Re: draft-igoe-secsh-x509v3-00

[Joseph Galbraith <[e-mail]>]

Igoe, Kevin M. wrote:
> Colleagues:
>  
>   I'd like to call your attention to the following draft submission for 
> X.509 certificates in Secure Shell:
>  
>  http://www.ietf.org/id/draft-igoe-secsh-x509v3-00.txt
>  
>  
> Your comments, suggestions and insights are appreciated!

Thanks for picking this work up.  I'm definitely interested
in seeing this move forward, and I know I've had a number of
people express interest in this work as well.

I think this draft needs to spell out, explicitly, various formats.

For example,

 > The key format has the following specific encoding:
 >
 >      string    "x509v3-ssh-dss" / "x509v3-ssh-rsa" /
 >                "x509v3-ecdsa-sha2-*" / "x509v3-ecmqv-sha2"
 >      string    certificate-chain
 >
 > A certificate-chain is a DER-encoded ASN.1 SEQUENCE of certificates.

I suppose you might omit the second string and embed the ASN.1 sequence
directly, but usually SSH wraps that kind of data in a string.

Notice the algorithm name is encoded into the publickey.  This is
the way ssh-dss and ssh-rsa work, but not the way the (undocumented)
de-facto x509 implementation that is currently floating around works.
(And in case it isn't obvious, I think it should work like the existing
algorithms and encode the algorithm name in the publickey.)

It might be more SSH friendly to encode the chain using SSH rather
than ASN.1:

 > The "x509v3-ssh-dss" key format has the following specific encoding:
 >
 >      string    "x509v3-ssh-dss" / "x509v3-ssh-rsa" /
 >                "x509v3-ecdsa-sha2-*" / "x509v3-ecmqv-sha2"
 >      uint32    certificate-count
 >      string    certificate[1..certificate-count]

I'd also include an example, just because we've had a number of
implementation problems in this area (the string nesting can be a
little confusing.)

 >  byte      SSH_MSG_KEXDH_REPLY
 >  string    0x00 0x00 0xXX 0xXX
 >      0x00 0x00 0x00 0x0D "x509v3-ssh-dss"
 >      0x00 0x00 0x00 0x02
 >      0x00 0x00 0xXX 0xXX DER-encoded senders certificate
 >      0x00 0x00 0xXX 0xXX DER-encoded issuer certificate
 >  mpint     f
 >  string    signature of H

I think we also need to document signature algorithms.

 > Signing and verifying using the "x509v3-ssh-dss" key format
 > is done according to the Digital Signature Standard [FIPS-186-2]
 > using the SHA-1 hash [FIPS-180-2].
 >
 > The resulting signature is encoded as follows:
 >
 >      string    "ssh-dss"
 >      string    dss_signature_blob
 >
 > The value for 'dss_signature_blob' is encoded as a string containing
 > r, followed by s (which are 160-bit integers, without lengths or
 > padding, unsigned, and in network byte order).
 >
 > Signing and verifying using the "x509v3-ssh-rsa" key format
 > is performed according to the RSASSA-PKCS1-v1_5 scheme in
 > [RFC3447] using the SHA-1 hash.
 >
 > The resulting signature is encoded as follows:
 >
 >     string    "ssh-rsa"
 >     string    rsa_signature_blob
 >
 >  The value for 'rsa_signature_blob' is encoded as a string containing
 >  s (which is an integer, without lengths or padding, unsigned, and in
 >  network byte order).
 >
 >  These formats are that same as "ssh-rsa" and "ssh-dss", see RFC 4253,
 >  6.6. Public Key Algorithms

You'll probably also need details for "x509v3-ecdsa-sha2-*"
and "x509v3-ecmqv-sha2" signature encodings.

(And I assumed you wanted to use the existing encodings for signatures--
if not, you'll need to spell something different out.)

In addition, in your "IANA Considerations", you specified that there
were new entries in the key exchange algorithms, but I don't think
you actually introduced any new key exchange algorithms did you? These
are just new public key types that can be used in combination with any
of the existing key exchange algorithms.

Other miscellaneous thoughts:

o Do we want to require processing of (or give guidance with regards to)
   any extensions, such as BasicConstraints, KeyUsage, and
   SubjectAltName?

o Do we need language about respecting critical extensions or rejecting
   the certificate?

o Do we want to define any extended key usage key purpose ids?

o When we were working on x509v3 before, people seemed to want to
   include revocation data as well as chain data; I was never quite
   sure whether this was really needed or if it was gold plating, so
   to speak.

I'm more than happy to see any of these discarded as unneeded fluff...
to be honest, I know a lot about SSH, but not so much about x509, so
these thoughts mostly reflect feedback I was given when working on
the (now abandoned) x509v3 draft
  (http://tools.ietf.org/html/draft-ietf-secsh-x509-03)

o We should require the first certificate in a chain to use the correct
   type of key (i.e., it must have a rsa key if it is a
   "x509v3-ssh-rsa".)

Thanks,

Joseph