swift ----- md5加密

func md5() ->String!{
    let str = self.cString(using: String.Encoding.utf8)
    let strLen = CUnsignedInt(self.lengthOfBytes(using: String.Encoding.utf8))
    let digestLen = Int(CC_MD5_DIGEST_LENGTH)
    let result = UnsafeMutablePointer<CUnsignedChar>.allocate(capacity: digestLen)
    CC_MD5(str!, strLen, result)
    let hash = NSMutableString()
    for i in 0 ..< digestLen {
        hash.appendFormat("%02x", result[i])
    }
    result.deinitialize()
    return String(format: hash as String)
}