[Ndn-interest] Describe the HMAC algorithm in SignatureHmacWithSha256?

christian.tschudin at unibas.ch christian.tschudin at unibas.ch
Mon Jun 1 09:48:44 PDT 2015

I understood Jeff wanting to make the spec as self-contained as 
possible, but I doubt that a reference to an HMAC-internal variable is 

(It translate into "Dear NDN implementor, to implement this two-line 
feature you need to read the following RFC and locate that Keyval 
variable in your favorit shrinked-wrapped library." As a probe: the 
Python hashlib API page does not reference it).

So better write at least that part out?

Regarding how to choose the KeyId (hash, ISAKMP SPI etc): I see, so it 
make sense to point out the many possibilities to pick your way. At 
least an advantage of the currently discussed hashing approach would 
be to not add another document to read 8-)

best, c

On Mon, 1 Jun 2015, Marc.Mosko at parc.com wrote:

> The proposed spec already defines KeyValue as the right-padded (for short keys) or sha256 (for long keys) of the Key.  I would define the KeyDigest as the SHA256(KeyValue) and not repeat the same algorithm used to compute KeyValue.
> With regard to repeating the definition of HMAC, I would not do that but simply say that the ‘text’ input to HMAC is {Name, MetaInfo, Content, SignatureInfo} TLVs.
> For symmetric key systems, like HMAC, I think it is also acceptable to use an agreed upon integer identifier for the shared secret, as determined by a key exchange protocol (e.g. an ISAKMP SPI).  I don’t think that symmetric key KeyDigests need to be derived from the key.  That’s different than public key systems, where the the KeyDigest is used like the Subject Key Identifier (RFC 5280 and derived from the actual public key.
> Marc
> On Jun 1, 2015, at 1:45 PM, <christian.tschudin at unibas.ch> <christian.tschudin at unibas.ch> wrote:
>> On Wed, 27 May 2015, Thompson, Jeff wrote:
>>> Hello Christian,
>>> Two questions:
>>> 1. Can you repeat your suggestion that the spec allow a truncated form of the key digest, for example the first 8 bytes of the key digest instead of the full 32 bytes?
>> Hi Jeff,
>> This is a trick that Marc Mosko used when we presented the 1+0 encoding at the January ICNRG interim meeting: Just reduce then number of KeyID-Bits (2 Bytes, in our case), if need be.
>> The observation is that the key digest is for convenience, not security reasons: If you have many parties for which you have to maintain different symmetric keys, it is nice to quickly identify which key to use for validating the signature. But theoretically, you could just try out all symmetric keys you have for some peer. Rivest's "chaffing and winnowing" even relies on not sending any KeyID bits, on purpose.
>>> 2. If you want this suggestion on the spec, what is the best way to describe the computation of the key digest if we don't describe the HMAC algorithm in the spec?
>> First, it could be made optional. But then I think it would be good to write down how the digest is computed, as you suggest (because RFC 2104 does not cover it, right?)
>> Adapted from previous text of yours:
>> "The Key must not be included in the signature but optionally a full or partial KeyDigest can be put in the KeyLocator block of the SignatureInfo field. If the byte length of Key is less than or equal to the SHA256 block length (64 bytes) then the full length KeyDigest is SHA256(Key). But if the byte length of Key is greater than 64 bytes, the KeyValue is already SHA256(Key) with zeros appended, so in this case the full length KeyDigest is SHA256(SHA256(Key)). The optional KeyDigest bits consist of 2 to 32 of the most-significant (=left-most) bytes of the full length KeyDigest. For the convenience of the validator and if packet size permits, it is recommended to include the full 32 bytes."
>> Would that be precise enough?
>> best, c
>>> Thanks,
>>> - Jeff T
>>> From: Junxiao Shi <shijunxiao at email.arizona.edu>
>>> Date: Wednesday, May 27, 2015 at 5:10
>>> To: Jeff Thompson <jefft0 at remap.ucla.edu>
>>> Cc: "ndn-interest at lists.cs.ucla.edu" <Ndn-interest at lists.cs.ucla.edu>
>>> Subject: Re: [Ndn-interest] Describe the HMAC algorithm in SignatureHmacWithSha256?
>>> Dear folks
>>> Details about HMAC algorithm, or any other crypto algorithm, SHOULD NOT appear in NDN Packet Format
>>> spec.
>>> Instead, the implementer should be referred to RFC.
>>> Those details are duplication of RFC, and they would make the spec unnecessary long.
>>> They also increase the probability of incorrect implementations because the implementer is unsure
>>> whether it's exactly same as what she/he has in the library, and would have to implement it again.
>>> "don't have HMAC in their crypto library" is not a valid argument - it's easier to find an
>>> RFC-compliant library or snippet for most languages than to implement according to the (duplicate of
>>> RFC in) spec.
>>> Yours, Junxiao
>>> On Mon, May 18, 2015 at 4:00 PM, Thompson, Jeff <jefft0 at remap.ucla.edu> wrote:
>>>      The proposed SignatureHmacWithSha256 spec (below) repeats the details of the HMAC
>>>      algorithm from RFC 2104. But should the details be removed and just refer to RFC 2104?
>>>      Arguments for keeping the details are that it provides details for the discussion of
>>>      creating the KeyDIgest and also because some applications don't have HMAC in their crypto
>>>      library and need to implement it directly. An argument against keeping the details is that
>>>      the info is in RFC 2104 so an application writer can read the RFC if needed, and that we
>>>      don't repeat the details of other algorithms like SHA-256.
>>> Any opinions on removing the algorithm details?
>>> - Jeff T
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