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

Christian Huitema <huitema@microsoft.com> Wed, 23 December 2015 22:19 UTC

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From: Christian Huitema <huitema@microsoft.com>
To: "tls@ietf.org" <tls@ietf.org>
Thread-Topic: A small detail in HMAC key generation for Finished message
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Date: Wed, 23 Dec 2015 22:19:10 +0000
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Subject: [TLS] A small detail in HMAC key generation for Finished message
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In the current 1.3 draft, section 6.3.4.3 specifies the content of the Finished message. It contains this specification for key computation:

client_finished_key =
    HKDF-Expand-Label(BaseKey, "client finished", "", L)

server_finished_key =
    HKDF-Expand-Label(BaseKey, "server finished", "", L)

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?

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?

Similarly, in the HKDF-Expand-Label, do we assume a final null byte for the "label"?

-- Christian Huitema

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