C25519 Is Not Allowed In Fips Mode

Issue

kex_gen_client: Key exchange type c25519 is not allowed in FIPS mode

Solution

1. Check the server for accepted KEX

   ssh -Q kex contoso.com -p 22
   diffie-hellman-group1-sha1
   diffie-hellman-group14-sha1
   diffie-hellman-group14-sha256
   diffie-hellman-group16-sha512
   diffie-hellman-group18-sha512
   diffie-hellman-group-exchange-sha1
   diffie-hellman-group-exchange-sha256
   ecdh-sha2-nistp256
   ecdh-sha2-nistp384
   ecdh-sha2-nistp521
   curve25519-sha256
   curve25519-sha256@libssh.org
   sntrup761x25519-sha512@openssh.com

2. Omit the issue KEX, since you have working ones that can be used

   ssh -o KexAlgorithms=-curve25519-sha256 contoso.com