Date: Tue, 26 May 2009 19:02:10 +0200 From: Roland Smith <rsmith@xs4all.nl> To: Jeffrey Goldberg <jeffrey@goldmark.org> Cc: Kelly Jones <kelly.terry.jones@gmail.com>, freebsd-questions@freebsd.org Subject: Re: Secure unsalted or fixed salt symmetric encryption? Message-ID: <20090526170210.GB75202@slackbox.xs4all.nl> In-Reply-To: <43F89C0B-370E-4E29-9214-E447768C97A3@goldmark.org> References: <26face530905242257m7030933cy4a1171de7a06ee59@mail.gmail.com> <20090525190039.GA39139@slackbox.xs4all.nl> <43F89C0B-370E-4E29-9214-E447768C97A3@goldmark.org>
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--QKdGvSO+nmPlgiQ/ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, May 26, 2009 at 09:31:25AM -0500, Jeffrey Goldberg wrote: > On May 25, 2009, at 2:00 PM, Roland Smith wrote: >=20 > > You could use the -S option and specify a constant salt. It might make > > the encrypted materials easier to break, though. You can generate a > > random salt with openssl as well: >=20 > > Or you can use the -nosalt option. But as explained in > > [http://www.openssl.org/docs/apps/enc.html], using a random salt by > > default is a design decision because: "Without the -salt option it is > > possible to perform efficient dictionary attacks on the password". =20 > > That > > doesn't sound good, does it? >=20 > This is being used for file encryption, not password encryption.=20 Of course. > So a dictionary attack isn't all that likely unless the encrypted > files are of a specific nature Suppose you are encrypting a tarfile that includes /usr/src/. There are definitely files in that tree that haven't changed in a long time. These could be used as (partial) cribs.=20 > (known template which remains constant while only small parts of the > file vary).=20 Or if you have the case of a 'known-plaintext' attack. It happens more often than you would think:=20 [http://en.wikipedia.org/wiki/Known-plaintext_attack]=20 Note that using a random salt would be a good protection against such an attack! I agree that in this case such an attack seems unlikely.=20 =46rom the original posters' questions I get the feeling that he is looking for an incremental encrypted backup solution for a large file or files. All possible solutions involve trade-offs between ease of use, robustness and security. And as you've said making a good choice requires more insight into the constraints. Roland --=20 R.F.Smith http://www.xs4all.nl/~rsmith/ [plain text _non-HTML_ PGP/GnuPG encrypted/signed email much appreciated] pgp: 1A2B 477F 9970 BA3C 2914 B7CE 1277 EFB0 C321 A725 (KeyID: C321A725) --QKdGvSO+nmPlgiQ/ Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.11 (FreeBSD) iEYEARECAAYFAkocIJIACgkQEnfvsMMhpyV2hACgnNdtMEaWCoNkFn8PDZPch3k4 qWsAn0ytWNIxZx2AASCBAR9rhq0e9Hk+ =3fRk -----END PGP SIGNATURE----- --QKdGvSO+nmPlgiQ/--
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