Re: Interpretation of KeyUsage?

["Housley, Russ" <housley@xxxxxxxxxx>]

My opinion:

1. Authentication is a 'digitalSignature' key.

2. Computing digital signatures (e.g. signing legal contracts) is both a 
'digitalSignature' and a 'nonRepudiation' key.

3. Encryption is a 'keyEncipherment' or 'keyAgreement' key depending on the 
algorithm you use.  RSA is a 'keyEncipherment' key.  Diffie-Hellman is a 
'keyAgreement' key.

Russ


______________________________ Reply Separator _________________________________
Subject: Interpretation of KeyUsage?
Author:  Lars Johansson <Lars.Johansson@psab.posten.se> at internet
Date:    1/17/97 3:06 AM


Hi all,

This is a topic that has confused us a great deal 
in the swedish SEIS project (http://www.seis.se). 
One goal of SEIS is to agree on a standard of 
electronic ID-cards (i.e. smart cards primarly 
used for identification). The electronic ID-card 
shall be capable of performing the following 
cryptographic functions (all based on RSA):

1. Authentication ("signing" a random challenge).

2. Computing digital signatures (e.g. signing 
   legal contracts).

3. Encryption

For each function there is a separate key with a 
corresponding certificate. These three certificates 
must therefore include the X.509v3 extension keyUsage.

It's quite clear that the encryption key can have the 
usage 'keyEncipherment' but what about the other two?

After reading the X.509 DAM over and over (and even 
calling Warwick Ford) it was decided that the key we
use for (what we call) digital signatures (function no. 2) 
would be 'nonRepudiation' in the X.509 terminology.

This left us with the KeyUsage 'digitalSignatures'
for the key we use for authentication. Although I think 
this interpretation of X.509v3 is correct it still
worries me somehow. As we interpret the term authentication, 
it means encrypting some random data with your private key. 
Since the protocol uses random data, this type of signature 
mustn't be mixed with the ones performed on legally binding 
contracts (supporting non-repudiation).

Now I'd like to know for what other purposes are people 
using keys with the X.509v3 extension 'digitalSignature'?
As I see it there is a potential risk that the intepretation 
differs from country to country or even from application to 
application.

Suppose that one service provider on the Internet accepts 
a digitally signed payment order using the extension 
'digitalSignature' in the corresponding certificate.
Do you all see the potential risk of fraud to our happily 
unworried swedish inhabitants that use their electronic 
ID-cards for authetication puposes?

Please comment!
/Lars Johansson
Sweden Post