Re: [TLS] A small detail in HMAC key generation for Finished message

Michael StJohns <msj@nthpermutation.com> Fri, 25 December 2015 16:47 UTC

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From: Michael StJohns <msj@nthpermutation.com>
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Date: Fri, 25 Dec 2015 11:47:12 -0500
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Subject: Re: [TLS] A small detail in HMAC key generation for Finished message
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On 12/23/2015 8:04 PM, Christian Huitema wrote:
> On Wednesday, December 23, 2015 3:05 PM, Eric Rescorla wrote:
>>> I wonder what the zero length string actually means. Is it a null-terminated string
>>> that would encode in binary as a one octet byte string of value 0, or an empty
>>> string that would encode in binary as a zero length string?
>> I see what you mean about the ambiguity here. What I meant was 0 bytes
>> (i.e., no trailing '\0').
> OK, makes sense.
>
>>> There is one example of encoding a string in section 4.8.1, and the binary representation
>>> shows the encoding of the final null byte. Is that a common assumption?
>> No.
>>
>>> Similarly, in the HKDF-Expand-Label, do we assume a final null byte for the "label"?
>> No. I wonder if we should instead add the '\0' explicitly in the 4.8.1 for maximal clarity.
> Either that, or just remove the trailing 00 from the binary description.

Or even better.  Express the label as a specific sequence of octets as 
the normative form for each and every label.   That then avoids 
questions of the form "C String"?  "Ascii"?, "Wide characters"?, etc.

Mike

>
> -- Christian Huitema
>
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