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RFC 8268 defines a new set of Diffie-Hellman key exchange methods for use in SSH, using fixed prime moduli larger than the previous fixed ones, and SHA-512 as the hash function:
For comparison, the older fixed groups used in key exchange were
group14 at 2048 bits (used with SHA-256 and SHA-1), and
the original SSH-2 specification's
group1 at 1024 bits
(used with SHA-1).
PuTTY now supports all of these key exchange methods, for both basic SSH key exchange and GSS-API key exchange.