You must connect to your Cisco device from another machine using an earlier version of OpenSSH as the reason for this error is:
- The new version of OpenSSH does not support RSA Host Keys less than 1024 bits in length
- The switch by default has an RSA Host Key of less than 1024 bits
The solution is as follows:
Step 1. Connect from another device
You can actually get away with (for now) using Bitvise SSH on Windows and enabling the disabled Algorithms, etc. to allow you to connect to the Cisco device.
Step 2. Zeroize
Run the following commands to zeroize and re-generate your host RSA key:
crypto key zeroize crypto key generate rsa modulus 2048
Step 3. Reconnect from your Mac
Now you should be able to successfully SSH from your Mac.