This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
In conventional cryptosystems, such as the US Federal Data Encryption
Standard (DES), a single key is used for both encryption and
decryption. This means that a key must be initially transmitted via
secure channels so that both parties have it before encrypted
messages can be sent over insecure channels. This may be
inconvenient. If you have a secure channel for exchanging keys, then
why do you need cryptography in the first place?
In public key cryptosystems, everyone has two related complementary
keys, a publicly revealed key and a secret key. Each key unlocks the
code that the other key makes. Knowing the public key does not help
you deduce the corresponding secret key. The public key can be
published and widely disseminated across a communications network.
This protocol provides privacy without the need for the same kind of
secure channels that a conventional cryptosystem requires.
Anyone can use a recipient's public key to encrypt a message to that
person, and that recipient uses her own corresponding secret key to
decrypt that message. No one but the recipient can decrypt it,
because no one else has access to that secret key. Not even the
person who encrypted the message can decrypt it.
Message authentication is also provided. The sender's own secret key
can be used to encrypt a message, thereby "signing" it. This creates
a digital signature of a message, which the recipient (or anyone
else) can check by using the sender's public key to decrypt it. This
proves that the sender was the true originator of the message, and
that the message has not been subsequently altered by anyone else,
because the sender alone possesses the secret key that made that
signature. Forgery of a signed message is infeasible, and the sender
cannot later disavow his signature.
These two processes can be combined to provide both privacy and
authentication by first signing a message with your own secret key,
then encrypting the signed message with the recipient's public key.
The recipient reverses these steps by first decrypting the message
with her own secret key, then checking the enclosed signature with
your public key. These steps are done automatically by the
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