A puzzle, spies ... and a beheading (5)

Private keys...public keys

What is Emma's method to get round their problems of messages being intercepted and read? Their main weakness was that they had to send the key as well as the locked message - if the key was intercepted then the lock was worthless. The alternative way that involves not sending keys anywhere is the following. Suppose Emma wants to send a message to Amy. She first asks Amy to post her notebook (without the key but left open). Emma writes the message in Amy's book then snaps it locked shut and posts it back. Amy who has kept the key safe all along opens it secure in the knowledge that the key has never left her possession. This is essentially the same as a method known by computer scientist's as public key encryption - the method used on the internet for secure message exchange that allows the Internet to be secure. In this scheme, keys come in 2 halves a "private key" and a "public key". Each person has a secret "private key" of their own that they use to read all messages sent to them. They also have a "public key" that is the equivalent to Amy's open padlock. If someone wants to send me a message, they first get my public key - which anyone who asks for it can have. It is used to encrypt the message (close the padlock), but is no use to decrypt it (reopen the padlock). Only the person with the private key (the key to the padlock) can get at the message. So messages can be exchanged without the important decryption key going anywhere. It remains safe from interception.

Would this have helped Mary? No. Her problem was not in exchanging keys but that she used a method of encryption that was easy to crack - in effect the lock itself was not very strong and could easily be picked. Walsingham's code breakers were better at decryption than Babington was at encryption.