Digital Signatures: what are they, and how do they protect me?

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Summary: Digital Signatures use public key cryptography to validate both message sender and message contents. Digital Signatures rely on simple concepts you should know, built on top of very complex mathematics you don't need to worry about.

Digital Signatures are pretty much what they sound like: the digital equivalent of signing a document. If you digitally sign a message, it proves that you, and only you, actually did the signing.

In addition, it also allows the recipient of the message to confirm that the message wasn't changed after you signed it.

There is, of course, a catch. And we need to understand just a little about "public key cryptography".

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Don't let the phrase fool you. While public key cryptography is a really, really complex science, the concepts you need to know to use it are really, really simple, once you get your brain around one concept.

You're probably already aware of simple encryption - you take a file, run it through an encryption tool of some sort that takes a password, and the file is scrambled using that password. In order to read the file, you need to run the file through the decryption tool, and you must supply the correct password again. This is called "symmetric" encryption, because you use the same password to encrypt, and then to decrypt the file.

Enter asymmetric encryption. With this type of encryption, you use one password (now called a key) to encrypt your data, but then you use a different key to decrypt it.

Think about that for a moment, because it's not at all obvious.

That means that I might encrypt a file using a key, say "rubberduck", but in order to decrypt that file, you need to supply a different key, say "platypus". ("rubberduck" and "platypus" are just simple examples, and not actual, working, keys.)

Now, not just any two keys will do. These two keys go together. Only "platypus" can decrypt something that was encrypted using "rubberduck". And, very conveniently, the opposite works as well: if you encrypt something using "platypus", only "rubberduck" can decrypt it. These keys must be chosen, or rather, mathematically generated as a pair, in order for this relationship to hold. Together they are a key pair.

Here's where things get interesting.

What if I generated my pair of keys, and I keep one secret, and told the world the other one? Let's say I keep "rubberduck" a secret - no one knows that's the key that matches "platypus". But I tell world that my "public key" is "platypus". Here's what that enables:

• Someone can encrypt a message to me, using my public key ("platypus"), and know that only I can decrypt it. Because only I know that "rubberduck" is the matching key that will decrypt something encrypted with "platypus".

• I can encrypt a message using my private key ("rubberduck"), and anyone can decrypt it with my public key ("platypus"). Sounds pointless if anyone can decrypt it, until you realize that since they can, they know the message came from me. Only I have the private key that matches, only I can encrypt something that would be decryptable with my public key.

But so far, that's all been "encryption", not "signing". One more concept is required.

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We now need to understand something called a "hash". As a VERY simple example, let's say I assigned a value from 0 to 255 for every character in the alphabet, every symbol, and every number that might appear in a text message. Now, I take a message and I add up all the numbers corresponding to all the characters in that message. The resulting number is a "hash value" of that message.

Now, just adding up the numbers happens to be a very poor hash algorithm, because it's easy to get two different messages to generate the same hash value, and it's also easy to alter a message to get the hash value you want. Change one 'a' to a 'b', and anther 'z' to a 'y', and the hash of the message might well calculate to exactly the same value.

Enter the "sha" algorithm. This is a complex mathematical algorithm that has the following two properties:

• It's statistically extremely unlikely that an sha hash of two different messages would ever generate the same hash value

• It's effectively impossible to generate or alter a message to generate a specific, desired, sha hash value.

There are other hash algorithms besides sha - md5 being the most common.

Now let's pull this together for signing...

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After you've composed an email message, and you digitally sign it, this is what happens:

• an sha hash of your email message is calculated

• that sha hash is then encrypted using your private key

• the encrypted sha hash is then appended below your email message.

Your recipient can then validate your message by:

• decrypting the sha hash using your public key - if that works, they know the message came from you

• calculating the sha hash of the message, and comparing their result to the decrypted sha hash that you included as your signature. If the hash values match, they know that the message was not altered.

One question that typically comes up is this: why not just encrypt the entire message?

Absolutely you can do that. In fact, if you want to maintain absolute security about the contents of your message, that's exactly what you must do.

But for situations where that type of security is not required, digital signing of this form adds very little overhead, gets you the validation you want, and allows your recipient to still read the message immediately, without requiring decryption.

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Here's an example of a signed message:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

This is an example of signed email.

Leo
- ------------------------------------------------------------------------
Tech Questions? Get Answers:                          http://ask-leo.com
Suffering from Email Overload?                http://www.tamingemail.com
Sooner or later, the jokes stop here:    http://www.forwardedfunnies.com
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.3 (MingW32)

iD8DBQFErFtgCMEe9B/8oqERAupmAKCLH0gSQUJjXQd/SYfjAWAaP/I6mwCgiAT1
1Rpc2RK7GB29LToJfPrYOwg=
=z7A1
-----END PGP SIGNATURE-----

You can see that I'm using GnuPG as my encryption tool. It includes the original message, and below it, the appended digital signature.

And here's my public key (my real-world equivalent to "platypus", above):

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.4.0 (MingW32)
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=0j0h
-----END PGP PUBLIC KEY BLOCK-----

(You can see that public/private keys used for digital signing are much more involved that simple passwords like "rubberduck" or "platypus".)

Using that public key, you can now verify my signature. Since only I have the matching private key, only I could have encrypted the hash of the message, and hence only I could have signed it.

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Public key encryption, and digital signing is some heavy duty computing. Heavy duty enough that when used properly it's considered one of the most secure ways to encrypt data and verify identity in the digital world.

The good news, though, is that while the mathematics is complex and the computing is quite involved, you only really need to deal with the much more simple, high level concepts.

Related:

• Ask Leo! - How do I send encrypted email?

• Ask Leo! - Just how secure is email, anyway?

• Ask Leo! - Email Encryption - Failure to Launch?

Article C2713 - July 5, 2006