Re: Options, was Re: To Be, or NR To Be ...

[Ed Gerck <egerck@xxxxxxx>]

Peter Williams wrote:

>
> > Thus, the value of bit 0 does not depend on the value of bit 1; they are
> > in fact independent.
>
> That is quite correct. In PKIX, ISO NR and use of the NR bit
> does not depend on use of a digital signature
> mechanism (except as it is is to issue certs and CRLs.)

Peter:

If I say that  "I want an ice-cream other than vanilla", this means that I want ANY
ice-cream EXCEPT vanilla.  Thus, when the spec says that bit_0 is TRUE when
the subject public key is used with a digital signature mechanism to support
security services OTHER THAN non-repudiation (bit 1), then the spec is saying
that any service can be used with bit_0 EXCEPT non-repudiation.    For a logical
derivation and further comments, please see [1].

Is the spec wrongly written in this regard?  Well, this much is certain because
2459 also says that it does not restrict the combinations of bits that may be
set in an instantiation of the keyUsage extension -- which is in contradiction with
the above.  Thus, either passage must be corrected; and this is NOT the only
example (check the cRLSign bit for instance).

The assumption that the certificate can be used for signing (bit_0 = TRUE) also
when "bit_1 = TRUE" due to system elements outside of those specified in PKIX
is a simple fallacy because bit_1 is defined as a signed bit by the PKIX
specification and cannot be asserted *after* bit_0 is signed.  The entire certificate is
signed and "frozen" at the time of issuance and must thus conform to the spec at that
time, which affirms that bit_0 is TRUE when the subject public key is used with a digital
signature mechanism to support security services EXCEPT non-repudiation (bit_1).

Cheers,

Ed Gerck



[1]   As I quoted, the sentence in 2459 specifies:

       The digitalSignature bit is asserted when the subject public key
       is used with a digital signature mechanism to support security
       services other than non-repudiation (bit 1)

Now, suppose I define two boolean variables A and B (where "bit_1" means both
the service of "non-repudiation" and the bit that indicates the service):

A = (the subject public key is used with a digital signature mechanism to support ANY security services)
B = (the subject public key is used with a digital signature mechanism to support security services other than bit_1)

which variables I define  to be TRUE if the subject public-key "IS" used in the context
specified or FALSE if it is NOT used in that context.

The logical expression that is equivalent to the quoted phrase in 2459 would read:

bit_0 =  A and not  (bit_1)

because, in logic notation,  B = A and not (bit_1).  The set of security services in A
is larger than the set of security services in B -- because B excludes ("other than")
the non-repudiation service.

In other words, 2459 says that bit_0 is asserted when a subject's public-key is used
with a digital signature mechanism to support services which are in mathematical
complement to non-repudiation.  Hence, bit_0 and bit_1 can't be TRUE at the same
time if the spec text is correct.