Let's have a break.

I think we really do need a rest from politics.  I'm sure there will be plenty to say after Thursday, so I'm declaring an election-free blog until then.  So, what to talk about? Well, it did occur to me that I could have done one more post on my "It's a secret" series, so here it is.  You'll remember that we ended up with public-private-key cryptography, whereby you have a public key which you tell everybody about and which they can then use to encrypt a message to you.  But the message can only be decrypted by you using your private key.  But the system works just as well the other way round and so has another use.  Suppose I send a message to you and I want to convince you that the message has indeed come from me.  I can add something to my message which I encrypt using my private key. You - or for that matter anybody else - can decrypt it using my public key and as only I have my private key, it must have come from me.  This is known as a digital signature and has proved one of the most useful and used aspect of PPK.