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Seth Goodman wrote:
Julian Mehnle wrote on Saturday, December 02, 2006 6:10 PM -0600:
How is "strong cryptography" any more secure an assertion method than
IP address authorization?
From the standpoint of the recipient, you can determine that a PGP key
belongs to an individual to a degree of certainty that you understand.
You know the trust relationship in detail and make of it what you will.
It is much more tenuous for designated hosts. A recipient has no way to
evaluate the extent to which a host is secure and the strength of the
measures used to discourage forgery. Neither does a recipient have any
way to evaluate how many machines have submission rights to that host
and their level of security.
The same does apply to the security of a PGP key. How do you know whether
the private key is stored protected with a passphrase or not? How do you
know whether the private key is not readable for all users on the system
where it is stored? How do you know if the private key isn't deliberately
being shared among a group (of unknown size) of users? How do you know
that PGP private keys aren't stored on a central server of the sender
organization and all messages sent are being signed on that central
server?
All a recipient can say is that the return-path domain is verified
according to the senders wishes.
... according to the sender's wishes, exactly. Here, the sender is the
authority with regard to what constitutes an acceptable assertion. That
is a general concept of authentication, not specific to any single method.
The grade of security of any assertion method only depends on the
odds of it being plausibly reproduced against the will of the
authority, not on some inherent magical properties.
I agree. Saying you possess the secret key corresponding to a public
key that was signed by numerous individuals a recipient trusts after
verification in person is a much stronger assertion than saying you
designate a given host as permitted to send mail on behalf of your
domain.
I agree that it is generally stronger. But who can say that the one is
generally "sufficient" for a given purpose while the other isn't?
In a strong system, the prover makes an assertion that he is actually in
a position to prove.
How would I prove to you that I haven't given my PGP private key away to
others? Or, more realistically put, how would I prove to you that my key
hasn't been stolen?
A person can prove his identity to a high degree of certainty to others
who do not possess special skills,
Well, yeah, that's the theory. :-)
but proving that a given mail host and the network behind it are secure
is pretty difficult.
True.
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