hex.swift

    private func stringFromHexString(string:String) -> String? {
        guard (string.length % 2) != 0 else { return nil }

        var hex     = string
        var result  = ""
    
        while(hex.length > 0) {
            let advance     = hex.startIndex.advancedBy(2)
            let substring   = hex.substringToIndex(advance)
            hex             = hex.substringFromIndex(advance)

            var char:UInt32 = 0

            NSScanner(string: substring).scanHexInt(&char)
            
            result = result.stringByAppendingString(String(format:"%c", char))
        }

        return result
    }