Re: [lamps] [Pqc] Multiple drafts with PQ algorithm key encodings that are not compatible.

["Massimo, Jake" <[e-mail]>]

Hi John,

Thanks for reaching out, this email is quite timely as Panos and I had been discussing your feedback (among all others from the WG on the Dilithium I-D) just today. Here are my thoughts.

Regarding the Public Key:

There is a reason as to why I went with:


DilithiumPublicKey ::= OCTET STRING

In https://datatracker.ietf.org/doc/draft-massimo-lamps-pq-sig-certificates/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-massimo-lamps-pq-sig-certificates/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlou-rKXm$>, and not:


   DilithiumPublicKey ::= SEQUENCE {

       rho         OCTET STRING,

       t1          OCTET STRING

   }

As found in https://datatracker.ietf.org/doc/draft-uni-qsckeys/01/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-uni-qsckeys/01/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlnAfHtmn$>.

This is due to the discussions from Peikert and Markku on the NIST PQC forum here https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/eAaiJO1qzkA/m/K66R_ftNBwAJ. The unambiguous public key serialization provides additional security properties (message binding and non-resignability). I attempted to outline this in the last paragraph of my security recommendations.

Regarding the Private Key:

We’re not too far off here, I was actually in the process of changing the private key structure today from the feedback from the WG. In particular, I agree with the addition of the version parameter – the other differences are just naming conventions.

However, upon digesting a comment from Markku:

The secret key lengths in Appendix B match with v3.1 (v3.0 has 16 bytes longer private keys), but do not account for ASN.1 encoding of the SEQUENCE in Section 5 of the same I-D. Even section 5 itself does not seem to account for this as it reports "the size necessary to hold all private key elements." There is a de facto key transport encoding for secret keys, defined by the algorithm designers and used in KAT tests, that doesn't have ASN.1 encoding of individual components. It can be simply taken as an OCTET STRING, just like the public key in this I-D. The lengths in Appendix B match that encoding, not the completely new encoding in Section 5.

Seconded by Simon Guggi,

I also support the byte array encoding for the private key proposed by the algorithm designers. It is an already defined encoding, so I think it makes sense to take that one.

And this comment from Ilari Liusvaara:

Don't split the private key into fields. Standard Dilithium implementation takes the private key as octet string (as this is cryptographic BCP), so any splitting causes application to need to perform redundant merging/splitting in order to adapt what is in private key with what Dilithium implementation takes.

And things get even worse as tr is SHAKE output which gets sticked into a BIT STRING... Which is a cursed combination.


I was considering changing the DilithiumPrivateKey ASN.1 to match other private key formats (https://datatracker.ietf.org/doc/draft-ietf-lamps-kyber-certificates/ and https://www.rfc-editor.org/rfc/rfc5915). This means proposing the new sequence structure:

     DilithiumPrivateKey ::= SEQUENCE {
         version                               Version,
         privateKeyAlgorithm      PrivateKeyAlgorithmIdentifier,
         privateKey                        OCTET STRING,
         publicKey                            [0] IMPLICIT DilithiumPublicKey OPTIONAL
     }

i.e., the parameters are defined/contained within privateKeyAlgorithm. This means we can support the byte array encoding for the private key proposed by the algorithm designers in privateKey an OCTET STRING.

Thoughts?

Cheers,
Jake

From: Mike Ounsworth <[e-mail]>
Date: Wednesday, 28 September 2022 at 13:26
To: John Gray <John.Gray=[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Cc: "Massimo, Jake" <[e-mail]>, "Panos Kampanakis (pkampana)" <[e-mail]>, Sean Turner <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: RE: [EXTERNAL][Pqc] Multiple drafts with PQ algorithm key encodings that are not compatible.


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My curiosity is about the procedural aspect of draft-uni-qsckeys. Is this attached to a working group? Are you indenting to publishing this as an RFC, or is this just to bridge prototyping work until NIST publishes real standards?

---
Mike Ounsworth
Software Security Architect, Entrust

From: Pqc <[e-mail]> On Behalf Of John Gray
Sent: September 28, 2022 1:34 PM
To: [e-mail]; [e-mail]; [e-mail]
Cc: Massimo, Jake <[e-mail]>; Panos Kampanakis (pkampana) <[e-mail]>; Sean Turner <[e-mail]>; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: [EXTERNAL] [Pqc] Multiple drafts with PQ algorithm key encodings that are not compatible.

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We are doing some interoperability testing with different vendors using the new Dilithium, Falcon and SPHINCS+ algorithms.   We have come across at least two drafts which are trying to specify the ASN.1 encoding formats for these algorithms.   However, the encoding formats are not compatible with each other.   I imagine the authors of these drafts should get together and come up with a common format (I have copied them on this email).   This means we must choose one or the other, or even worse, support multiple formats (which can lead to bugs).   Initially I started more than a year ago using my own encoding format for internal prototyping, but now need to interoperate with others outside of our organization, so a common format is definitely needed at this point.   😊

We fully realize the OID values will be changing once official OIDs are registered, (changing those are trivial), but the ASN.1 formats of the public and private keys is kind of important as well…  😊

For example:

The LAMPS group has a specification of Dilithium public keys in this draft:
https://datatracker.ietf.org/doc/draft-massimo-lamps-pq-sig-certificates/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-massimo-lamps-pq-sig-certificates/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlou-rKXm$>


the public key format is this:



The Dilithium public key MUST be encoded using the ASN.1 type

   DilithiumPublicKey:



     DilithiumPublicKey ::= OCTET STRING



The private key format is this:



     DilithiumPrivateKey ::= SEQUENCE {

         rho         BIT STRING,         - nonce/seed

         K           BIT STRING,         - key/seed

         tr          BIT STRING,         - PRF bytes (CRH in spec.)

         s1          BIT STRING,         - vector l

         s2          BIT STRING,         - vector k

         t0          BIT STRING,         - encoded vector

         PublicKey   IMPLICIT DilithiumPublicKey OPTIONAL

     }



In this draft:
https://datatracker.ietf.org/doc/draft-uni-qsckeys/01/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-uni-qsckeys/01/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlnAfHtmn$>

Dilithium keys have this encoding:




   DilithiumPublicKey ::= SEQUENCE {

       rho         OCTET STRING,

       t1          OCTET STRING

   }





  DilithiumPrivateKey ::= SEQUENCE {

       version     INTEGER {v0(0)}     -- version (round 3)

       nonce       BIT STRING,         -- rho

       key         BIT STRING,         -- key/seed/D

       tr          BIT STRING,         -- PRF bytes (CRH in spec)

       s1          BIT STRING,         -- vector(L)

       s2          BIT STRING,         -- vector(K)

       t0          BIT STRING,

       publicKey  [0] IMPLICIT DilithiumPublicKey OPTIONAL

                                       -- see next section

   }

The draft-uni-qsckeys does not cover SPHINCS+,  it does cover Falcon, but I don’t know of another draft that specifies Falcon.

There are also encodings for Kyber mentioned in two documents that I see.  There is an early   https://datatracker.ietf.org/doc/draft-ietf-lamps-kyber-certificates/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-ietf-lamps-kyber-certificates/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlubO7POn$>  draft which mentions it is up to the document defining Kyber to give more details.    In the draft-uni-qsckeys draft it is more specific.

https://datatracker.ietf.org/doc/draft-uni-qsckeys/01/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-uni-qsckeys/01/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlnAfHtmn$>

   KyberPrivateKey ::= SEQUENCE {
       version     INTEGER {v0(0)}   -- version (round 3)
       s           OCTET STRING,     -- sample s
       publicKey   [0] IMPLICIT KyberPublicKey OPTIONAL,
                                     -- see next section
       hpk         OCTET STRING      -- H(pk)
       nonce       OCTET STRING,     -- z
   }

Partial public key encoding:


KyberPrivateKey ::= SEQUENCE {

       version     INTEGER {v0(0)}   -- version (round 3)

       s           OCTET STRING,     -- EMPTY

       publicKey   [0] IMPLICIT KyberPublicKey OPTIONAL,

                                     -- see next section

       hpk         OCTET STRING      -- EMPTY

       nonce       OCTET STRING,     -- d

   }

Full public key encoding:


   KyberPublicKey ::= SEQUENCE {

       t           OCTET STRING,

       rho         OCTET STRING

   }

Is https://datatracker.ietf.org/doc/draft-uni-qsckeys/01/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-uni-qsckeys/01/__;!!FJ-Y8qCqXTj2!eSTwME-1IjQkfjJFS0X7lRzbYJ1PhEzSelmmUcLwF9JaceAAKFQwvLRif4O6vS8ijYlK0TG2cYO4rpBHtq5AG1OZQAKxlnAfHtmn$> meant to become the document that defines the key formats for all the PQ keys that will be standardized?    If not, then it should probably just refer to whatever documents will define the formats so that we can at least agree on one common format for the PQ keys.


Cheers,

John Gray
Entrust
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