Question

More specifically, I put a breakpoint in my code after
NSString *signStr = [NSString stringWithCString:text encoding:NSASCIIStringEncoding];
I am calling the function with secret= @"12345678901234567890" and movingFactor = 1
I noticed that at the breakpoint, signStr is an empty string,
also are there any other noticeable errors in the code?
Thanks

-(NSString*) generateOTP:(NSString*) secret with:(uint64_t) movingFactor
{
    NSString* result;

    const int DIGITS = 6;
    const int SIX_DIGITS = 1000000;

    char text[8];
    for (int i = 7; i >= 0; i--) {
        text[i] = movingFactor & 0xff;
        movingFactor >>= 8;
    }

    NSString *signStr = [NSString stringWithCString:text encoding:NSASCIIStringEncoding];

    NSString* hash = [self sha1UseKey:secret toSign:signStr];

    int offset = [hash characterAtIndex:[hash length] - 1] & 0xf;

    int binary = (([hash characterAtIndex:offset] & 0x7f) << 24) |
        (([hash characterAtIndex:offset+1] & 0xff) << 16) |
        (([hash characterAtIndex:offset+2] & 0xff) << 8) | ([hash characterAtIndex:offset+3] & 0xff);

    int otp = binary % SIX_DIGITS;

    result = [NSString stringWithFormat:@"%d", otp];

    NSString* zero = @"0";
    while ([result length] < DIGITS) {
        result = [zero stringByAppendingString:result];
    }
    return result;

}

Answer 1

To be a “C string” it has to be terminated by a zero ('\0'). If movingFactor is 1 then the last character will be 0x01, and the first (and every other byte) is zero due to shifting. This makes the first character zero, which makes the string “empty”.