How to add more exchange algorithms to Putty SSH session?

Question

I have a device that accept the following exchange algorithms for SSH connections:

curve25519-sha256,[e-mail],diffie-hellman-group14-sha256,[e-mail]

I downloaded the latest version of Putty, and it says it supports: diffie-hellman group exchange, diffie-hellman group 14, diffie-hellman group 1 and RSA-based key exchange.

I guess it should work because in both configurations I see the diffie-hellman group 14. However, when I try to connect to the device I get the error:

Couldn't agree on a key exchange algorithm (available: curve25519-sha256,[e-mail],diffie-hellman-group14-sha256,[e-mail]) 

Answer 1

That's not the latest version of PuTTY. ECDH key exchange (including Curve25519) has been supported since 0.68 (February 2017), and the latest release as of writing this post is already 0.70 (July 2017).

(The reason DH group-14 doesn't work is because PuTTY only supports it with SHA-1. In current versions, only DH group-exchange is supported with SHA-256.)

Alternatively, you could connect via OpenSSH (standard on Windows 10.1803) or Bitvise Tunnelier (compatible with Windows Vista+); both support all of the above mechanisms.