sFTP Plugin fails - Error Message: Algorithm negotiation fail

Question

We are a cloud customer attempting to connect from Appian to an internal sFTP server over the VPN tunnel to retrieve a file for processing. We are receiving the following error message within Appian when we attempt to connect: 
 Error message: 16:22:40,745 INFO [stdout] (Appian Work Item - 41212 - ProcessExec01 : UnattendedJavaActivityRequest) com.appiancorp.suiteapi.process.exceptions.SmartServiceException: userMsg[error.sftp_conn=Error establishing SFTP connection. Confirm your hostname, username and password. See application server log for more details. Error Message: Algorithm negotiation fail] 
 Has anyone experienced this issue?

ajaved · Answer

It took us a while to narrow down the issue but it was eventually resolved. 
1) We opened a case with Appian support to increase logging level on the sFTP plugin 
2) In the logs we saw that there was an issue with the compression libraries being used (in our case it was ZLib). Our servers were expecting ZLib compression but Appian sFTP plugin did not support that compression 
3) Appian worked with the plugin team to enable the ZLib compression and redeploy 
 
This resolved the Algorithm failure issue for us. My recommendation would be to open a case with Appian support and increase logging level of the plugin.