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There are two answers to this question, depending on exactly what key
you are talking about -- private or public. First, let's look at what PGP
is: a public key encryption system.
In such systems there are two keys
involved, public and private. It is assumed that any user's public key
is readily available but that their private key is only known to them.
PGP uses these public and private keys to ensure that messages between
users are encrypted and, hence, private. An attacker capturing a message
will not be able to decipher and read the message.
To provide
this privacy, PGP uses a mathematical encryption algorithm to combine or
encrypt your message with your recipient's public key. The recipient
then has to use their private key to decipher and read your message. If
you create keys of a sufficient size (generally a minimum of 2,048-bits
these days) it's not practically feasible (warning - I am by no means a
mathematician or a cryptographer) to break PGP encryption short of
someone discovering a mathematical shortcut or possibly being the NSA.
PGP private keys can't be 'forged' as such but they can be stolen or
duplicated. The weakness of PGP is the private key and the passphrase
used to protect that key (you can't use your private key unless you
input your pass phrase first). If you are careless with your private
key then anyone with a copy of this key and the passphrase for the key
can de-crypt your messages. So, keys can't be forged as such but if
someone gets access to it then you risk having your security compromised.
So how to protect your keys? Well here are some starting points:
- Choose a good, strong passphrase
- Don't use the passphrase for anything else (login IDs, etc)
- Don't write down or share your passphrase
- Store your private key somewhere secure -- if it's on a host you share,
then lock down the permissions on your key ring so only you can access it
- Don't email your private key
You can find an excellent and much more detailed discussion of this
topic at http://senderek.de/security/secret-key.protection.html.
Now onto to public keys. Public keys can be forged and are vulnerable
to what is called a 'man in the middle attack'. With PGP, it is a tricky
attack to implement but it is feasible.
Let's take the scenario with Bob, who
wants to exchange some private information with Alice. To do this, Bob
is going to give Alice his public key, have her encrypt the message
with his public key and deliver it to him. Bob can then de-crypt it
with his private key and read the message. To get his public key to
Alice, Bob emails it to her. An attacker, Peter, intercepts the email
and substitutes a forged public key for Bob's real public key. Alice
receives the forged public key, combines it with her message and emails
it back to Bob. Peter intercepts the email and de-crypts and reads the
message (having the private key for the forged public key). If Peter
wants to maintain the ruse he could then encrypt the message with Bob's
real public key and let it continue onto Bob. Otherwise he has the
information he requires and can make whatever use of it. In either
case, Bob or Alice would probably be unaware that Peter has read the
message.
If you are worried about someone compromising your public key like this,
then use a trusted and secure mechanism to exchange it. For example,
exchange keys personally or via some secure medium. This can become
very cumbersome if you have to exchange keys with large numbers of
people or across large distances.
Another approach is to have your
public key signed by a trusted third party. In this model, PGP users
digitally sign each other's keys to validate the key belongs to the
correct person. Usually this signing requires the person whose key is
being signed to somehow prove their identity like with a
driver's license or passport. Many organizations (open source groups
for example) host key-signing parties and some companies (in banking or finance) even organize meetings to exchange/sign keys in a
secure manner. Obviously, this only works if you trust the person or
people who have signed the key.
You can read about key signing parties and related information at
http://cryptnet.net/fdp/crypto/keysigning_party/en/keysigning_party.html.
This is a very simplistic explanation of the topic of PGP key security.
I strongly recommend you read about PGP, security of keys and the
secure use of PGP in more details before implementing anything. Bruce
Schneier's excellent "Practical Cryptography" (ISBN 0-471-22357-3) is a
good starting point.
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