Analysis of information sources in references of the Wikipedia article "Public-key cryptography" in English language version.
Now, certain types of information cannot reasonably be assumed to be concealed. For instance, an upper bound on the total volume of a party's sent or received communication (of any sort) is obtainable by anyone with the resources to examine all possible physical communication channels available to that party.
Since PGP, beyond compressing the messages, does not make any further attempts to hide their size, it is trivial to follow a message in the network just by observing its length.
The scenario just described would seem to be secure, because all data is encrypted before being passed to the communications processors. However, certain control information must be passed in cleartext from the host to the communications processor to allow the network to function. This control information consists of the destination address for the packet, the length of the packet, and the time between successive packet transmissions.
The scenario just described would seem to be secure, because all data is encrypted before being passed to the communications processors. However, certain control information must be passed in cleartext from the host to the communications processor to allow the network to function. This control information consists of the destination address for the packet, the length of the packet, and the time between successive packet transmissions.
Why is naïve Sign & Encrypt insecure? Most simply, S&E is vulnerable to "surreptitious forwarding:" Alice signs & encrypts for Bob's eyes, but Bob re-encrypts Alice's signed message for Charlie to see. In the end, Charlie believes Alice wrote to him directly, and can't detect Bob's subterfuge.
