SSH Cipher Updates in Cisco ASA 9.4(3)12

After upgrading our Cisco ASAs from 9.4(3)11 to 9.4(3)12, Rancid could no longer log in.  Debugging by manually running clogin, the problem was clear: incompatibility with SSH ciphers.  Rancid wanted to use 3DES (“Triple DES”), but the ASA only supported AES.

rancid@localhost:~$ clogin
spawn ssh -c 3des -x -l rancid
no matching cipher found: client 3des-cbc server aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr

By default, the ASA’s SSH server uses the “medium” cipher list.  Sure enough, 3DES is no longer in the list:

ciscoasa/pri/act# show ssh ciphers 
Available SSH Encryption and Integrity Algorithms
Encryption Algorithms:
  low: 3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr 
  medium: aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr
  fips: aes128-cbc aes256-cbc  
  high: aes256-cbc aes256-ctr 

A quick and dirty work-around: configure the ASA to use the “low” cipher list.  However, since it’s time to start phasing out 3DES anyway (it’s from the 90s), I instead wanted to tell Rancid to prefer AES and only use 3DES as a last resort.  The first step was finding the possible cipher names, which were in /etc/ssh/ssh_config:

# Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc

I simplified this a bit and added this line to Rancid’s .cloginrc:

add cyphertype * aes128-ctr,aes128-cbc,3des-cbc

This preference matches most of my devices since AES-CTR is supported in IOS 15 and is preferred over AES-CBC and 3DES.  Good enough for me.


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