[OpenWrt-Devel] [PATCH] [package] dropbear: enable sha2-based hmac by default.

Alive alive4ever at live.com
Sat Sep 27 13:29:07 EDT 2014

On 27/09/2014 23:34, Weedy wrote:
> I got bit by this last week when I audited my ssh config and dropped all
> weak/slow ciphers.
> Couldn't log into my routers and had to add md5 ciphers and macs back.
You must rebuild dropbear package cleanly to test this patch

Save the patch as file located on
relative to OpenWrt source directory. The patch is available here

Do "make clean" to clean up the package, and then "make" to build the
Flash the resulting firmware and test connecting to dropbear ssh daemon.

To test using hmac-sha2-256, run this command
ssh -o MACs=hmac-sha2-256 root at OpenWrt
To test using hmac-sha2-512, run this command
ssh -o MACs=hmac-sha2-512 root at OpenWrt

For your information, there is almost no difference between message
authentication algorithm in time needed to establish ssh connection.

I perform the test on my TL-MR3220 (AR9330, mips, dropbear 2014.65,
r42321). The identity file is cached via ssh-agent. The test commands
are bellow.
time ssh -o MACs=hmac-md5 OpenWrt /bin/true
time ssh -o MACs=hmac-sha1 OpenWrt /bin/true
time ssh -o MACs=hmac-sha2-256 OpenWrt /bin/true
time ssh -o MACs=hmac-sha2-512 OpenWrt /bin/true

Here are time needed to establish ssh connection
|time\hmac|    md5   |   sha1   |  sha256  |  sha512  |
| real    | 0m3.526s | 0m3.631s | 0m3.519s | 0m3.543s |
| user    | 0m0.015s | 0m0.061s | 0m0.015s | 0m0.031s |
| sys     | 0m0.077s | 0m0.046s | 0m0.093s | 0m0.092s |

Putty and Bitvise will try connecting using hmac-sha2-256 when
available, whereas OpenSSH will use hmac-md5 by default.

I hope this information will be a good reason for enabling sha2-based
hmac on dropbear by default.
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