Question

I am getting following error when I am trying to use Convert.FromBase64String

“The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or a non-white space character among the padding characters.”

Dim payloadBytes = Convert.FromBase64String(payloadBase64)

Basically when my facebook registration form (http://developers.facebook.com/docs/plugins/registration/) phone field has a dash in it and encoded string is posted to other page and I am trying to decode it there which creates this error. Basically I am trying to extract data from Facebook Signed Request.

Answer 1

The issue is that the dash is not a valid character in the Base64String. Here is a quote from MSDN:

The base-64 digits in ascending order from zero are the uppercase characters “A” to “Z”, the lowercase characters “a” to “z”, the numerals “0” to “9”, and the symbols “+” and “/”. The valueless character, “=”, is used for trailing padding.

You can either take the dash out (which might not be what you want) or you need to figure out what format the data is truly coming in as since it doesn’t seem like it is Base-64 string data.

http://msdn.microsoft.com/en-us/library/system.convert.frombase64string.aspx

The issue is that the Facebook Signed Request is using a modified Base-64 request for URL that changes a few things. Here is a quote on what it does:

For this reason, a modified Base64 for URL variant exists, where no padding ‘=’ will be used, and the ‘+’ and ‘/’ characters of standard Base64 are respectively replaced by ‘-‘ and ‘_’, so that using URL encoders/decoders are no longer necessary and have no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general.

I believe you could solve your problem by simply replace the dash with a plus and replace the underscore with a backslash and you should be able to then decode it from Base-64.

Here is the link to the Facebook developers page that indicates that the value you are trying to decode is base64url encoded:

http://developers.facebook.com/docs/authentication/signed_request/