Facebook Signed Request for iOS (HMAC SHA256)

facebook, ios, iphone, objective-c, sha256

Solution

The problem was with the Base64 encoder. It needs to be encoded as Base64Url see: http://en.wikipedia.org/wiki/Base64#URL_applications

Here's the modifed base64EncodedString category method:

//NSData+Base64.h
 - (NSString *)base64EncodedString
 {
    size_t outputLength;

    char *outputBuffer = NewBase64Encode([self bytes], [self length], true, &outputLength);
    NSString *result = [[[NSString alloc] initWithBytes:outputBuffer length:outputLength encoding:NSASCIIStringEncoding] autorelease];
    free(outputBuffer);

     NSString *b64PayloadClean = [[result componentsSeparatedByCharactersInSet:[NSCharacterSet newlineCharacterSet]] componentsJoinedByString:@""];

     //do URL encoding by replacing "+" and "/" to "-" and "_" respectively
     b64PayloadClean = [b64PayloadClean stringByReplacingOccurrencesOfString:@"=" withString:@""];
     b64PayloadClean = [b64PayloadClean stringByReplacingOccurrencesOfString:@"+" withString:@"-"];
     b64PayloadClean = [b64PayloadClean stringByReplacingOccurrencesOfString:@"/" withString:@"_"];

    return b64PayloadClean;
 }

Problem

I've been trying to generate HMAC SHA256 for Facebook signed request on iOS for weeks now. I am desperate need of help. Facebook signed requests have two parts which are separated by a period. First part is an HMAC256 of the payload while the 2nd part is Base64 encoded string of the payload. I've only been able to recreate the second part. ``` vlXgu64BQGFSQrY0ZcJBZASMvYvTHu9GQ0YM9rjPSso.eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsIjAiOiJwYXlsb2FkIn0 ``` I've been using the following code that everybody uses but it's generating a different hash: ``` #import <CommonCrypto/CommonHMAC.h> #import "NSData+Base64.h" +(NSString*) hmacForSecret:(NSString*)secret data:(NSString*)data { const char *cKey = [secret cStringUsingEncoding:NSASCIIStringEncoding]; const char *cData = [data cStringUsingEncoding:NSASCIIStringEncoding]; unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH]; CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC); NSData *HMAC = [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)]; return [HMAC base64EncodedString]; } ``` According to Facebook docs: https://developers.facebook.com/docs/authentication/signed_request/ Correct HMAC256 output should be when using "secret" as the key: ``` vlXgu64BQGFSQrY0ZcJBZASMvYvTHu9GQ0YM9rjPSso ``` The payload to be encoded: ``` { "algorithm": "HMAC-SHA256", "0": "payload" } ``` NOTE: You can Base64 decode the 2nd part of the signed request to get this payload.

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