Hi John, On Wed, Jul 15, 2020 at 22:49:09 CEST, John Wiersba wrote: > 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 distinguish between > 1. A file which contains a human-readable passphrase which could also > be entered interactively, and > 2. A file which contains random bytes. You are overthinking this, I think. If you create the keyfile via a text-editor, make sure you have no trailing newling (unless you _want_ that traling newline to be part of the passphrase). Many UNIX editors add that trailing newline when saving a file automatically and then you have a character in there you do not see but which is part of the passphrase. This comment just simplifies debugging the problem. If you create a new random keyfile, whatever bytes are in there are fine. A random keyfile will contain (almost certainly ;-) a lot of characters you cannot enter interactively anyways, hence this does not have "interactive entry" as use-case. [...] > 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. Examples > are the value 256 above, and the parameters bs=512 count=8 for dd. If > I understand the FAQ correctly, the actual advice is > > Plain dm-crypt: Use > 80 bit. ... If paranoid, add at least 20 bit. > > This implies (taking the worst case) that > > head -c 13 /dev/random > > should be sufficient (13 * 8 bytes = 104 bits > 81+20 bits), and 256 > bytes is "overkill". I do understand that some reasonable amount of > overkill is essentially "free" and therefore can be used "just in > case". Yes. When it costs you nothing, use more. When it costs you something, what you quoted gives you a generally reasonable trade-off. Regards, Arno > Did I understand these two concepts correctly, and if so, could you > clarify the FAQ? > Thanks! > -- John Wiersba > _______________________________________________ > dm-crypt mailing list > dm-crypt@xxxxxxxx > https://www.saout.de/mailman/listinfo/dm-crypt -- Arno Wagner, Dr. sc. techn., Dipl. Inform., Email: arno@xxxxxxxxxxx GnuPG: ID: CB5D9718 FP: 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718 ---- A good decision is based on knowledge and not on numbers. -- Plato If it's in the news, don't worry about it. The very definition of "news" is "something that hardly ever happens." -- Bruce Schneier _______________________________________________ dm-crypt mailing list dm-crypt@xxxxxxxx https://www.saout.de/mailman/listinfo/dm-crypt
