Problem and Root Cause
When attempting to establish an SSH connection using an iOS device, the connection may fail due to the older version of the libssh library not supporting the ecdh-sha2-nistp256 algorithm for key exchange. The error message would be like this:
NMSSH: Socket connection to xxx.xxx.xx.xx on port 22 succesful
NMSSH: Failure establishing SSH session
Solution
To resolve this issue, I have updated the libssh library in this fork to a version that supports the ecdh-sha2-nistp256 algorithm. Simply use this fork in your project to benefit from the fix:
pod 'NMSSH', :git => 'https://github.com/speam/NMSSH.git'