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National Cyber Alert System
Cyber Security Tip ST04-018
Understanding Digital Signatures
Digital signatures are a way to verify that an email message is really
from the person who supposedly sent it and that it hasn't been
changed.
What is a digital signature?
You may have received emails that have a block of letters and numbers
at the bottom of the message. Although it may look like useless text
or some kind of error, this information is actually a digital
signature. To generate a signature, a mathematical algorithm is used
to combine the information in a key with the information in the
message. The result is a random-looking string of letters and numbers.
Why would you use one?
Because it is so easy for attackers and viruses to "spoof" email
addresses (see Using Caution with Email Attachments for more
information), it is sometimes difficult to identify legitimate
messages. Authenticity may be especially important for business
correspondence--if you are relying on someone to provide or verify
information, you want to be sure that the information is coming from
the correct source. A signed message also indicates that changes have
not been made to the content since it was sent; any changes would
cause the signature to break.
How does it work?
Before you can understand how a digital signature works, there are
some terms you should know:
* Keys - Keys are used to create digital signatures. For every
signature, there is a public key and a private key.
+ Private key - The private key is the portion of the key you
use to actually sign an email message. The private key is
protected by a password, and you should never give your
private key to anyone.
+ Public key - The public key is the portion of the key that is
available to other people. Whether you upload it to a public
key ring or send it to someone, this is the key other people
can use to check your signature. A list of other people who
have signed your key is also included with your public key.
You will only be able to see their identities if you already
have their public keys on your key ring.
* Key ring - A key ring contains public keys. You have a key ring
that contains the keys of people who have sent you their keys or
whose keys you have gotten from a public key server. A public key
server contains keys of people who have chosen to upload their
keys.
* Fingerprint - When confirming a key, you will actually be
confirming the unique series of letters and numbers that comprise
the fingerprint of the key. The fingerprint is a different series
of letters and numbers than the chunk of information that appears
at the bottom of a signed email message.
* Key certificates - When you select a key on a key ring, you will
usually see the key certificate, which contains information about
the key, such as the key owner, the date the key was created, and
the date the key will expire.
* "Web of trust" - When someone signs your key, they are confirming
that the key actually belongs to you. The more signatures you
collect, the stronger your key becomes. If someone sees that your
key has been signed by other people that he or she trusts, he or
she is more inclined to trust your key. Note: Just because someone
else has trusted a key or you find it on a public key ring does
not mean you should automatically trust it. You should always
verify the fingerprint yourself.
The process for creating, obtaining, and using keys is fairly
straightforward:
1. Generate a key using software such as PGP, which stands for Pretty
Good Privacy, or GnuPG, which stands for GNU Privacy Guard.
2. Increase the authenticity of your key by having your key signed by
co-workers or other associates who also have keys. In the process
of signing your key, they will confirm that the fingerprint on the
key you sent them belongs to you. By doing this, they verify your
identity and indicate trust in your key.
3. Upload your signed key to a public key ring so that if someone
gets a message with your signature, they can verify the digital
signature.
4. Digitally sign your outgoing email messages. Most email clients
have a feature to easily add your digital signature to your
message.
_________________________________________________________________
Authors: Mindi McDowell, Allen Householder
_________________________________________________________________
Produced 2007 by US-CERT, a government organization.
Note: This tip was previously published and is being re-distributed
to increase awareness.
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