[Ndn-interest] Adding HMAC to available NDN signature types

Adeola Bannis abannis at ucla.edu
Sat Sep 20 11:18:12 PDT 2014


On Fri, Sep 19, 2014 at 11:18 PM, Yingdi Yu <yingdi at cs.ucla.edu> wrote:

>
> On Sep 19, 2014, at 10:44 PM, Wentao Shang <wentaoshang at gmail.com> wrote:
>
>
>
> On Fri, Sep 19, 2014 at 9:45 PM, Yingdi Yu <yingdi at cs.ucla.edu> wrote:
>
>> Hi Adeola,
>>
>> It is great that we have a proposal for HMAC, a few comments about the
>> doc.
>>
>> 1. I think you should mentioned in the spec that how to handle keys that
>> are longer than the hash output.
>>
>
> Hi Yingdi,
>
> Correct me if I'm wrong: I thought the key should have the same length as
> the hash output.
>
>
> Not necessarily. At least the RFC does not prevent the usage of longer
> key.
>
> What people usually do is to provide some kind of secret (e.g., a
> password) and use key derivation function to get the actual HMAC key.
>
>
> This is only a way to derive a symmetric key, but HMAC key does not have
> to be derived in this way. You do not want to impose such an restriction
> here.
>
>
Yes, "standard" HMAC does not explicitly restrict the size of the key, and
it also recommends that some key derivation function (bcrypt, PBKDF2, etc)
be used with any passphrase that is not the same size as the hash output. I
think Tai-Lin's point is fair, and we should let people do whatever they
would have done with their HMAC key.


>
> @Adeola, you probably want to forbid KeyDigest in KeyLocator for this HMAC
> signature. Because if key size is longer than hash output, the key digest
> is used instead. If we allow KeyDigest in KeyLocator, then some careless
> programmers may leak the secret.
>

Well, a careless programmer could put a passphrase, or something used to
derive the key, into a KeyLocator KeyName as well. We can go ahead and make
the restriction, but there are other ways a programmer could shoot herself
in the foot here.


> Yingdi
>
>
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