Re: Last Call: <draft-ietf-lamps-hash-of-root-key-cert-extn-02.txt> (Hash Of Root Key Certificate Extension) to Informational RFC

["Paul Hoffman" <[e-mail]>]

On 2 Jan 2019, at 11:57, Russ Housley wrote:

>> On Jan 2, 2019, at 1:26 PM, Paul Hoffman <[e-mail]> 
>> wrote:
>>
>> This extension seems useful to CAs that understand the increased 
>> risks that using it incurs, but those risks are not mentioned in the 
>> document.
>>
>> The document implicitly assumes that the CA will in fact use the key 
>> named in the extension next. Using this extension increases the risk 
>> of a bad trust anchor rollover at the same time that it makes good 
>> rollover more secure. If the key listed in this extension cannot be 
>> used when the CA eventually changes the trust anchor, relying parties 
>> will mistrust the new trust anchor. There are many reasons why a CA 
>> might think they know the next key but cannot use that key when they 
>> change trust anchors, such as if the HSM that holds the next key 
>> fails or is destroyed. Given the last sentence in Section 2, this 
>> could mean that a CA might never be able to issue a new trust anchor, 
>> even if the key for its current trust anchor is compromised.
>>
>> Given the severity of the new risks of using this extension, they 
>> need to be introduced at the beginning of the document and then 
>> discussed in more detail in the Security Considerations. Note that 
>> this risk affects the last paragraph of the Security Considerations 
>> section as well.
>
> The point is to facilitate the transition from one Root CA certificate 
> to the next.

To be clear, the transition is from one public key to the next, not from 
one certificate to the next. But, more importantly, the point of this 
extension is to facilitate the transition from one Root CA certificate 
to what is supposed to be the next key. However, if that next key is not 
seen by every relying party during the transition, the extension 
prevents the CA from ever making the transition for the relying parties 
that do not see the new key in a revised trust anchor. This seems like a 
huge restriction that is only mentioned in the document in exactly one 
sentence:

> The last sentence in Section 2 says:
>
>    If either check fails, then potential Root CA
>    certificate is not a valid replacement, and the recipient continues
>    to use the current Root CA certificate.
>
> Indeed, these check are necessary to make sure that the relying party 
> makes the transition to the proper replacement.  Any failure of the 
> checks leave the trust anchor unchanged, which seems like the right 
> result to me.

It seems right to me as well, but it still seems to be wholly 
insufficient to not talk about the risks of using the extension early in 
the document.

>
> Recall the definition of trust anchor from RFC 5280, Section 6.1.1:
>
>       (d)  trust anchor information, describing a CA that serves as a
>            trust anchor for the certification path.  The trust anchor
>            information includes:
>
>          (1)  the trusted issuer name,
>
>          (2)  the trusted public key algorithm,
>
>          (3)  the trusted public key, and
>
>          (4)  optionally, the trusted public key parameters associated
>               with the public key.
>
> The checks make sure that the replacement self-signed certificate 
> contains the intended information.

That is all fine, but it does not address the significant risk a CA is 
undertaking by promising what the next key will be.

>> Editorial:
>>
>> The abstract says:
>>   This document specifies the Hash Of Root Key certificate extension.
>>   This certificate extension is carried in the self-signed 
>> certificate
>>   for a trust anchor, which is often called a Root Certification
>>   Authority (CA) certificate.  This certificate extension 
>> unambiguously
>>   identifies the next public key that will be used by the trust 
>> anchor
>>   at some point in the future.
>> The term "trust anchor" is used as the concept, not the bits in the 
>> certificate. As such, the last sentence is confusing because the 
>> trust anchor will change when the key changes. A possible fix is to 
>> replace "will be used by the trust anchor at some point in the 
>> future" with "will be used in a trust anchor at some point in the 
>> future".
>
> I think I understand your point.  Does this text resolve the concern?
>
>    This document specifies the Hash Of Root Key certificate extension.
>    This certificate extension is carried in the self-signed 
> certificate
>    for a trust anchor, which is often called a Root Certification
>    Authority (CA) certificate.  This certificate extension 
> unambiguously
>    identifies the next public key that will be used at some point in 
> the
>    future as the next Root CA certificate, replacing the current one.

Not really. A key will not be used as a certificate: it is just a key. A 
key might be used as the signing key for a certificate, but not as the 
certificate itself. Maybe instead: "will be used to sign a trust anchor 
at some point in the future"?

>> The first list in Section 2 would be clearer if the order was R1, R2, 
>> H2, C1; this would also then match the order in the second list.
>
> Okay.  I changed that in my edit buffer.
>
>> Later in Section 2, a sentence appears to be missing its subject. 
>> "That is, verify the signature on the self-signed certificate..." 
>> should probably be "That is, the recipient verifies the signature on 
>> the self-signed certificate...".
>
> Okay.  I changed that in my edit buffer.
>
> In addition, I added a bit more detail about the relationship to 
> certification path validation, which I hope adds clarity around your 
> first comment.  It now reads:
>
>    The successor to the Root CA self-signed certificate can be
>    delivered by any means.  Whenever a new Root CA certificate is
>    received, the recipient is able to verify that the potential Root 
> CA
>    certificate links back to a previously authenticated Root CA
>    certificate with the hashOfRootKey certificate extension.  That is,
>    the recipient verifies the signature on the self-signed certificate
>    and verifies that the hash of the DER-encoded SubjectPublicKeyInfo
>    from the potential Root CA certificate matches the value from the
>    HashOfRootKey certificate extension of the current Root CA
>    certificate.  Checking the self-signed certificate signature 
> ensures
>    that the certificate contains the subject name, public key 
> algorithm
>    identifier, and public key algorithm parameters intended by the key
>    owner intends; these are important inputs to certification path
>    validation as defined in Section 6 of [RFC5280].  Checking the hash
>    of the SubjectPublicKeyInfo ensures that the certificate contains 
> the
>    intended public key.  If either check fails, then potential Root CA
>    certificate is not a valid replacement, and the recipient continues
>    to use the current Root CA certificate.

Yes, adding this clarifies how all the trust anchor information is 
linked through the validation process. This is a good addition.

--Paul Hoffman