gNbvXgSvxdyRThQkDxS<span>mOA=
$ cat file
BZh91AY..(binary file)

Google BZh91AY, find bz2

$ bzip2 -d file
bzip2: Can't guess original name for file -- using file.out
$ cat file.out
9afa828748387b6ac0a393c00e542079

The key is: </span>9afa828748387b6ac0a393c00e542079