The common drawback of all ciphers that had been used for thousands of years up until the 1970s was that they were all symmetric ciphers, meaning that the encryption and decryption keys were essentially the same. Whether it was the simple alphabet-shift cipher of Julius Caesar, or the complex Enigma Cipher of the Second World War, they all suffered from the common weakness that once an adversary learned how you were encoding your messages, they could decode them just as well as you. In order to make use of a symmetric cipher, the communicating partners needed to exchange the cipher key in a secure way.