Re: LC comments on draft-laurie-pki-sunlight-05

Ben Laurie <> Fri, 25 January 2013 11:16 UTC

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Date: Fri, 25 Jan 2013 11:16:18 +0000
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Subject: Re: LC comments on draft-laurie-pki-sunlight-05
From: Ben Laurie <>
To: =JeffH <>
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Apologies for responding to recent comments in random order: I'm
travelling and have accumulated something of a backlog.

On 22 January 2013 03:11, =JeffH <> wrote:
> apologies for latency, many meetings and a conference in the last couple of
> weeks.
> BenL replied:
>> On 1 January 2013 21:50, =JeffH <> wrote:
> [ in the below discussion:
>  "the spec", "this spec" refers to draft-laurie-pki-sunlight-05.
> "TLS-CT client"  refers to a TLS client capable of processing CT information
> that is included in the TLS handshake in any of the specified manners.
> "ok" means in general: "ok, will check this in next rev of the spec..".
> ]
> <snip>
>>> comments on draft-laurie-pki-sunlight-05
>>> substantive comments (in somewhat arbitrary order)
>>> --------------------------------------------------
> [ I demoted the comments wrt "JSON object" terminology and put them down at
> the end of this msg ]
>>> Also, the syntax for GETS isn't fully specified. Are the URL parameters
>>> to
>>> be encoded as (order independent) key-value pairs, or just as
>>> order-dependent values?  Which separator character is to be used between
>>> parameters? RFC3986 should be cited.
>> RFC 3986 says nothing about parameter format, though
> correct, it doesn't, and I wasn't trying to imply that it did, sorry.  I was
> just trying to say that RFC3986 should be cited in this spec because this
> spec normatively employs URLs, but perhaps referencing RFC2616 HTTP is
> better because it defines "http_URL".


>>  - is there a
>> standard reference for that? I've refereced HTML 4.01, but perhaps
>> there's a better one?
> hm, AFAICT, there is not a standard for URI query component formating and
> thus parameter encoding, so this spec will have to explicitly specify
> something. Section 3.4 of RFC3986 gives allowed chars for the query
> component, but that's about it.
> Have you mocked up code that parses the log client messages? If so, what
> query component syntax does it handle?

I have specified the "standard" format via HTML 4.01.

>>> 3. There appear to be three defined methods for TLS servers to provide
>>> TLS
>>> clients with CT data, in S3.2.  For this experiment, which approach is
>>> mandatory to implement for servers and clients?  Or, is it the case that
>>> participating TLS clients (ie web browsers etc) implement all three
>>> methods,
>>> and TLS servers can choose any of them?
>> The latter.
> That should be made very clear. Is the reason for doing so to obtain
> operational experience wrt the three defined methods such that they perhaps
> can be narrowed down in the future, or is the expectation that TLS-CT
> clients will need to support all three methods in perpetuity?

I think I made that clear.

The reason three methods exist are as follows (I don't intend to get
into this in the RFC, but for your edification):

1. TLS extension is the right way to do it, but requires a server s/w
change - this adds many years to full deployment.

2. So, alternatives must be provided. One is to put the stuff in the
certificate, but...

3. ... some CAs have said they'd rather not gate issuance on the log,
so alternatively, wedge the stuff in an OCSP response (which must be
stapled - servers exist that support this option already).

>>> 5. The recursive equations in S2.1 describe how to calculate a Merkle
>>> Tree
>>> Hash (MTH) (aka "root hash"), and thus as a side effect generate a Merkle
>>> Tree, for a given set of input data. However, there doesn't seem to be a
>>> defined algorithm (or even hints, really) for adding further inputs to an
>>> existing tree. Even though this may be reasonably left as an exercise for
>>> implementers, it should probably be discussed to some degree in the spec.
>>> E.g., note that leaf hashes are "frozen" and various interior tree node
>>> hashes become "frozen" as the tree grows. Is it not sub-optimal to employ
>>> the obvious default brute-force mechanism of rebuilding a tree entirely
>>> from
>>> scratch when new inputs are available?  Would not a recursive algorithm
>>> for
>>> adding new inputs to an existing tree be straightforward to provide?
>> I dunno about straightforward.
> yeah, agreed.
>> I'll think about it.
> ok. At least providing some hints would be useful it seems.

We do provide working code :-)

>>> 6. Signed tree heads (STHs) are denoted in terms of "tree size" (number
>>> of
>>> entries), but SCTs are denoted in terms of a timestamp.  Should there be
>>> a
>>> log client message supporting the return of the nearest STH (and thus
>>> tree
>>> size) to a given timestamp?
>> I'm not sure why? Any STH (that includes that SCT) will do.
> Hm, it was sort of a gut feel that it might be useful, but perhaps not.
> S5.2. Auditor says..
>    A certificate accompanied by an SCT can be verified against any STH
>    dated after the SCT timestamp + the Maximum Merge Delay by requesting
>    a Merkle Audit Proof using Section 4.5.
> S4.5 get-proof-by-hash stipulates tree_size as an input,  but
> if a log auditor doesn't already have tree_size, then I suppose it first
> calls S4.3 get-sth, which will return a timestamp and a tree_size, which if
> generated max merge delay (MMD) after the SCT was gen'd, ought to be
> sufficient, yes?


> I don't see in the spec where/how MMD is published.  Does MMD vary per log
> service?  The latter isn't stipulated in the spec it seems AFAICT ?

We have not really figured out how MMD is specified. I suspect it is
something that will be agreed between browser vendors and logs.

>>> 7. S3 paragraph 2 states that "TLS clients MUST reject certificates that
>>> do
>>> not have a valid SCT for the end-entity certificate" (i.e., hard-fail).
>>> Presummably this requirement is only for TLS clients participating in the
>>> CT
>>> experiment and that understand this protocol.
>> Of course - what other way could it be? In other words, all RFCs can
>> only say what implementations that conform with them do.
>>> This, or whatever the
>>> requirement actually is, should be further explained.
> I guess what I was getting at is that CT-conformant TLS clients should be
> differentiated from non-CT-conformant ones. Stipulating a name for
> CT-conformant TLS clients would clarify this (seems to me), e.g. "TLS-CT
> client" or something similar.

I understand what you're getting at, but not why. CT conformant TLS
clients obey all the MUSTs and non-conformant ones do not. That is
what MUST means.

>>> 8. The spec implies, but doesn't clearly describe, especially in S3.1,
>>> that
>>> the hashes are "labels" for tree entries, and that given a leaf hash, the
>>> log implementation should be able to look up and present the LogEntry
>>> data
>>> defined in that section.
>> We actually only require an entry to be retrievable by hash (in
>> effect) for the message in 4.7, which we (at least currently) label as
>> a debugging message - so I am not sure that logs really are required
>> to be able to do that - certainly the system would work fine if they
>> couldn't, I believe (other than being unable to provide the debugging
>> data).
> I think what I was getting at was that this is stated at the end of S3.2..
>    The leaves of the Merkle Tree are the hashes of the corresponding
>    "MerkleTreeLeaf" structures.
> ..but it is somewhat confusing because (abstractly) each leaf also contains
> (or has a reference to) the LogEntry data (that's defined back in S3.1)
> ...if I understand this correctly.

I suppose this reflects our own confusion :-)

A LogEntry shows that someone we know can be blamed for the creation
of the certificate - perhaps indirectly. This is desirable because:

a) We want to limit spam, and

b) If someone starts misissuing, we probably want to know who that is...

However, this information is not need to _fix_ the misissuance, it
turns out. It also is not necessarily the information TLS clients may
see (because of cross-signing). So, I think we;re a bit ambivalent
about the precise role of this information.

>>> While in CT, if the input set is an odd number of entries, then the hash
>>> of
>>> the final single leaf is at layer 1, and is calculated as a leaf hash
>>> using
>>> 0x00. Thus CT "interior nodes" always have two children, but if the tree
>>> has
>>> an odd number of entries, the rightmost hash at layer 1 ("j" in the
>>> "binary
>>> Merkle tree with 7 leaves" figure) is a leaf node hash rather than an
>>> interior node hash.
>>> O-8. [CrosbyWallach] discusses auditing and gossiping and could be cited
>>> as
>>> a source for further discussion on those topics.
>>> O-9. The notion of "commitments" isn't well defined, and where "add a
>>> commitment to D[k:n]"  couldn't  "add an interior/intermediate node to
>>> D[k:n}"  be used?
>>> Is not the term "commitment" used in [CrosbyWallach] equivalent to the
>>> sha256_root_hash (an STH component) in the spec?
>>> [CrosbyWallach] uses the term "interior node(s)" while the spec uses
>>> "intermediate nodes" (in one place).
>> CT is not actually derived from [CrosbyWallach], we just mention it as
>> a useful reference.
> yeah, it is useful, it would've been much harder to figure out the spec
> without it. Is there an existing paper that the spec is based more closely
> on?

No, we started afresh :-)

> The differences between the spec and [CrosbyWallach] doesn't seem to be that
> large, and so if there aren't more closely matching paper(s) to cite (?), it
> may be worth summarizing the differences between the spec and
> [CrosbyWallach] e.g. in an appendix.
>> "commitment" is a term of cryptographic art.
> so it is, but its definitions in the literature seem to vary somewhat
> depending on context.  IIUC, the spec uses "commitment" to refer to the hash
> labels of intermediate nodes ('interior nodes' in [CrosbyWallach]) as well
> as the root hash, yes?
> In S2.1.1, where it says..
>          We prove that the left subtree entries D[0:k] are consistent
>    and add a commitment to D[k:n]:
>'s not clear just what "add a commitment" means. add it (an
> intermediate-node hash?) to the list of hashes that the recursive algorthm
> is constructing?
> Overall, I'm finding S2.1.1 pretty difficult to parse and sort out.
> E.g., it isn't clear to me how/why/when the apparently boolean 3d parameter
> of SUBPROOF is either true or false, and whether there's an implied "if b
> then ... else ..."  in there somewhere.

Its reasonably hard to define this thing rigorously, and so I'm not
surprised you find it hard to follow - but as I think I've said
before, we did try various different phrasings and did not find an
easier one. Suggestions welcome.

>>> O-10. The recursive algorithms in S2 are dense and take effort to work
>>> through, perhaps adding simplistic example code (in an appendix) which
>>> implements, and/or actually working through the algorithms to arrive at
>>> some
>>> of the audit paths and consistency proofs in S2.1.3, would be helpful.
>> We have actual working code - would a reference to that be better?
> I don't know if it would be better (I've compiled it and am poking through
> the code). I sorta think pseudo code in an appendix that would generate the
> trees in S2.1.3. would be helpful.

I suspect this is more relevant to a non experimental RFC - right now
I have no evidence that anyone is planning to actually write code
other than us - AFAIK, so far everyone who has played with it has used
our code.

>>> O-14. Detailed comments on S2...
>>> ------------------------------
>>>> 2. Cryptographic components
>>>> 2.1. Merkle Hash Trees
>>>>    Logs use a binary Merkle hash tree for efficient auditing.  The
>>>>    hashing algorithm is SHA-256 (note that this is fixed for this
>>>>    experiment but it is anticipated that each log would be able to
>>>>    specify a hash algorithm).  The input to the Merkle tree hash is a
>>>>    list of data entries; these entries will be hashed to form the leaves
>>>>    of the Merkle hash tree.  The output is a single 32-byte root hash.
>>>>    Given an ordered list of n inputs, D[n] = {d(0), d(1), ..., d(n-1)},
>>>>    the Merkle Tree Hash (MTH) is thus defined as follows:
>>>>    The hash of an empty list is the hash of an empty string:
>>>>    MTH({}) = SHA-256().
>>> This MTH({}) construct doesn't appear to be used anywhere else in the
>>> spec
>>> (yes?), and so does it really need mentioning?
>> If it is not defined, then we cannot represent an empty tree.
> yeah, I agree from a mathematical completeness perspective.  but I still
> found it sort of distracting in that I don't know that it's actually germane
> from an implementor's perspective. maybe it is because
>>>>    The hash of a list with one entry is:
>>>>    MTH({d(0)}) = SHA-256(0x00 || d(0)).
>>> The immediately above equation is for leaf entries (yes?),
>> Yes.
>>> where in this
>>> notation n = 1, perhaps it should be stated explicitly:
>>>     When n = 1, a leaf entry is denoted, and D[1] = {d(0)}. The leaf hash
>>>     (LH) for a leaf entry is calculated as:
>>>     MTH(D[1]) = LH(D[1]) = SHA-256( 0x00 || d(0) )
>> Ugh. LH(D[1]) seems meaningless to me. A leaf hash is always of a "1
>> entry tree".
> yeah, for those who have grokked this stuff. for others it isn't immediately
> obvious. also,  the above comment was motivated in part due to the missing
> formal definition for "Leaf Hash" noted in item (4) above. even if the
> LH(D[1]) notation isn't used, some additional prose along the lines
> suggested above would be helpful to less initiated readers imv.
>>>>    For n > 1, let k be the largest power of two smaller than n.
>>> The unqualified "power of two" phrase is arguably ambiguous.
>> It is?
> uh, yeah.
> but "let k be a number which is the largest power of two such that..."
> isn't.
>>> Suggested rephrase for this where it occurs throughout section 2..
>>>     For n > 1, let k be a number which is the largest power of two
>>>     such that k = 2^i, 0 <= i < n, and k < n.
>> If we're going to go down that path, then it should say:
>> For n > 1, let k be the largest number such that k = 2^i and k < n.
>> or
>> For n > 1, let k = 2^i s.t. k < n and 2k >= n.
>> surely?
> well, yes (i prefer the former), but i think it's also important to
> explicitly state 0 <= i < n

Why? Its weird! its also not generally true - e.g. n=2 and i=1.

>>>>    The Merkle Tree Hash of an n-element list D[n] is then defined
>>>>    recursively as
>>> The above statement applies to the combination of the n = 1 equation
>>> above
>>> and the equation below, and so should perhaps be moved up above the n = 1
>>> equation.
>> ? It says n > 1, so doesn't apply to n = 1?
> oh yeah huh.
> in any case, I found the prose separation of the "list with one entry" and
> the "n > 1" case to be confusing because the former is part of the recursive
> definition of MTH(), yes?

OK, so D[0] = {} and D[1] = {d[0]} and we should make that more
explicit as you say which I expect will make this a lot clearer.

>>>>    MTH(D[n]) = SHA-256(0x01 || MTH(D[0:k]) || MTH(D[k:n])),
>>>>    where || is concatenation and D[k1:k2] denotes the length (k2 - k1)
>>>>    list {d(k1), d(k1+1),..., d(k2-1)}.
>>> The above phrase doesn't parse well and is somewhat ambiguous, here it is
>>> extracted for clarity:
>>>  "D[k1:k2] denotes the length (k2 - k1) list {d(k1), d(k1+1),...,
>>> d(k2-1)}"
>>> How about rephrasing it along the lines of this:
>>>     D[k1:k2] denotes a sublist {d(k1), d(k1+1),..., d(k2-1)}, having
>>>     (k2 - k1) elements, of the original input list D[n]. When (k2 - k1)
>>>     is 1, a leaf hash is calculated.
>> We tried lots of different ways of saying this and they were all a
>> little messy. Yours mixes concerns and is rather verbose, so not
>> convinced it is actually an improvement.
> well, the way it's presently said in the spec is (to me) pretty darn hard to
> understand.
> which concerns are missed in the suggested reformulation?

I said "mixes" :-)

That is, it mixes the hashing up with the definition of the list.
Other than that, I'm OK with the rephrasing.

>>>>    Anyone can submit certificates to certificate logs for public
>>>>    auditing, however, since certificates will not be accepted by clients
>>>>    unless logged, it is expected that certificate owners or their CAs
>>>>    will usually submit them.  A log is a single, ever-growing, append-
>>>>    only Merkle Tree of such certificates.
>>>>    When a valid certificate is submitted to a log, the log MUST
>>>>    immediately return a Signed Certificate Timestamp (SCT).  The SCT is
>>>>    the log's promise to incorporate the certificate in the Merkle Tree
>>>>    within a fixed amount of time known as the Maximum Merge Delay (MMD).
>>>>    If the log has previously seen the certificate, it MAY return the
>>>>    same SCT as it returned before.
>>> What if the submitted end entity cert is the same, but the certificate
>>> chain
>>> is different (yet valid)?
>> The purpose of the chain is to:
>> a) Prevent spam, and
>> b) Identify who to blame in the event of a misissue.
>> Alternate chains presumably don't actually change the direct blame,
>> and so I see no reason to do other than what the I-D says - i.e.
>> return the same SCT as before.
> ok. tho i wonder if there's any value in caching the alternate chain or the
> new portions thereof.

Logs are free to keep additional information, of course :-)

We probably will.

Its not clear that its particularly useful.

>>>>                                     TLS servers MUST present an SCT from
>>>>    one or more logs to the client together with the certificate.  TLS
>>>>    clients MUST reject certificates that do not have a valid SCT for the
>>>>    end-entity certificate.
>>> [ see comment (7) above ]
>>>>    Periodically, each log appends all its new entries to the Merkle
>>>>    Tree, and signs the root of the tree.  Clients and auditors can thus
>>> Should "Clients and auditors" actually be "TLS Clients, log monitors, and
>>> log auditors" ?
>> Bearing in mind that these are actually roles rather than distinct
>> entities, it should probably just say "auditors".
> I dunno, that may loose some readers.  in any case, the distinction between
> roles and distinct entities should probably be mentioned/discussed e.g. in
> the Introduction or thereabouts.


> <snip>
>>>> 3.1. Log Entries
>>>>    Anyone can submit a certificate to any log.  In order to enable
>>>>    attribution of each logged certificate to its issuer, the log SHALL
>>>>    publish a list of acceptable root certificates (this list might
>>>>    usefully be the union of root certificates trusted by major browser
>>>>    vendors).  Each submitted certificate MUST be accompanied by all
>>>>    additional certificates required to verify the certificate chain up
>>>>    to an accepted root certificate.  The root certificate itself MAY be
>>>>    omitted from this list.
>>>>    Alternatively, (root as well as intermediate) Certificate Authorities
>>> Additionally?  which manner is the experiment going to operate, or is it
>>> TBD
>>> ?
>> Not sure what you mean? The log will accept either type of submission.
> oh, sorry, I meant s/Alternatively/Additionally/

Probably the other way round? In any case, as I said, the log will
accept either type - some CAs seem to prefer one, some the other.

>>>>    Structure of the Signed Certificate Timestamp:
>>> The SCT discussion here should probably be its own subsection.
>> OK.
>>>>        enum { certificate_timestamp(0), tree_hash(1), 255 }
>>>>          SignatureType;
>>>>        enum { v1(0), 255 }
>>>>          Version;
>>>>          struct {
>>>>              opaque key_id[32];
>>>>          } LogID;
>>>>          opaque CtExtensions<0..2^16-1>;
>>>>    "key_id" is the SHA-256 hash of the log's public key, calculated over
>>>>    the DER encoding of the key represented as SubjectPublicKeyInfo.
>>> I'd place the above paragraph regarding "key_id" down below the
>>> SignedCertificateTimestamp definition.
>>>>        struct {
>>>>            Version sct_version;
>>>>            LogID id;
>>>>            uint64 timestamp;
>>>>            CtExtensions extensions;
>>>>            digitally-signed struct {
>>>>                Version sct_version;
>>>>                SignatureType signature_type = certificate_timestamp;
>>>>                uint64 timestamp;
>>>>                LogEntryType entry_type;
>>>>                select(entry_type) {
>>>>                    case x509_entry: ASN.1Cert;
>>>>                    case precert_entry: ASN.1Cert;
>>>>                } signed_entry;
>>>>               CtExtensions extensions;
>>>>            };
>>>>        } SignedCertificateTimestamp;
>>>>    The encoding of the digitally-signed element is defined in [RFC5246].
>>> I would add a few words here summarizing that what happens here is that
>>> the
>>> digitally-signed struct here is replaced in the actual serialized binary
>>> structure by a struct DigitallySigned and cross-ref to S4.7 of RFC5246.
>> Except it isn't :-)
> ok, then I don't understand what's going on here.  RFC5246 S4.7 sez...
>    A digitally-signed element is encoded as a struct DigitallySigned:
>       struct {
>          SignatureAndHashAlgorithm algorithm;
>          opaque signature<0..2^16-1>;
>       } DigitallySigned;
> ..and I take the "digitally-signed struct" within SignedCertificateTimestamp
> to be a "digitally-signed element".  Please help me understand what am I
> missing?

We've actually significantly revamped this whole thing, so hopefully
you'll find it clearer in the next version.

>>>> 5.1. Monitor
>>>>    Monitors watch logs and check that they behave correctly.  They also
>>>>    watch for certificates of interest.
>>> "Monitor" should be "Log Monitor" ?
>> There's no other kind of monitor :-)
> in the explicit context of this spec, agreed.  but in general I
> favor/advocate creation and use of more fully descriptive words/phrases so
> at least one is more likely to find them with a search when you're wondering
> what sort of "monitor" someone down the road is yammering on about.
> [ I would add a terminology section to the spec ]
>>>> 5.2. Auditor
>>>>    Auditors take partial information about a log as input and verify
>>>>    that this information is consistent with other partial information
>>>>    they have.  An auditor might be an integral component of a TLS
>>>>    client, it might be a standalone service or it might be a secondary
>>>>    function of a monitor.
>>> "Auditor" should be "Log Auditor" ?
>> And there's no other kind of auditor.
> see above :)
> e.g. in the overall webpki world, there /are/ other forms of auditors (e.g.
> the ones noted here <>) and
> so it's worth using a more descriptive term, imv.
>>>> 8. Efficiency Considerations
>>>>    The Merkle tree design serves the purpose of keeping communication
>>>>    overhead low.
>>>>    Auditing logs for integrity does not require third parties to
>>>>    maintain a copy of each entire log.  The Signed Tree Heads can be
>>>>    updated as new entries become available, without recomputing entire
>>>>    trees.  Third party auditors need only fetch the Merkle consistency
>>>>    proofs against a log's existing STH to efficiently verify the append-
>>>>    only property of updates to their Merkle Trees, without auditing the
>>>>    entire tree.
>>> The above could be explained in more detail, and S5.1 should be
>>> cross-referenced. Is the last sentence above essentially a summary of
>>> step
>>> #8 in S5.1? Or are there differences?
>> It is a summary of 5.1
> ok, but it'd be helpful to state that and crossref S5.1.

Actually, its the auditor not the monitor, but I have referenced it now.

> wrt he JSON stuff...
>>> 1. The client messages S4 don't explicitly lay out the syntax for request
>>> messages or responses. E.g., for S4.1 "Add Chain to Log", is the input a
>>> stand-alone JSON text array, or a JSON text object containing a JSON text
>>> array?
>>> The term "JSON object" as used in the first paragraph is ambiguous and
>>> perhaps what is mean is simply "JSON texts" or "JSON text objects or JSON
>>> text arrays". RFC4627 clearly defines "JSON text", and should be cited.
>>> But
>>> RFC4627 is a little ambiguous itself regarding "JSON object" and so I
>>> suggest these definitions:
>>>     JSON text object:   A JSON text matching the "object" ABNF production
>>>        in Section 2.2 of [RFC4627].
>>>     JSON text array:   A JSON text matching the "array" ABNF production
>>>        in Section 2.3 of [RFC4627].
>> I agree that RFC 4627 should be cited and I will correct that.
> ok.
>> The
>> rest of this confuses me: JSON is a textual representation of
>> structured data, as it states in the RFC. It defines an object quite
>> clearly
>> " An object is an unordered collection of zero or more name/value
>>    pairs, where a name is a string and a value is a string, number,
>>    boolean, null, object, or array."
> well, yes, but that's in just the introduction of RFC 4627.
>> Defining a "JSON text object" seems pointless to me - clearly a JSON
>> object is an object as defined by JSON, surely? Introducing another
>> term seems like to add confusion rather than remove it.
> note that "JSON text" is very explicitly defined in RFC4627 at the beginning
> of S2 as..
>    A JSON text is a sequence of tokens.  The set of tokens includes six
>    structural characters, strings, numbers, and three literal names.
>    A JSON text is a serialized object or array.
>       JSON-text = object / array
> the reason I flagged this issue is that I just recently reviewed a different
> internet-draft where they'd confused a JSON object -- in the sense of a JSON
> text matching the object production of S2.2 RFC4627 -- and a "JSON object"
> in the sense of an abstract programming construct, and they didn't
> understand that in the protocol on-the-wire world a JSON object /is a
> string/  (i.e. it is string-serialized according to the grammar of RFC4627).
> It seems the term "JSON object" is ambiguous depending on whether you're
> looking at it from a programming perspective or an on-the-wire protocol
> perspective (eg see <>
> which talks about "internal form" and "external form" (ugh)), hence my
> (perhaps feeble) attempt to invent a more explicit term for the on-the-wire
> protocol form.
> So maybe a more palatable definition would be..
>   JSON object: A JSON text matching the "object" ABNF production
>                in Section 2.2 of [RFC4627].


> ?
> ---
> end