Couldn't agree a client-to-server MAC (available: hmac-sha2-512)

[Magicman8508]

I get this error when i try to connect to some devices. Any way i can fix this? I have tried the latest release and also the current nightly build.

Thanks

[Simon Tatham]

This is the first I've heard of any server _only_ speaking HMAC-SHA-512.
It's not really a recommended configuration, because HMAC-SHA-512 is
specified as OPTIONAL, which does mean there's a risk of clients not
supporting it. What is this server, anyway?

I've added HMAC-SHA-512 to PuTTY. Try today's nightly build.

--
import hashlib; print((lambda p,q,g,y,r,s,m: (lambda w:(pow(g,int(hashlib.sha1(
m.encode('ascii')).hexdigest(),16)*w%q,p)*pow(y,r*w%q,p)%p)%q)(pow(s,q-2,q))==r
and s%q!=0 and m)(12342649995480866419, 2278082317364501, 1670428356600652640,
5398151833726432125, 645223105888478, 1916678356240619, "<ana...@pobox.com>"))

[Magicman8508]

I just tried the recent nightly version and it works! Perfect. Many thanks. Didn't thought it could be resolved so fast.

In my case it affects a cisco 9800 series. Not sure why it is configured this way. Maybe a company policy only allows hmac-sha2-512.

Thanks again.
Have a great day.

[Austin Harsh]

In my case this is based on the new US Government CNSA V2.0 policy (this is what is replacing FIPS stuff, kind of). CNSA V2.0 states you must use HMAC-SHA2-384 or HMAC-SHA2-512. Cisco switches do not support the 384 variant, so you have to use 512. In the future (~5 years) PuTTY will eventually need to support a new hashing algorithm called CRYSTALS-Kyber. https://media.defense.gov/2022/Sep/07/2003071834/-1/-1/0/CSA_CNSA_2.0_ALGORITHMS_.PDF