From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from sonic316-55.consmr.mail.gq1.yahoo.com (sonic316-55.consmr.mail.gq1.yahoo.com [98.137.69.31]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.server123.net (Postfix) with ESMTPS for ; Wed, 15 Jul 2020 22:49:45 +0200 (CEST) Date: Wed, 15 Jul 2020 20:49:09 +0000 (UTC) From: John Wiersba Reply-To: John Wiersba Message-ID: <269697254.1965946.1594846149229@mail.yahoo.com> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_1965945_422032307.1594846149227" References: <269697254.1965946.1594846149229.ref@mail.yahoo.com> Subject: [dm-crypt] Cryptsetup FAQ is confusing re: key-file length and newlines List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "dm-crypt@saout.de" ------=_Part_1965945_422032307.1594846149227 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable The FAQ as of today (2020/7/15) states Make sureno trailing newline (0x0a) is contained in the input key file, or = thepassphrase will not work because the whole file is used as input. But then a few lines later it suggests head -c 256 /dev/random > keyfile Obviously if /dev/random is used, it's possible that the keyfile will end w= ith a trailing newline. I think you're trying to distinguish between=20 =20 - A file which contains a human-readable passphrase which could also be = entered interactively, and =20 - A file which contains random bytes.=C2=A0 =20 In order for a passphrase to be entered interactively, the passphrase must = not end with a newline.=C2=A0 Therefore, when intending to use both a keyfi= le containing a passphrase and also enter that same passphrase interactivel= y, the keyfile should not contain a newline (anywhere), including a trailin= g newline.=C2=A0 If a keyfile does contain a newline (anywhere), the text o= f that keyfile will not be able to be entered interactively as a passphrase= . Additionally, I see lots of guidance on the length of a keyfile which uses = magic numbers, both on the internet and also in the FAQ.=C2=A0 Examples are= the value 256 above, and the parameters bs=3D512 count=3D8 for dd.=C2=A0 I= f I understand the FAQ correctly, the actual advice is Plain dm-crypt: Use > 80 bit.=C2=A0 ...=C2=A0 If paranoid, add at least 20 = bit. This implies (taking the worst case) that=20 head -c 13 /dev/random should be sufficient (13 * 8 bytes =3D 104 bits > 81+20 bits), and 256 byte= s is "overkill".=C2=A0 I do understand that some reasonable amount of overk= ill is essentially "free" and therefore can be used "just in case". Did I understand these two concepts correctly, and if so, could you clarify= the FAQ? Thanks!-- John Wiersba ------=_Part_1965945_422032307.1594846149227 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

The FAQ as of today (2020/7/15) states

Make sure no trailing newline (0x0a) is contained in the input key file, or the passphrase will not work because the whole file is used as input.
But then a few= lines later it suggests
head -c 256 /dev/random > keyfile
Obviously if /dev/random is used, it's possible that the keyfile will end = with a trailing newline.

I think you're trying to disting= uish between
A file which contains a human-readable passphr= ase which could also be entered interactively, and
A file whic= h contains random bytes.
In order for a passphrase to b= e entered interactively, the passphrase must not end with a newline. = Therefore, when intending to use both a keyfile containing a passphrase and= also enter that same passphrase interactively, the keyfile should not cont= ain a newline (anywhere), including a trailing newline. If a keyfile = does contain a newline (anywhere), the text of that keyfile will not be abl= e to be entered interactively as a passphrase.

Additional= ly, I see lots of guidance on the length of a keyfile which uses magic numb= ers, both on the internet and also in the FAQ. Examples are the value= 256 above, and the parameters bs=3D512 count=3D8 for dd. If I unders= tand the FAQ correctly, the actual advice is
<= div dir=3D"ltr" data-setdir=3D"false">
Plain dm-crypt: Use > 80 bit. ... If paranoid, add at l= east 20 bit.

This implies (= taking the worst case) that

head -c 13 /dev/random

should be sufficient = (13 * 8 bytes =3D 104 bits > 81+20 bits), and 256 bytes is "overkill".&n= bsp; I do understand that some reasonable amount of overkill is essentially= "free" and therefore can be used "just in case".

Did= I understand these two concepts correctly, and if so, could you clarify th= e FAQ?

Thanks!

-- John Wiersba

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