after completing all challenges, you get an encoded message like `FU4QGg8CFgMSUkwOFRNXQgsIF0hAQVQTDhkAUVRTRVVJSVlPSwQABAQQAVFRExwQSQwFCQMTBwNG VVYUEl1eUxwMBwYODRZXTVVLVVZcWVUYDA4KAhVUUFtVS0FbWF9TBQwHSEBBVAIAFw5dQUcXEFRJ RBwNBxZXTVVLUlpbFxBUSUQYBQ9SVxw=`

The = at the end can indicates the message is base64-encoded.

I eventually found the key to be my account name (the one displayed as the user in google foobar).
Found the code [here](https://www.oasys.net/posts/google-foobar-programming-challenge/#for-your-eyes-only)