[Bug 1808194] [NEW] paramiko can't parse the default key format generated by ssh-keygen

[Dan Watkins]

Public bug reported:

The default key format used by OpenSSH changed in version 7.8[0] to the
openssh format (rather than the previous "pem" format).  (disco is the
first Ubuntu version to include an OpenSSH version later than 7.8; it's
currently at 7.9p1.)

paramiko doesn't have full support for the openssh key format[1] (it
will only accept it for Ed25519 keys, currently), which means anything
using paramiko with keys generated on disco and later will stop working.

A simple reproducer:

```
$ ssh-keygen -f fail.rsa -t rsa -N ''
...
$ python3 -c 'from paramiko.rsakey import RSAKey; RSAKey.from_private_key_file("fail.rsa")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 206, in from_private_key_file
    key = cls(filename=filename, password=password)
  File "/usr/lib/python3/dist-packages/paramiko/rsakey.py", line 48, in __init__
    self._from_private_key_file(filename, password)
  File "/usr/lib/python3/dist-packages/paramiko/rsakey.py", line 169, in _from_private_key_file
    data = self._read_private_key_file('RSA', filename, password)
  File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 279, in _read_private_key_file
    data = self._read_private_key(tag, f, password)
  File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 289, in _read_private_key
    raise SSHException('not a valid ' + tag + ' private key file')
paramiko.ssh_exception.SSHException: not a valid RSA private key file

$ ssh-keygen -f pass.rsa -t rsa -N '' -m pem
...
$ python3 -c 'from paramiko.rsakey import RSAKey; RSAKey.from_private_key_file("pass.rsa")'
$ echo $?
0
```

[0] https://www.openssh.com/txt/release-7.8
[1] https://github.com/paramiko/paramiko/issues/602

** Affects: paramiko (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1808194

Title:
  paramiko can't parse the default key format generated by ssh-keygen

Status in paramiko package in Ubuntu:
  New

Bug description:
  The default key format used by OpenSSH changed in version 7.8[0] to
  the openssh format (rather than the previous "pem" format).  (disco is
  the first Ubuntu version to include an OpenSSH version later than 7.8;
  it's currently at 7.9p1.)

  paramiko doesn't have full support for the openssh key format[1] (it
  will only accept it for Ed25519 keys, currently), which means anything
  using paramiko with keys generated on disco and later will stop
  working.

  A simple reproducer:

  ```
  $ ssh-keygen -f fail.rsa -t rsa -N ''
  ...
  $ python3 -c 'from paramiko.rsakey import RSAKey; RSAKey.from_private_key_file("fail.rsa")'
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 206, in from_private_key_file
      key = cls(filename=filename, password=password)
    File "/usr/lib/python3/dist-packages/paramiko/rsakey.py", line 48, in __init__
      self._from_private_key_file(filename, password)
    File "/usr/lib/python3/dist-packages/paramiko/rsakey.py", line 169, in _from_private_key_file
      data = self._read_private_key_file('RSA', filename, password)
    File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 279, in _read_private_key_file
      data = self._read_private_key(tag, f, password)
    File "/usr/lib/python3/dist-packages/paramiko/pkey.py", line 289, in _read_private_key
      raise SSHException('not a valid ' + tag + ' private key file')
  paramiko.ssh_exception.SSHException: not a valid RSA private key file

  $ ssh-keygen -f pass.rsa -t rsa -N '' -m pem
  ...
  $ python3 -c 'from paramiko.rsakey import RSAKey; RSAKey.from_private_key_file("pass.rsa")'
  $ echo $?
  0
  ```

  [0] https://www.openssh.com/txt/release-7.8
  [1] https://github.com/paramiko/paramiko/issues/602

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