> If I should encrypt a message by using a public key, and then transmit the
> message to you, then there is nothing secret about it, because the key is
> publicly available. On the other hand, if I should encrypt a message
> by use of a private key, and if only you and I know what our private key is,
> then we can encrypt and decrypt secret messages to each other. A public key
> has no security value whatsoever.
The explanation in PGP is that messages encrypted with the private key can
only be decrypted with the public key, and messages encrypted with the
public key can only be decrypted with the private key.
So, someone can send me a message encrypted with my public key and only I
can read it by applying my private key. Anyone trying to decrypt the
message with the public key will get garbage.
I can also send out a message and "sign" it (apply my private key to an
authentication header but not encrypt it). Thus, anyone can read the
message, and can verify it's from me by checking the signature against my
public key. Nobody can *modify* my message however, because the
authentication header also contains a unique value calculated off the
content of the message which will change if the message is modified.
