NSData won't accept valid base64 encoded string

Question

I'm implementing JSON Web Token authentication on the iOS (7) cient-side. It's working nicely. My app rceives tokens, and can make authenticated calls to my server with them.

Now, I want my client side code to check for an expiration date on the token so it can know when to re-authenticate. Checking for the expiration date on a JWT auth token is straightforward. The authorization token is 3 base64 encoded JSON blobs, separated by a '.' - The expiration timestamp is in the middle blob, in a field called ext. It's seconds since unix epoch.

So my code's looking like so:

- (NSDate*) expirationDate {     if ( !_tokenAppearsValid ) return nil;      if ( !_parsedExpirationDate )     {         //         //  Token is three base64 encoded payloads separated by '.'         //  The payload we want is the middle one, which is a JSON dict, with         //  'exp' being the unix seconds timestamp of the expiration date         //  Returning nil is appropriate if no 'exp' is findable         //          NSArray *components = [self.token componentsSeparatedByString:@"."];          NSString *payload = components[1];          NSData* payloadJsonData = [[NSData alloc]             initWithBase64EncodedString:payload             options:NSDataBase64DecodingIgnoreUnknownCharacters];          NSError* jsonError = nil;         NSDictionary* payloadJson = [NSJSONSerialization JSONObjectWithData:payloadJsonData options:0 error:&jsonError];         if ( payloadJson )         {             if ( payloadJson[@"exp"] )             {                 NSTimeInterval timestampSeconds = [payloadJson[@"exp"] doubleValue];                 _expirationDate = [NSDate dateWithTimeIntervalSince1970:timestampSeconds];             }         }          _parsedExpirationDate = YES;     }      return _expirationDate; } 

The problem is simple. The middle base64 blob, when parsed by NSData -initWithBase64EncodedString is nil - and that's bad.

I've checked the base64 blob and it seems to be valid. My server's returning dummy data for the moment so here's an example blob: eyJlbWFpbCI6ImZvb0BiYXIuYmF6IiwiYWNjb3VudElkIjoiMTIzNDUtNjc4OTAtYmFyLWJheiIsImV4cCI6MTM5MDkxNTAzNywiaWF0IjoxMzkwOTE0MTM3fQ

It decodes to:

{"email":"[email protected]","accountId":"12345-67890-bar-baz","exp":1390915037,"iat":1390914137} 

I tested it here: http://www.base64decode.org

I've used NSData's base64 methods elswhere in my app with success - I don't think I'm doing anything particularly broken here. But I'm all ears! Any ideas?

Answer 1

Your Base64 string is not valid. It must be padded with = characters to have a length that is a multiple of 4. In your case: "eyJlbWFp....MTM3fQ==".

With this padding, initWithBase64EncodedString decodes the Base64 string correctly.

Answer 2

Although Martin's answer is correct, here is a quick and correct(!) way to fix the problem:

NSString *base64String = @"<the token>"; NSUInteger paddedLength = base64String.length + (4 - (base64String.length % 4)); NSString* correctBase64String = [base64String stringByPaddingToLength:paddedLength withString:@"=" startingAtIndex:0];