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

Christian Huitema <huitema@microsoft.com> Thu, 24 December 2015 01:05 UTC

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From: Christian Huitema <huitema@microsoft.com>
To: Eric Rescorla <ekr@rtfm.com>
Thread-Topic: [TLS] A small detail in HMAC key generation for Finished message
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Date: Thu, 24 Dec 2015 01:04:59 +0000
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Subject: Re: [TLS] A small detail in HMAC key generation for Finished message
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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.

-- Christian Huitema

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