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“No explicit cipher (aes128-cb)

Merkur:jens jens$ scp jens@helios:/export/home/jens/XXXXX /dev/null
XXXXX 6% 143MB 23.7MB/s 01:22 ETA^CKilled by signal 2.

This is the default behaviour on my machines. Other setups might choose a different default algorithm. According to the OpenSSH homepage the list of availabe candidates is (cited from the man page):

Ciphers
Specifies the ciphers allowed for protocol version 2 in order of
preference. Multiple ciphers must be comma-separated. The default is
“aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc”

Using 3des

I expected 3des to be quite slow, and I wasn’t disappointed:

Merkur:~ jens$ scp -c 3des jens@helios:/export/home/jens/XXXXX /dev/null
XXXXX 3% 82MB 11.7MB/s 02:51 ETA^CKilled by signal 2.

As expected, 3des will not be our performance king today.
Using Blowfish

The next candidate was Blowfish. I expected at least aes performance from it. Let’s see, how it worked out:

Merkur:jens jens$ scp -c blowfish jens@helios:/export/home/jens/XXXXX /dev/null
XXXXX 29% 615MB 36.0MB/s 00:40 ETA^CKilled by signal 2.

Final candidate: RC4

The final candidate is the only stream cipher in the suit: arcfour.

Merkur:~ jens$ scp -c arcfour jens@helios:/export/home/jens/XXXXX /dev/null
XXXXX 9% 202MB 51.7MB/s 00:36 ETA^Killed by signal 2.

Conclusion

Speedwise arcfour wins hands down. I did not expect a more than doubled performance over the default aes128-cbc. Who would have thought that switching the algorithm makes that much of a difference.”
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The default cipher after OpenSSH v4.7 is blowfish, which as you can see is slower than arcfour. Sadly, I am right, and you still have crappy hardware. I will note however that arcfour is MUCH less secure, but if you’re looking for speed it cant be beat. Except by http://www.psc.edu/networking/projects/hpn-ssh/theory.php .