4.19.1 Key exchange algorithm selection

PuTTY supports a variety of SSH-2 key exchange methods, and allows you to choose which one you prefer to use; configuration is similar to cipher selection (see section 4.21).

PuTTY currently supports the following key exchange methods:

• ‘ECDH’: elliptic curve Diffie-Hellman key exchange.
• ‘Group 14’: Diffie-Hellman key exchange with a well-known 2048-bit group.
• ‘Group 1’: Diffie-Hellman key exchange with a well-known 1024-bit group. We no longer recommend using this method, and it's not used by default in new installations; however, it may be the only method supported by very old server software.
• ‘Group exchange’: with this method, instead of using a fixed group, PuTTY requests that the server suggest a group to use for key exchange; the server can avoid groups known to be weak, and possibly invent new ones over time, without any changes required to PuTTY's configuration. We recommend use of this method instead of the well-known groups, if possible.
• ‘RSA key exchange’: this requires much less computational effort on the part of the client, and somewhat less on the part of the server, than Diffie-Hellman key exchange.

If the first algorithm PuTTY finds is below the ‘warn below here’ line, you will see a warning box when you make the connection, similar to that for cipher selection (see section 4.21).