How to give hex string to Hmac SHA256 objective C? - ios

Question

I am trying to generate hmac SHA256 in objective C in the following way:
const char *cKey = [key cStringUsingEncoding:NSUTF8StringEncoding];
const char *cData = [data cStringUsingEncoding:NSUTF8StringEncoding];
unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);
This works fine as long as the key is a string. Problem is on the external server command used to generate mac is:
openssl dgst -binary -sha256 -mac HMAC -macopt hexkey:$key
There hmac is getting generated considering key as hexkey . So obviously hmac generated is different.
How do i tell the CCMac function in objective C to consider the key as hex key? I already tried converting the string key to byte array and passing it to CCMAC but still didn't work.
In android I have achieved the same by converting the hex number to a Big Integer and then doing getBytes on it, use it to create the secret key.
Hope some of you will be able to guide me.

Answer 1

Is this what you're looking for?
- (NSData *)dataFromHexString:(NSString *)sHex {
const char *chars = [sHex UTF8String];
int i = 0;
NSUInteger len = sHex.length;
NSMutableData *data = [NSMutableData dataWithCapacity:len / 2];
char byteChars[3] = {'\0','\0','\0'};
unsigned long wholeByte;
while (i < len) {
byteChars[0] = chars[i++];
byteChars[1] = chars[i++];
wholeByte = strtoul(byteChars, NULL, 16);
[data appendBytes:&wholeByte length:1];
}
return data;
}
- (NSData *)hmacForHexKey:(NSString *)hexkey andStringData:(NSString *)data
{
NSData *keyData = [self dataFromHexString:hexkey];
const char *cKey = [keyData bytes];
const char *cData = [data cStringUsingEncoding:NSUTF8StringEncoding];
unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
CCHmac(kCCHmacAlgSHA256, cKey, keyData.length, cData, strlen(cData), cHMAC);
return [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
}
- (NSData *)hmacForKey:(NSString *)key andStringData:(NSString *)data
{
const char *cKey = [key cStringUsingEncoding:NSUTF8StringEncoding];
const char *cData = [data cStringUsingEncoding:NSUTF8StringEncoding];
unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);
return [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
}
- (void)testIt {
NSString *key = #"This is my random key.";
NSString *hexKey = #"54686973206973206d792072616e646f6d206b65792e";
NSString *data = #"This is a data string.";
NSData *hmac1 = [self hmacForKey:key andStringData:data];
NSLog(#"hmacForKey : %#", hmac1);
NSData *hmac2 = [self hmacForHexKey:hexKey andStringData:data];
NSLog(#"hmacForHexKey: %#", hmac2);
}