Needed to convert this java method to Objective-C. It turnes out that it's only doing starndard HmacSHA1. Don't ask how long it took me to figure it out.

some_hash_method.java

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

public static String someHashMethod(String data, String hash) {
    byte[] hashBytes = hash.getBytes();
    SecretKeySpec signKey = new SecretKeySpec(hashBytes, "HmacSHA1");

    Mac mac = Mac.getInstance("HmacSHA1");
    mac.init(signKey);

    byte[] rawHmac = mac.doFinal(data.getBytes());

    BigInteger number = new BigInteger(1, rawHmac);
    String returnHash = number.toString(16);

    if (returnHash.length() == 39) {
        returnHash = "0" + returnHash;
    }
    return returnHash;
}

the_objective_c_equivalent.m

#import	"GHNSString+HMAC.h"
[payload gh_hmacSha1:hashKey];