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From: Nimmi Srivastav on 6 Jun 2008 17:40
Hi,

Apologies if this is OT.

Is there a solution for asymmetric cryptography (using public and
private keys) between two end points (say Point A and Point B) when
Point A does not have the liberty to choose its own public key but is
forced by a third party, that wants to exercise administrative rights,
to use an assigned public key? Is it still possible to establish
secure communication between the two end points, A and B, such that
the third party (that wants to exercise administrative rights) cannot
decipher the communication?

Thanks,
Nimmi
From: Ronny Schuetz on 6 Jun 2008 18:20
Hi,

> Is there a solution for asymmetric cryptography (using public and
> private keys) between two end points (say Point A and Point B) when
> Point A does not have the liberty to choose its own public key but is
> forced by a third party, that wants to exercise administrative rights,
> to use an assigned public key? Is it still possible to establish
> secure communication between the two end points, A and B, such that
> the third party (that wants to exercise administrative rights) cannot
> decipher the communication?

You could use the keys for example for authentication only and use the
Diffie-Hellman key exchange protocol [1] to generate the session key to
communicate securely.

Ronny

[1] http://en.wikipedia.org/wiki/Diffie-Hellman_key_exchange
From: Kristian Gj�steen on 7 Jun 2008 04:34
Nimmi Srivastav <nimmi_srivastav(a)yahoo.com> wrote:
>Is it still possible to establish
>secure communication between the two end points, A and B, such that
>the third party (that wants to exercise administrative rights) cannot
>decipher the communication?

If someone else knows A's private keys, then that someone else can always
impersonate A.

One method is to let A and B have signing keys, then use a signed
Diffie-Hellman key exchange to establish session keys.

PS. Followup-To set to sci.crypt.
--
Kristian Gj�steen
From: Joseph Ashwood on 7 Jun 2008 07:16
[had to drop comp.sources.d from group list to meet server requirements.
Stupid server, feel free to add it back it you want]

"Nimmi Srivastav" <nimmi_srivastav(a)yahoo.com> wrote in message
news:4fbac648-0b9a-4240-8613-e6e53bc2a077(a)m73g2000hsh.googlegroups.com...
> Is there a solution for asymmetric cryptography (using public and
> private keys) between two end points (say Point A and Point B) when
> Point A does not have the liberty to choose its own public key but is
> forced by a third party, that wants to exercise administrative rights,
> to use an assigned public key? Is it still possible to establish
> secure communication between the two end points, A and B, such that
> the third party (that wants to exercise administrative rights) cannot
> decipher the communication?

If there is a way where A and B can communicate privately for a short period
of time (bootstrapping problem) then yes it is possible for B to supply A's
private key, and to maintain security.

Also of interest may be using the assigned private key to only sign an
ephemeral DH key. This would have many security behaviors (e.g. A could
always be certain it was B, and privacy would be maintained), but some
security disadvantages (e.g. B could not determine between A and E, since E
could use the same private key).
Joe

From: CWhizard on 8 Jun 2008 01:28
For authentication, there is no way around it. If A encrypts something for
B and C has A's private key, then C can impersonate A.

As far as sucure communicatiosn go once authentication is through, DH is
probably the easiest PKE to impliment.

You could always generate your own public/private key pairs.


"Nimmi Srivastav" <nimmi_srivastav(a)yahoo.com> wrote in message
news:4fbac648-0b9a-4240-8613-e6e53bc2a077(a)m73g2000hsh.googlegroups.com...
> Hi,
>
> Apologies if this is OT.
>
> Is there a solution for asymmetric cryptography (using public and
> private keys) between two end points (say Point A and Point B) when
> Point A does not have the liberty to choose its own public key but is
> forced by a third party, that wants to exercise administrative rights,
> to use an assigned public key? Is it still possible to establish
> secure communication between the two end points, A and B, such that
> the third party (that wants to exercise administrative rights) cannot
> decipher the communication?
>
> Thanks,
> Nimmi


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