Re: More on learning "Public Key Authentication"

Tom Stiller <tomstiller@xxxxxxxxxxx> wrote:

> In article <1h2h4qp.16l3r5zkr6rf2N%navn@xxxxxxxxxxxxxxx>,
> navn@xxxxxxxxxxxxxxx (Jon Aalborg) wrote:
>
> > You use a password, preferably long and complex, to generate a
> > public/private keypair. They relate in such a way that you can always
> > decrypt stuff coded with the public key by using the private one and
> > that only, never vice versa (there are good mathematical reasons why
> > this is so, don't ask me for details as I wouldn't be able to understand
> > them anyway).
>
> I'm not sure if this is just an awkwardly constructed sentence or if
> it's just plain wrong. In any event, let me say that in public key
> encryption, the public key and private keys can be applied in either
> order to transform a message from cleartext to ciphertext and back to
> cleartext. However, the intervening ciphertext will be different
> depending on the order of keu application.
>
> Specifically:
> cleartext -(private key)-> ciphertext1 -(public key)-> cleartext and
> cleartext -(public key)-> ciphertext2 -(private key)-> cleartext both
> work but ciphertext1 and ciphertext2 are not the same.
>
> A simple digital transaction can be constructed as follows:
> Let Bob encrypt a message with his own private key (which he owns) and
> encrypt the result with Alice's public key (which he knows). The result
> is sent to Alice who decrypts the message with her private key (which
> she owns) and decrypts the result with Bob's public key (which she
> knows). The resulting message could only have come from Bob and can
> only be read by Alice.
>
> Public key enciphering is to slow to be practical for large messages so
> it is frequently only used to exchange a "session" key which is used
> with a fast, robust, encryption scheme to the main data exchanges.

This can't be right. I hope it isn't.

A simple thought experiment:
I store my public key on a server for anyone to retrieve as needed, so
that they can encrypt messages for me. If someone then can steal a
message from me (e.g., by physically accessing my disk), one that I
encrypted with my private key and they can then decrypt it with the
publicly available key, floating around on the internet like it should
be, then my security is really, _really_ compromised.

OTOH, they might be able to use my public key to _verify_ that "I am me"
if they use it to check a _signed_ message. That is very different from
an _encrypted_ one, although a message may easily be both signed and
encrypted. I think that is what you mean? If so, I understand what you
are talking about, at least. I could sign a message using my private key
so that someone could verify that a mesage came from me.

If I want to send something to Anne that only she should read, I need
her public key. I will then encrypt the message using that, certain that
only she, using her _private_ key, can decrypt it. I would never encrypt
it with my private key and ask her to just find a copy of my public key
to decrypt it. How can that be safe?

It should not, cannot, be enough for someone to get a copy of my public
key from somewhere to decrypt stuff I really want to keep secret and
that I encrypt using my private key. That would negate the whole
concept, as far as I am able to understand. Or am I totally at sea here?
--
/Jon
Put "jaalborg" for "navn", remove ".invalid".
.

Relevant Pages

  • Re: Public - Private key
    ... As to what could be a very small private key and veyr small public key so ... decrypt A1 and likewise A1 can only decrypt P1. ... When PC1 communicates securely with PC2, PC1 will encrypt the data using the ...
    (microsoft.public.security)
  • Re: DECRYPT with PUBLIC key (how to?)
    ... values in my application which would be decoded with my own public key which ... This is a very stupid thing to think that you can encrypt with private ... Private key operations often uses CRT ... Signature schemes and Encryption schemes have completely different ...
    (microsoft.public.dotnet.security)
  • RSA frustrations - encrypt with private, decrypt with public - possible?
    ... -User with name "Foo" requests license. ... -User has public key, ... sufficient - I want to encrypt / decrypt a small amount of arbitrary ... "distribute private key, ...
    (microsoft.public.security)
  • RSA frustrations - encrypt with private, decrypt with public - possible?
    ... -User with name "Foo" requests license. ... -User has public key, ... sufficient - I want to encrypt / decrypt a small amount of arbitrary ... "distribute private key, ...
    (microsoft.public.dotnet.security)
  • Re: CryptExport private key only
    ... >supposed to use the public key to encrypt data and private key to always ... >decrypt data. ... What you encrypt with one key may only be decrypted with the other ... No - if you encrypt using your private key, anyone can decrypt it, because ...
    (microsoft.public.platformsdk.security)
Quantcast