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Re: [babel] hmac info model elements

Toke Høiland-Jørgensen <toke@toke.dk> Mon, 07 January 2019 19:11 UTC

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From: Toke Høiland-Jørgensen <toke@toke.dk>
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To: Dave Taht <dave.taht@gmail.com>
Cc: "STARK, BARBARA H" <bs7652@att.com>, Juliusz Chroboczek <jch@irif.fr>, Mahesh Jethanandani <mjethanandani@gmail.com>, Babel at IETF <babel@ietf.org>
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Date: Mon, 07 Jan 2019 20:11:39 +0100
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Subject: Re: [babel] hmac info model elements
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Dave Taht <dave.taht@gmail.com> writes:

> On Mon, Jan 7, 2019 at 8:35 AM Toke Høiland-Jørgensen <toke@toke.dk> wrote:
>>
>> "STARK, BARBARA H" <bs7652@att.com> writes:
>>
>> > Toke said: "Bird uses an unencoded string..."
>> > <bhs> I'm not sure what "unencoded" means here? But this sounds like
>> > maybe the entered string is a "passphrase" / PSK, like what's used for
>> > Wi-Fi, which is then used to derive the HMAC key using Unicode
>> > encoding?
>>
>> Nope, no derivation, just the raw ASCII bytes from the string used as an
>> HMAC key, zero-padded to the block size. Unless the supplied ASCII
>> string is longer than the block size, in which case it is hashed
>> first.
>
> ^^^^^^ ???? So you are saying an overlong key is transmuted into
> something else entirely, not truncated?

Yup, that would appear to be the case:

https://gitlab.labs.nic.cz/labs/bird/blob/master/lib/mac.c#L105


-Toke

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