After upgrading to macOS High Sierra (10.13), I noticed some of my Transmit bookmarks to connect to SFTP servers via SSH were no longer working. By using the command line to try to connect I received an error message along the lines of:

Unable to negotiate with [IP_ADDR] port 22: no matching cipher found. Their offer: twofish256-cbc,twofish-cbc,twofish128-cbc,blowfish-cbc,3des-cbc,arcfour,cast128-cbc,aes256-cbc,aes128-cbc

Turns out the release of macOS disabled support for old ciphers (likely for security reasons. Since I don’t administer the servers I was needing to connect to, I had little choice but to re-enable support for older Ciphers. I did this by editing /etc/ssh/ssh_config adding the following line:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
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