I am using paramiko to connect to a remote server over ssh. When authenticating using a public key, the performance is slow (~90 secs). Here is the relevant logging output:
2012-05-14 17:37:21,378 Ciphers agreed: local=aes128-ctr, remote=aes128-ctr
2012-05-14 17:37:21,378 using kex diffie-hellman-group1-sha1; server key type ssh-rsa; cipher: local aes128-ctr, remote aes128-ctr; mac: local hmac-sha1, remote hmac-sha1; compression: local none, remote none
2012-05-14 17:37:21,481 Switch to new keys ...
2012-05-14 17:37:21,483 Adding ssh-rsa host key for 10.12.34.56: 8a05c68a0707a9fad19290c22542a338
2012-05-14 17:37:21,485 Trying discovered key 3793c7b9c500f52c12e190e92e21713f in /home/david/.ssh/id_rsa
2012-05-14 17:37:21,513 userauth is OK
2012-05-14 17:38:54,370 Authentication (publickey) successful!
Note the delay between the last two lines of log output. When using ssh to connect from the command line for the same user and remote server, the connection is instantaneous. Any ideas what is causing the delay?
After a fair amount of experimentation, it appears that the delay is related to the length of the key. The first key I was using was 4096 bits. When I switched to a shorter (2048 bit) key, the amount of time required to authenticate dropped dramatically (< 2 seconds).