scp

scp is the command-line tool included with the OpenSSH suite of tools, it is designed to securely transfer files to and from remote hosts.

Compression

If you add the following to ~/.ssh/config your files will be compressed in-transit which will reduce the amount of data which is required to transfer

Host *
    Compression      yes
    CompressionLevel 4

Cipher Choices

ssh and scp both support a large number of ciphers, which are used to encrypt your content over the network. You can see the ciphers enabled in your copy of OpenSSH by running "man ssh_config".

For my personal Debian GNU/Linux machine I see the following ciphers are supported:

• 3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr arcfour128 arcfour256 arcfour blowfish-cbc cast128-cbc

To choose a particular cipher run:

$ ssh -o Cipher=arcfour [email protected]

or

scp -o Cipher=arcfour local-file [email protected]:

The different ciphers have different performance characteristics, and you can test the timings if you have a large file named test.img by repeatedly copying the file to a remote host using a different cipher each time:

$ for i in aes128-cbc arcfour ; do time scp -o Cipher=$1 test.img [email protected]: ; done

For my personal systems the following were rough timings:

• 3des-cbc 45.48 seconds.
• aes128-cbc 45.39 seconds.
• aes192-cbc 45.01 seconds.
• aes256-cbc 45.50 seconds.
• aes128-ctr 45.29 seconds.
• aes192-ctr 44.40 seconds.
• aes256-ctr 45.88 seconds.
• arcfour128 45.29 seconds.
• arcfour256 45.94 seconds.
• arcfour 42.32 seconds.
• blowfish-cbc 46.02 seconds.
• cast128-cbc 47.07 seconds.

So for my particular setup it seems that arcfour is the fastest cipher to choose. (For reference disabling compression increased the time to transfer my sample file to over 30 minutes!)

 

Optimizing

Testing

© 2023 Steve Kemp