title: configuring git/ssh to access bitbucket results in keyfile rejected - format error?
body: <p>I have been trying for 3 days now to set up git/ssh to work with bitbucket.org on windows 10. 
  Made a little progress. Now the log is...</p>
  
  <pre><code>   Ian@Helike MINGW64 /m/websites/coachmaster (master)
  $ ssh -v git@bitbucket.org
  OpenSSH_8.0p1, OpenSSL 1.1.1c  28 May 2019
  debug1: Reading configuration data /c/Users/Ian Hobson/.ssh/config
  debug1: /c/Users/Ian Hobson/.ssh/config line 1: Applying options for bitbucket.org
  debug1: Reading configuration data /etc/ssh/ssh_config
  debug1: Connecting to bitbucket.org [18.205.93.1] port 22.
  debug1: Connection established.
  debug1: identity file D:\\Ian\\keyfiles\\ianhobson-bitbucket type -1
  debug1: identity file D:\\Ian\\keyfiles\\ianhobson-bitbucket-cert type -1
  debug1: Local version string SSH-2.0-OpenSSH_8.0
  debug1: Remote protocol version 2.0, remote software version conker_e350bda95a app-131
  debug1: no match: conker_e350bda95a app-131
  debug1: Authenticating to bitbucket.org:22 as 'git'
  debug1: SSH2_MSG_KEXINIT sent
  debug1: SSH2_MSG_KEXINIT received
  debug1: kex: algorithm: curve25519-sha256@libssh.org
  debug1: kex: host key algorithm: ssh-rsa
  debug1: kex: server-&gt;client cipher: chacha20-poly1305@openssh.com MAC: &lt;implicit&gt; compression: none
  debug1: kex: client-&gt;server cipher: chacha20-poly1305@openssh.com MAC: &lt;implicit&gt; compression: none
  debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
  debug1: Server host key: ssh-rsa SHA256:zzXQOXSRBEiUtuE8AikJYKwbHaxvSc0ojez9YXaGp1A
  debug1: Host 'bitbucket.org' is known and matches the RSA host key.
  debug1: Found key in /c/Users/Ian Hobson/.ssh/known_hosts:1
  debug1: rekey out after 134217728 blocks
  debug1: SSH2_MSG_NEWKEYS sent
  debug1: expecting SSH2_MSG_NEWKEYS
  debug1: SSH2_MSG_NEWKEYS received
  debug1: rekey in after 134217728 blocks
  debug1: Will attempt key: D:\\Ian\\keyfiles\\ianhobson-bitbucket  explicit
  debug1: SSH2_MSG_SERVICE_ACCEPT received
  debug1: Authentications that can continue: publickey
  debug1: Next authentication method: publickey
  debug1: Trying private key: D:\\Ian\\keyfiles\\ianhobson-bitbucket
  sign_and_send_pubkey: no mutual signature supported
  debug1: No more authentication methods to try.
  git@bitbucket.org: Permission denied (publickey).
  
  Ian@Helike MINGW64 /m/websites/coachmaster (master)
  </code></pre>
  
  <p>I can see it is using the correct keyfile. That keyfile, and ~/.ssh/config have been copied from the old machine where it works. </p>
  
  <p>Ian</p>