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Re: I-D ACTION:draft-ietf-smime-escertid-00.txt
2006-04-05 05:52:03
Jim,
After more thoughs and exchange of arguments I am inclined to accept
your position.
I believe that the rule should be:
if SHA-1 is used, then ESSCertID MUST be used.
So, it would make sense to have SHA-256 as the default algorithm
for the new structure.
In this thread, some people advocated implicit signaling of the algorithm.
It made me think that we should provide some additional guidance for
implementors. For example, if SHA-256 with RSA is the signature algorithm,
then it would make sense, *not* to use ESSCerID :-) but rather to use the
new structure with SHA-256.
Now, the case where SHA-1 with RSA is the signature algorithm is more
complicated. ESSCertID should be used. Should we, in addition, recommend
to use the new structure with SHA-256 ? It would not hurt, but in case
SHA-1 is broken, does it really provide additional security ? I leave the
floor open to cryptographers.
Finally, I have a minor naming problem: what is it called ESSCertIDEx
and not ESSCertIDv2 ? What means "Ex" ?!?
I would rather prefer ESSCertIDv2 which is more meaningful
without any explanation.
Denis
Denis,
I am well aware that the issue was debated on the PKIX mailing list, and I
ignored it because it was not of immediate importance to me at the time.
I am not sure that I agree that the PKIX proposal is really backwards
compatable. What can be said is that if you use it in a specific manner
then it produces the same bits on the wire, however consider what the
reality of things is.
In the S/MIME world the new structure is identified by a new OID, thus an
old client would see that there is an attribute that it does not understand.
This means that the new structure is really backwards compatable, if an old
client gets a message from a new client the item is ignored and the old
client can successfully process the message. Likewise if an old client
sends a message and includes this attribute the new client should still
understand and process the extension correctly. (This would also be true
with the timestamp protocol in RFC 3161.)
Now lets look at what happens with this in RFC 3029 if you simply replace
ESSCertID with your proposed ESSCertIDEx. If an old client sends a message
to the new client, it works since the new client can correctly decode it.
If a new client sends a message to an old client, it MUST use the SHA-1 hash
algorithm as any other would cause the old client to have a decode
algorithm. But it is not clear that the new client can recognize in advance
that it is dealing with an old client so it would ALWAYS use SHA-1 just in
case. TO me this means that you might as well not change the protocol.
(Side note - there is not versioning in the low level structures that I
could see. If you update the version number then there is no problem with
changing the structure itself.
There is some logic to the argument that was put forward by Peter that
having the same effective structure would allow for writing of a single
decode/encode routine and thus potentially save code space.
I personally think that we should be moving out of SHA-1 and onto SHA-2 in
the standards as quickly as possible. If I belive this then I need to take
the position that either 1) size is not an issue or 2) the defaults should
be the algorithms that we think should be used in the immediate future. I
am a strong supporter of option 2.
I will look at your suggested re-write as part of the up-coming revision of
the document.
jim
-----Original Message-----
From: owner-ietf-smime(_at_)mail(_dot_)imc(_dot_)org
[mailto:owner-ietf-smime(_at_)mail(_dot_)imc(_dot_)org] On Behalf Of Denis
Pinkas
Sent: Thursday, March 30, 2006 6:59 AM
To: ietf-smime(_at_)imc(_dot_)org
Subject: Re: I-D ACTION:draft-ietf-smime-escertid-00.txt
Jim, and the working group
1 - This topic has been debated on the PKIX mailing list and a
proposal has been made. The proposed ASN.1 structure is different
from the one described in draft-ietf-smime-escertid-00.txt.
The current ASN.1 in RFC 2634 is:
ESSCertID ::= SEQUENCE {
certHash Hash,
issuerSerial IssuerSerial OPTIONAL
}
The proposal from draft-ietf-smime-escertid-00.txt is:
ESSCertIDEx ::= SEQUENCE {
certHash Hash,
hashAlg AlgorithmIdentifier
DEFAULT {id-sha256},
issuerSerial IssuerSerial OPTIONAL
}
The proposal made on the PKIX mailing list is:
ESSCertIDv2 ::= SEQUENCE {
certHash OCTET STRING,
issuerSerial IssuerSerial,
hashAlgorithm AlgorithmIdentifier DEFAULT { sha-1 }
The advantage of the last proposal is backward compatibility
with the existing structure. The disadvantage is that it is
less "natural" than invented from scratch, but some
explanations can easily overcome this drawback.
I would propose to adopt the proposal made on the PKIX mailing list.
In addition, I would take advantage of the rewriting of this
section to correct some past "errors" in section 5.4.1.
2 - Proposed rewritting:
"5.4.1. Certificate Identification
The best way to identify certificates is an often-discussed issue.
When an issuer/serial number pair is included in the SignedData
object, that information is not part of the message digest
calculation process may thus be changed without being detected.
That information may only be used as an hint and by itself, does
not allow to make sure that the right certificate is being
accessed.
A hash of the entire certificate allows a verifier to verify that
the certificate that was chosen by the signer is being
used when the
signature is verified. It permits a detection of certificate
substitution attacks.
Duplication of the issuer/serial number pair within the Signing
Certificate attribute is not fully necessary, but has the
advantage to get the whole information from that attribute.
Attribute certificates and additional public key certificates
containing authorization information do not have an issuer/serial
number pair represented anywhere in a SignerInfo object. When an
attribute certificate or an additional public key
certificate is not
included in the SignedData object, it becomes much more
difficult to
get the correct set of certificates based only on a hash of the
certificate. For this reason, these certificates SHOULD be
identified
by the IssuerSerial object."
3 - It is a matter of taste, but I would rather prefer:
SigningCertificatev2 instead of SigningCertificateEx
4 - It would be nice to correct the typos from RFC 2634, e.g.
"requred".
There are other typos like: "certficates", "subsiquent",
"Indentification", ...
Please use a spell checker.
Denis
A New Internet-Draft is available from the on-line
Internet-Drafts directories.
This draft is a work item of the S/MIME Mail Security
Working Group of the IETF.
Title : ESS Update: Adding CertID Algorithm Agility
Author(s) : J. Schaad
Filename : draft-ietf-smime-escertid-00.txt
Pages : 18
Date : 2006-3-24
In the original Enhanged Security Services for S/MIME draft, a
structure for cryptographically linking the certificate to
be used in
validation with the signature was introduced, this structure was
hardwired to use SHA-1. This document allows for the
structure to have
algorithm agility and defines new attributes to deal with
the updating.
A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-ietf-smime-escertid-00.txt
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Denis Pinkas
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