Generating an X-AvoSig string for signing API requests for Avocado

gistfile1.m

// Requires Security.framework on Cocoa and Cocoa-Touch
#import <CommonCrypto/CommonDigest.h>

- (NSString *)avoSigFromUserEmailHash:(NSString *)userEmailHash developerKey:(NSString *)developerKey developerId:(NSString *)developerId
{
    
    NSString *combo = [userEmailHash stringByAppendingString:developerKey];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    const char *combined = [combo UTF8String];
    CC_SHA256(combined, [combo lengthOfBytesUsingEncoding:NSUTF8StringEncoding], result);
    
    NSData *resultData = [NSData dataWithBytes:result length:CC_SHA256_DIGEST_LENGTH];
    NSString *hash = [self hexadecimalStringFromNSData:resultData];
    return [NSString stringWithFormat:@"%@:%@", developerId, hash];
}



// From: http://stackoverflow.com/questions/1305225/best-way-to-serialize-a-nsdata-into-an-hexadeximal-string
- (NSString *)hexadecimalStringFromNSData:(NSData *)data {
    /* Returns hexadecimal string of NSData. Empty string if data is empty.   */
    
    const unsigned char *dataBuffer = (const unsigned char *)[data bytes];
    
    if (!dataBuffer) {
        return [NSString string];
    }
    
    NSUInteger          dataLength  = [data length];
    NSMutableString     *hexString  = [NSMutableString stringWithCapacity:(dataLength * 2)];
    
    for (int i = 0; i < dataLength; ++i) {
        [hexString appendString:[NSString stringWithFormat:@"%02lx", (unsigned long)dataBuffer[i]]];
    }
    
    return [NSString stringWithString:hexString];
}