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  • What is the 'size' (bits in modulus) of your key? If you are using very recent OpenSSH (>=9.1) check if it is configured (in ~/.ssh/config or /etc/ssh/ssh_config or similar, or with a wrapping script or alias) to have option RequiredRSASize (in any case, but that's canonical) set too high for your key. Commented Jun 25, 2023 at 3:16
  • @dave_thompson_085 I use openssh 8.8p1 without RA keysize restriction and according to : openssl rsa -in privat.pem -text -noout 'Private-Key: (2047 bit, 2 primes) But I don't get it why it is working with putty - on Win10 and on Fedora- and not with openssh client Commented Jun 25, 2023 at 20:45
  • I don't understand either; 2047 bit should be valid. Assuming id_rsa is the same as privat.pem -- did you confirm that? Commented Jun 27, 2023 at 6:32
  • @dave_thompson_085 Yes. In fact, I've tried to specify it with 'ssh -i' as well. Odd is, if I generate a new key on my Fedora and copy the new public key to the target it works. But I guess, I confirmed it's an ssh client issue on Fedora as I've tried it from another laptop with Wim10 and MobaXterm IT WORKS. I also tried it from a CoreELEC Linux box CoreELEC:~ # ssh -V OpenSSH_9.2p1, OpenSSL 3.0.8 7 Feb 2023 and work from there as well. But I still don't know what could be the problem with 8.8.p1 as I 'm using with default config. Commented Jun 28, 2023 at 9:19
  • Gack! I didn't notice Fedora! Is your putty from the Fedora packagers or elsewhere? RHEL 8 up and Fedora I-don't-know-exactly-which have systemwide crypto policies that apply to most if not all distro packages and it appears now rejects RSA<2048 by default. If you run openssl genrsa 2047 does it generate a key and does that key work in OpenSSH? Commented Jun 28, 2023 at 11:56