The docs currently read: "A key’s length must be between 6 and 1024 characters and may only contain characters in the base64 set." This can lead to frustration, because mongodb won't handle the '=' character gracefully. '=' is used as a padding character to round out base 64 encodings that don't round to x % 3 chars.

Since '=' occurs in most base64 encodings, I believe we should mention that it is an invalid character for keyFiles.