[Ietf-dkim] Question regarding RFC 6376

[David Harris <[e-mail]>]

Apologies in advance if this is the wrong place for this message - if it is and 
you happen to know the right place, I'd be grateful for a correction.

I'm trying to implement DKIM as per RFC6376 and associated errata in my 
mail server, but am having all manner of problems establishing clarity on 
section 3.7 of the RFC.

I note in advance that I have read the erratum that corrects this part of the 
pseudocode in section 3.7

   data-hash    =  hash-alg (h-headers, D-SIG, body-hash)

to this instead:

   data-hash    =  hash-alg (h-headers, D-SIG)


Question 1:

My first question is the exact meaning of this piece of pseudocode:

   body-hash    =  hash-alg (canon-body, l-param)

Does this mean:

1:  Pass the canonicalized body to the hash algorithm, then pass the value of 
any l= tag to the hash algorithm as well.

-- or --

2:  Pass the canonicalized body to the hash algorithm, restricting the amount 
passed to the value of any "l=" tag.

>From the way the other two pseudocode items are written, (A) would seem to 
be the correct interpretation, but that raises the question of what should be 
passed - a binary form of the l= value? Or a string representation? And what 
should be done if there is no l= tag at all?


Question 2:

I am puzzled by the reference to an item called "a-hash-alg" in the NOTE 
part at the end of this section: "a-hash-alg" does not appear anywhere else in 
the document, and since it has not been mentioned in an erratum anywhere 
(as far as I could see), I assume it must have some specific meaning defined 
somewhere else. Could someone direct me to a reference explaining this 
term?

Question 3:

The main problem I am having is understanding exactly how the signing 
process is meant to be handled: traditionally, you would either use your hash 
algorithm (SHA-256 in this case) to generate a digest of the content, then 
have your crypto library (OpenSSL in my case) generate a signature for that 
digest. There are also "streaming" APIs available that perform the hashing 
and signing as a combined task.

I would have assumed from the pseudocode that the intended action here 
was to do one of the following:

*  Pass d-domain, selector, and data-hash in that order to an SHA-256 hash 
algorithm, then generate an RSA signature using the hash generated by that 
process as its digest.

*  Call a streaming RSA-SHA256 API passing each of the three items in the 
correct order then finalizing the signature.

Either of these approaches *should* generate the same signature. The 
problem comes from the NOTE section, where it says:

-------------------------- Cut here ----------------------------
   When using such an API, the last two steps in the
   algorithm would probably be combined into a single call that would
   perform both the "a-hash-alg" and the "sig-alg".
-------------------------- Cut here ----------------------------

It is difficult to interpret this without knowing what "a-hash-alg" means, but on 
the assumption that it is a typo, that suggests that the pseudocode sections 
"data-hash" and "sig-alg" would be rolled into one step -- but I can't see how 
that could work, since "sig-alg" requires "data-hash" (which I understand to 
be an actual hash result) as its input. The NOTE comment seems so at odds 
with the two intended actions I described above that I am not sure which 
approach is the one I need to take.

I have spent quite some time trying to perform internet searches to clarify this 
section, but it's historically something I do quite poorly, so if there is a nice 
clean explanation of this somewhere, I'd be really grateful if someone could 
send me a link to it.

I'm sorry to send such a long message, but hope someone will feel charitable 
enough to help me out on these issues.

Cheers!

-- David --

------------------ David Harris -+- Pegasus Mail ----------------------
Box 5451, Dunedin, New Zealand | e-mail: [e-mail]
              Phone: Number provided on request only.

Things the newspapers could have said better, #997
   "Mr Benjamin Porter visited the school yesterday and lectured
   on "Destructive Pests". A large number were present."