OpenPGP: How does PGP work?

How does it work

The magic, and I call it magic because I freely admit I do not have the mathematical background to explain it better, of this system is that if you encrypt something using the Public-key only the Private-key can decrypt it and vice versa. So there is no way for someone holding the Public-key to decrypt something encrypted using the Public-key, only the Private-key will decrypt it. The same is true in reverse. If something is encrypted using the Private-key only the Public-key can decrypt it again – in practice you won’t have a problem here, because if you hold the Private-key you already hold the Public-key as well.

Now when I write an email and want to sign it PGP looks at the message or file (for simplicity I’ll stick to email as my example) then runs a mathematical hash such as SHA256. A hash is a one way process. If you hash a block of text, using SHA256, you will get a string of what appears to humans as gibberish. The important part is, it is always the same. No matter how many times you run the same block of text through the SHA256 algorithm you will always get the same gibberish. PGP then uses my Private-key to encrypt that hashed result and includes that ether as an attachment to the email or at the bottom of the body.

To verify the integrity of a email the receiving PGP aware application uses the Public-key to decrypt the attached signature and reads the included hash. At this point you have already verified the signature was created using the Private-key because if it had been altered in any way after encryption the Public-key would no longer work. The next step is for the receiving copy of PGP to run the email through the same hash as before, SHA256, and then compare the hash encrypted in the email with the hash it just created. If the two match the email has been verified and you can be sure it has not been altered in transit.

How about encrypted messages

The process for full message encryption is slightly different. The problem with Public-key cryptography is it is incredibly expensive in computational power and CPU time and far large messages it is impractical to encrypt the whole message using a Private-key, so instead we use Symmetric-key encryption. Unlike Public-key encryption Symmetric-key encryption uses the same key to encrypt and decrypt a message.

So now when I send an encrypted message PGP signs the message in the same way detailed above then generates a large random password then uses this to encrypt the message. Now we have an encrypted block of text and a key to decrypt it again and we have to get both to the recipient without the decryption key becoming public, so we call on Public-key cryptography again. Using the recipients Public-key we can no encrypt our generated Symmetric-key and include it in the email header. At the other end the recipient uses their Private-key to decrypt the start of the email then can use the Symmetric-key we provided them to decrypt the message. This actually allows you to send the same email to multiple recipients as well, all we have to do is use the public-key of each person to encrypt a copy of the Symmetric-key.

Why Isn’t It Used More

PGP key management is hard work. Generating key, managing them and adding support to email applications that do not already support them is not for the faint hearted and the process is quite geeky. So while support is there its not easy to use, in simple terms it doesn’t yet pass the granny test.

I hope this will change in future and by signing the majority of my messages and writing these posts I would like to think that I can make it a little easier to get involved. Cryptography and PGP technique in particular is a subject I am interested in. I have given several talks to The Software Society (my local LUG) on the topic and plan to give another over the summer in the hopes of increasing awareness to my little corner of the universe.

If you have any questions or struggle to implement PGP in your own corner please drop me a line and I will do the best I can to help. Even if, like me, you see no reason to encrypt your emails the advantages of being able to sign your emails is huge deal in a world of spam and viruses being distributed by email – often appearing to come from an address you know.

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About the Author

Stuart McCulloch Anderson
For over a decade and a half Stuart has been in love with all things science fiction or technology and for almost fourteen of those years his operating system of choice has been one breed of Linux or another and despite some brief trips back into the world of Windows Stuart has never found him self wanting anything else.