Deprecating Old Crypto in a Linux Distro: A tale of something that looked obvious but .. there's a lesson in it somewhere.

While working on my Linux distro project at work, one of the things I recently wanted to do is phase out old crypto.

Yes we all read Bruce Schneider's text and how important it is, but nothing drives it home like reading The Guardian articles followed

by OpenSSL downgrade attacks in the last year or two.

Now, nothing should be defaulting to some of the antique crypto, but the only way to know 100% sure that the algorithms in question aren't being used, is to just not compile them into the various crypto libraries of your distro.

So.. step 1 was to look at the algorithm list of openssl:

arjan@clr:~$ openssl ciphers

ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:SRP-DSS-AES-256-CBC-SHA:SRP-RSA-AES-256-CBC-SHA:SRP-AES-256-CBC-SHA:DH-DSS-AES256-GCM-SHA384:DHE-DSS-AES256-GCM-SHA384:DH-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA256:DH-RSA-AES256-SHA256:DH-DSS-AES256-SHA256:DHE-RSA-AES256-SHA:DHE-DSS-AES256-SHA:DH-RSA-AES256-SHA:DH-DSS-AES256-SHA:DHE-RSA-CAMELLIA256-SHA:DHE-DSS-CAMELLIA256-SHA:DH-RSA-CAMELLIA256-SHA:DH-DSS-CAMELLIA256-SHA:ECDH-RSA-AES256-GCM-SHA384:ECDH-ECDSA-AES256-GCM-SHA384:ECDH-RSA-AES256-SHA384:ECDH-ECDSA-AES256-SHA384:ECDH-RSA-AES256-SHA:ECDH-ECDSA-AES256-SHA:AES256-GCM-SHA384:AES256-SHA256:AES256-SHA:CAMELLIA256-SHA:PSK-AES256-CBC-SHA:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:SRP-DSS-AES-128-CBC-SHA:SRP-RSA-AES-128-CBC-SHA:SRP-AES-128-CBC-SHA:DH-DSS-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:DH-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-SHA256:DHE-DSS-AES128-SHA256:DH-RSA-AES128-SHA256:DH-DSS-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA:DH-RSA-AES128-SHA:DH-DSS-AES128-SHA:DHE-RSA-SEED-SHA:DHE-DSS-SEED-SHA:DH-RSA-SEED-SHA:DH-DSS-SEED-SHA:DHE-RSA-CAMELLIA128-SHA:DHE-DSS-CAMELLIA128-SHA:DH-RSA-CAMELLIA128-SHA:DH-DSS-CAMELLIA128-SHA:ECDH-RSA-AES128-GCM-SHA256:ECDH-ECDSA-AES128-GCM-SHA256:ECDH-RSA-AES128-SHA256:ECDH-ECDSA-AES128-SHA256:ECDH-RSA-AES128-SHA:ECDH-ECDSA-AES128-SHA:AES128-GCM-SHA256:AES128-SHA256:AES128-SHA:SEED-SHA:CAMELLIA128-SHA:IDEA-CBC-SHA:PSK-AES128-CBC-SHA:ECDHE-RSA-RC4-SHA:ECDHE-ECDSA-RC4-SHA:ECDH-RSA-RC4-SHA:ECDH-ECDSA-RC4-SHA:RC4-SHA:RC4-MD5:PSK-RC4-SHA:ECDHE-RSA-DES-CBC3-SHA:ECDHE-ECDSA-DES-CBC3-SHA:SRP-DSS-3DES-EDE-CBC-SHA:SRP-RSA-3DES-EDE-CBC-SHA:SRP-3DES-EDE-CBC-SHA:EDH-RSA-DES-CBC3-SHA:EDH-DSS-DES-CBC3-SHA:DH-RSA-DES-CBC3-SHA:DH-DSS-DES-CBC3-SHA:ECDH-RSA-DES-CBC3-SHA:ECDH-ECDSA-DES-CBC3-SHA:DES-CBC3-SHA:PSK-3DES-EDE-CBC-SHA:EDH-RSA-DES-CBC-SHA:EDH-DSS-DES-CBC-SHA:DH-RSA-DES-CBC-SHA:DH-DSS-DES-CBC-SHA:DES-CBC-SHA

A few things stand out immediately.

RC4. This like seriously predates MD5, and MD5 is already suspect.

DES. Yes really. DES. in 1995 I worked at a company as an intern that made DES chips that you could use to brute force DES. In 1995, when Twin Peaks was on TV and you measured transistor sizes of a chip in micrometers not nanometers.

MD5. The general consensus seems to be that for crypto, you shouldn't use MD5 anymore. I'm not talking about SHA1, where one can argue that existing uses are still ok, but MD5.

I decided to draw my first line there, stick to the consensus and all that.

The good news is that OpenSSL is very configurable, and it's pretty easy to say

no-rc4 no-des no-md5

on the configure line (and for good measure, I added no-ssl2 and no-ssl3).

At this point, I thought I was on a roll, removing old crypto is easy, lets finish this 15 minute project before the project meeting starts.

So now on to the bad news. And sadly, there is plenty to be had.

openssl does not even

**compile** with the no-md5 option:

make[1]: Entering directory '/builddir/build/BUILD/openssl-1.0.2a/ssl'

In file included from s3_srvr.c:171:0:

../include/openssl/md5.h:70:4: error:

#error MD5 is disabled.

# error MD5 is disabled.

^

In file included from s3_clnt.c:158:0:

../include/openssl/md5.h:70:4: error:

#error MD5 is disabled.

# error MD5 is disabled.

^

....

Ok, so MD5 is technically not insane broken for small packets, and

it's just consensus not so much hard earned proof, so maybe deprecating md5 is a project for another day.

openssl does not even

**compile** with the no-des option:

make[2]: Entering directory '/builddir/build/BUILD/openssl-1.0.2a/apps'

../libcrypto.so: undefined reference to `EVP_des_ede3_wrap'

or when you fix that, it does not pass its test suite (I'll spare you the details).

Now here I had to draw a line. 20 years ago DES was not secure.. never mind today. I wouldn't be surprised if someone will chime in and say that their smartwatch can brute force DES in realtime now.

So.. fixing it is.

I suppose the good news is that no-rc4 went just fine.

The success story then, with the list of crypto from openssl after no-rc4 and no-des:

$ openssl ciphers

ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:SRP-DSS-AES-256-CBC-SHA:SRP-RSA-AES-256-CBC-SHA:SRP-AES-256-CBC-SHA:DH-DSS-AES256-GCM-SHA384:DHE-DSS-AES256-GCM-SHA384:DH-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA256:DH-RSA-AES256-SHA256:DH-DSS-AES256-SHA256:DHE-RSA-AES256-SHA:DHE-DSS-AES256-SHA:DH-RSA-AES256-SHA:DH-DSS-AES256-SHA:DHE-RSA-CAMELLIA256-SHA:DHE-DSS-CAMELLIA256-SHA:DH-RSA-CAMELLIA256-SHA:DH-DSS-CAMELLIA256-SHA:ECDH-RSA-AES256-GCM-SHA384:ECDH-ECDSA-AES256-GCM-SHA384:ECDH-RSA-AES256-SHA384:ECDH-ECDSA-AES256-SHA384:ECDH-RSA-AES256-SHA:ECDH-ECDSA-AES256-SHA:AES256-GCM-SHA384:AES256-SHA256:AES256-SHA:CAMELLIA256-SHA:PSK-AES256-CBC-SHA:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:SRP-DSS-AES-128-CBC-SHA:SRP-RSA-AES-128-CBC-SHA:SRP-AES-128-CBC-SHA:DH-DSS-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:DH-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-SHA256:DHE-DSS-AES128-SHA256:DH-RSA-AES128-SHA256:DH-DSS-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA:DH-RSA-AES128-SHA:DH-DSS-AES128-SHA:DHE-RSA-SEED-SHA:DHE-DSS-SEED-SHA:DH-RSA-SEED-SHA:DH-DSS-SEED-SHA:DHE-RSA-CAMELLIA128-SHA:DHE-DSS-CAMELLIA128-SHA:DH-RSA-CAMELLIA128-SHA:DH-DSS-CAMELLIA128-SHA:ECDH-RSA-AES128-GCM-SHA256:ECDH-ECDSA-AES128-GCM-SHA256:ECDH-RSA-AES128-SHA256:ECDH-ECDSA-AES128-SHA256:ECDH-RSA-AES128-SHA:ECDH-ECDSA-AES128-SHA:AES128-GCM-SHA256:AES128-SHA256:AES128-SHA:SEED-SHA:CAMELLIA128-SHA:IDEA-CBC-SHA:PSK-AES128-CBC-SHA

no DES, no RC4.

But, as it was a Monday, the misery only started there (Dave Jones should have taught me that misery is like lawyers, it always comes in pairs).

I threw the no-rc4/no-des package into our build system, and in no time the world came apart on me. Half the distro broke!

Well not half, but several very important pieces.

It turns out that components like curl, libcurl (so anything speaking http), wget, openssh, mariadb, ...

all hard-code DES usage. Now, I'll give curl credit, with creative use of configure options, you can make it not compile DES in, but you can't then make it pass its testsuite.

There must be a lesson in here somewhere.

One, our team will be fixing these projects to not require DES (or RC4), and we'll send those patches to the upstream projects of course.

But more, and this is a call to action: If you're working on an open source project that uses crypto, please please don't opencode crypto algorithm usage.

The algorithm may be outdated at any time and might have to go away in a hurry.

And if you have to use a very specific algorithm anyway (for compatibility or otherwise), at least be kind and make a

configure option for each algorithm in your project, so that when things go bad (be it in 5 or 20 years), its very feasible to disable the algorithm entirely.