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];

Answer 3

Here is a solution that pads the Base-64 string appropriately and works in iOS 4+:

NSData+Base64.h

@interface NSData (Base64)

/**
 Returns a data object initialized with the given Base-64 encoded string.
 @param base64String A Base-64 encoded NSString
 @returns A data object built by Base-64 decoding the provided string. Returns nil if the data object could not be decoded.
 */
- (instancetype) initWithBase64EncodedString:(NSString *)base64String;

/**
 Create a Base-64 encoded NSString from the receiver's contents
 @returns A Base-64 encoded NSString
 */
- (NSString *) base64EncodedString;

@end

NSData+Base64.m

@interface NSString (Base64)

- (NSString *) stringPaddedForBase64;

@end

@implementation NSString (Base64)

- (NSString *) stringPaddedForBase64 {
    NSUInteger paddedLength = self.length + (self.length % 3);
    return [self stringByPaddingToLength:paddedLength withString:@"=" startingAtIndex:0];
}

@end

@implementation NSData (Base64)

- (instancetype) initWithBase64EncodedString:(NSString *)base64String {
    return [self initWithBase64Encoding:[base64String stringPaddedForBase64]];
}

- (NSString *) base64EncodedString {
    return [self base64Encoding];
}

@end

Answer 4

A Swift version of Paul's answer

func paddedBase64EncodedString(encodedString: String) -> String
{
    let encodedStringLength = encodedString.characters.count
    let paddedLength = encodedStringLength + (4 - (encodedStringLength % 4))
    let paddedBase64String = encodedString.stringByPaddingToLength(paddedLength,
                                                                    withString: "=",
                                                                    startingAtIndex: 0)

    return paddedBase64String
}

Answer 5

I faced the same issue, but resolved it by adding == at end of string

base64UserStr = NSString(format: "%@%@", base64UserStr,"==") as String
let decodedData = NSData(base64EncodedString: base64UserStr, options: NSDataBase64DecodingOptions.init(rawValue: 0))


if (decodedData != nil)
{
    let decodedString = NSString(data: decodedData!, encoding: NSUTF8StringEncoding)

    print("Base 64 decode string is \(decodedString)")
}

This will definitely work.