Re: [Technical Errata Reported] RFC6960 (6165)

Russ Housley <[email protected]>
Newsgroups gmane.ietf.x509
Message-ID <[email protected]> 
We are talking about the DER encoding (not the BER encoding)

Russ

> On May 15, 2020, at 9:52 PM, Erwann Abalea <[email protected]> wrote:
> 
> I also think the errata is editorial.
> 
> In a BIT STRING encoding, the first octet (unused bits) is not part of the ASN.1 object, it's only part of its encoding.
> 
> The following hex strings are all valid BER encodings for a BIT STRING composed of 53 bits set to zero:
> 03 08 03 00000000000000
> 03 08 03 00000000000007
> 23 0D 03 02 00 00 03 07 03 000000000000
> 23 0D 03 02 00 00 03 07 03 000000000007
> 23 80 03 05 00 00000000 03 04 03 000000 0000
> 23 80 03 05 00 00000000 03 04 03 000007 0000
> 
> What is hashed is not the different encodings, but the BIT STRING value.
> In the preceding examples, only the first one is a valid DER encoding. And if you hash the 7 octets after the "unused bits" octet, you'll obtain a wrong hash, because the value is 53 bits long, not 56.
> 
> Le jeu. 14 mai 2020 à 19:11, <[email protected] <mailto:[email protected]>> a écrit :
> Hello All,
> 
> Let me explain some details. Firstly I need to say that I already wrote 
> a couple of mails to RFC6960 authors and discussed it a little. From 
> these discussions I got that before we would start our discussion I need 
> to provide you some technical details.
> 
> the main problem is that in RFC6960 there are two different description 
> for the same type KeyHash. First description is in 4.2.1 "ASN.1 
> Specification of the OCSP Response" of RFC6960:
> 
>     KeyHash ::= OCTET STRING -- SHA-1 hash of responder's public key
>     (excluding the tag and length fields)
> 
> And second is from Appendix B.1 "OCSP in ASN.1 - 1998 Syntax" of 
> RFC6960:
> 
> KeyHash ::= OCTET STRING -- SHA-1 hash of responder's public key
>                           -- (i.e., the SHA-1 hash of the value of the
>                           -- BIT STRING subjectPublicKey [excluding
>                           -- the tag, length, and number of unused
>                           -- bits] in the responder's certificate)
> 
> Then I need to clarify why these two descriptions are not the same. 
> Firstly I need to describe ASN.1 type BIT STRING. As all other ASN.1 
> types BIT STRING consists of three main parts: "Tag", "Length" and 
> "Value" (TLV). But a specific for BIT STRING is that "Value" itself 
> devided on two parts: first byte designates "unused bits" (this value 
> could be 0-7) and the rest is bits describing bits.
> So, when we are saying "SHA-1 hash of responder's key (excluding the tag 
> and length fields)" we do assume that SHA-1 hash should be for BIT 
> STRING ASN.1 value excluding the "Tag" and "Length" fields (responder's 
> key has type BIT STRING). Thus the SHA-1 hash should be for entire 
> "Value" block of BIT STRING, with "unused bits" byte (the very first 
> byte in BIT STRING "Value" block).
> 
> In opposite when we are saying " the SHA-1 hash of the value of the BIT 
> STRING subjectPublicKey [excluding the tag, length, and number of unused 
> bits] in the responder's certificate" we do assume that SHA-1 hash 
> should be for BIT STRING ASN.1 value excluding Tag, Length and the very 
> first byte from "Value" (unused bits).
> 
> Do you understand that two SHA-1 hashes made using these two 
> descriptions of KeyHash would be different? Or I would need to provide 
> you a real example with responder's public key values? Feel free to ask 
> me for the example.
> 
> As a conclusion: the RFC6960 has two descriptions of the same type and a 
> hash produced using these two descriptions would have different values. 
> This is a mistake. I do not really care how you would fix it and what 
> description for KeyHash is correct. But I want to have a correct 
> standard for OCSP.
> 
> As an addition I found that Appendix B.1 and Appendix B.2 have different 
> description for KeyHash. It is the same ASN.1 description, but different 
> in comment describing the SHA-1 calculation process. The Appendix B.2, 
> in fact, describes KeyHash calculation similar with RFC2560. But 
> Appendix B.1 describes new way of KeyHash calculation. Even if these two 
> appendicies are not aligned allows me to consider the errata change as 
> "technical".
> 
> Moreover, RFC6960 changes OCSP protocol in part of KeyHash calculation, 
> but it is never stated in any part of RFC6960. So, even if someone says 
> "calculation is obvious" but the problem that for KeyHash there is 
> "specifying comment". And exactly that "specifying comment" clearly 
> specify how to calculate KeyHash. But in RFC6960 there are two different 
> "ways of calculating KeyHash" (two different "specifying comments"). 
> This is also not an "editorial problem", this is clearly a problem in 
> technical description.
> 
> Best regards,
> Yury Strozhevsky
> 
> 
> 
> Stefan Santesson писал 2020-05-11 16:59:
> > I respond to all of these erratas collectivey (which I believe should
> > be consolidated to 1 and not 3 erratas)
> > 
> > I think the observation is correct, but I think it is editorial rather
> > than technical.
> > 
> > The normative text is that the KeyHash is the hash of the public key.
> > This is in itself clear an unambiguous.
> > The imperfection here is not in the normative text, but in the
> > explanation where the explanation in B.1 is the most accurate and
> > detaild.
> > The other shorter explanations are not wrong, but insufficient compared 
> > with B.1
> > 
> > Please also note that a similar definition exist for the 
> > "issuerKeyHash" like:
> > 
> > issuerKeyHash      OCTET STRING, -- Hash of issuer's public key
> > 
> > 
> > I think this is editorial
> > 
> > 
> > 
> > Stefan Santesson
> > 
> > On 2020-05-11, 14:16, "RFC Errata System" <[email protected] <mailto:[email protected]>> 
> > wrote:
> > 
> >     The following errata report has been submitted for RFC6960,
> >     "X.509 Internet Public Key Infrastructure Online Certificate
> > Status Protocol - OCSP".
> > 
> >     --------------------------------------
> >     You may review the report below and at:
> >     https://www.rfc-editor.org/errata/eid6165 <https://www.rfc-editor.org/errata/eid6165>
> > 
> >     --------------------------------------
> >     Type: Technical
> >     Reported by: Yury Strozhevsky <[email protected] <mailto:[email protected]>>
> > 
> >     Section: 1
> > 
> >     Original Text
> >     -------------
> >     ---
> > 
> >     Corrected Text
> >     --------------
> >        o  Appendix B.1 provides correct KeyHash type processing
> > description. Now SHA-1 hash must be calculated for responder's public
> > key ASN.1 value without tag, length and unused bits.
> > 
> > 
> >     Notes
> >     -----
> >     The RFC6960 changes OCSP protocol in part of KeyHash type
> > calculation. In RFC2560 there is the description:
> >        KeyHash ::= OCTET STRING -- SHA-1 hash of responder's public key
> >        (excluding the tag and length fields)
> > 
> >     But in Appendix B.1, which is the major OCSP descriptive module, 
> > stated:
> >     KeyHash ::= OCTET STRING -- SHA-1 hash of responder's public key
> >                              -- (i.e., the SHA-1 hash of the value of 
> > the
> >                              -- BIT STRING subjectPublicKey [excluding
> >                              -- the tag, length, and number of unused
> >                              -- bits] in the responder's certificate)
> > 
> >     The difference is in what would be under SHA-1 hash.. In RFC2560
> > KeyHash would be calculated for entire BIT STRING value, with "unused
> > bits" byte (first byte in BIT STRING value), but Appendix B.1 in
> > RFC6960 states that SHA-1 hash must be calculated for BIT STRING value
> > without "unused bits".
> > 
> >     Instructions:
> >     -------------
> >     This erratum is currently posted as "Reported". If necessary, 
> > please
> >     use "Reply All" to discuss whether it should be verified or
> >     rejected. When a decision is reached, the verifying party
> >     can log in to change the status and edit the report, if necessary.
> > 
> >     --------------------------------------
> >     RFC6960 (draft-ietf-pkix-rfc2560bis-20)
> >     --------------------------------------
> >     Title               : X.509 Internet Public Key Infrastructure
> > Online Certificate Status Protocol - OCSP
> >     Publication Date    : June 2013
> >     Author(s)           : S. Santesson, M. Myers, R. Ankney, A.
> > Malpani, S. Galperin, C. Adams
> >     Category            : PROPOSED STANDARD
> >     Source              : Public-Key Infrastructure (X.509)
> >     Area                : Security
> >     Stream              : IETF
> >     Verifying Party     : IESG
> 
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> -- 
> Erwann.
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