Provides HKDF with sha256 as hash function

hdkf.m

#import <CommonCrypto/CommonCrypto.h>
// ...

NSData * HKDF_SHA256(NSData *seed, NSData *info, NSData *salt, int outputSize) {
    char prk[kCCHmacAlgSHA256] = {0};
    CCHmac(CC_SHA256_DIGEST_LENGTH, [salt bytes], [salt length], [seed bytes], [seed length], prk);
    
    int             iterations = (int)ceil((double)outputSize/(double)CC_SHA256_DIGEST_LENGTH);
    NSData          *mixin = [NSData data];
    NSMutableData   *results = [NSMutableData data];
    
    for (int i=0; i<iterations; i++) {
        CCHmacContext ctx;
        CCHmacInit(&ctx, kCCHmacAlgSHA256, prk, CC_SHA256_DIGEST_LENGTH);
        CCHmacUpdate(&ctx, [mixin bytes], [mixin length]);
        if (info != nil) {
            CCHmacUpdate(&ctx, [info bytes], [info length]);
        }
        
        unsigned char c = i+1;
        CCHmacUpdate(&ctx, &c, 1);
        
        unsigned char T[CC_SHA256_DIGEST_LENGTH];
        memset(T, 0, CC_SHA256_DIGEST_LENGTH);
        CCHmacFinal(&ctx, T);
        NSData *stepResult = [NSData dataWithBytes:T length:sizeof(T)];
        [results appendData:stepResult];
        mixin = [stepResult copy];
    }
    
    return [NSData dataWithData:results];
}

line 5 should be char prk[CC_SHA256_DIGEST_LENGTH] and line 6 CCHmac(kCCHmacAlgSHA256... maybe a typo