Re: how to create a file with copy/read denied to it

David Schwartz <davids@xxxxxxxxxxxxx> writes:
On Apr 29, 9:35 am, Rainer Weikusat <rweiku...@xxxxxxxxxxx> wrote:

It was about encrypting something with a 'private key', as stated in
the text above the figure:

        The basic manner in which digital signatures are created is
        illustrated in Figure 1-6. Instead of encrypting information
        using someone else's public key, you encrypt it with your
        private key. If the information can be decrypted with your
        public key, then it must have originated with you.

Yeah, this is completely incorrect. In particular, this statement is
utterly false:

"The basic manner in which digital signatures are created is
illustrated in Figure
1-6. Instead of encrypting information using someone else’s public
key, you
encrypt it with your private key. If the information can be decrypted
with your
public key, then it must have originated with you."

The image, by the way, is correct. The text describing it doesn't
match the image and is incorrect.

It's a common misunderstanding that comes from the fact that this is
sort of true with RSA and you will hear similar statements from
experts in an RSA-specific context.

This refers to RSA. Another text on the same topic:

She then encrypts the message digest with her private key,
creating the digital signature she sends to Bob along with the
message itself. Bob, upon receiving the message and signature,
decrypts the signature with Alice's public key to recover the
message digest.
http://www.rsa.com/rsalabs/node.asp?id=2221

And for most practical purposes, 'public key cryptography' equals
'using RSA', so the chances that the OP was referring to RSA are
pretty high. It is actually possible to locate[*] yet another text on
this where the process of transforming some plaintext into some
ciphertext is called 'decryption' and the reverse process
'encryption'. This is, to some degree, technically true, because the
RSA decryption algorithm is used to *encrypt* the plaintext and vice
versa. But that's just the usual 'mathematician having troubles to
understand that normal people think of 'left' and 'right' as something
different instead of "essentially the same but with the sign
swapped"'-phenomenon and I would consider this usage for a discussion
in _layman_-terms inappropriate. Which is what we are doing here.
That the (academic?) experts, as isn't uncommon, cannot agree on
anything, is IMHO something better left to them.

[*] http://www.cs.ucdavis.edu/~rogaway/papers/exact.html
.

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