[Bug 1756031] Re: openssh-server doesn't accept aes256-cbc key

[ChristianEhrhardt]

AFAIK build time:
  sshkey.h:49:#define SSH_RSA_MINIMUM_MODULUS_SIZE        1024

And those short keys are really considered insecure, which is the reason
they went from deprecated to no more accepted.

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Title:
  openssh-server doesn't accept aes256-cbc key

Status in openssh package in Ubuntu:
  Confirmed

Bug description:
  I am using putty connection with RSA key cached in pageant and want to
  login without password.

  Private key on windows:
  PuTTY-User-Key-File-2: ssh-rsa
  Encryption: aes256-cbc
  ...

  On ubuntu there is public key in .ssh/authorized_keys
  ssh-rsa AAAAB3Nz...JBjQ== palo at winpgnotas

  This key works well in ubuntu versions 14.04...17.10
  When I tried 18.04 beta, I am getting sshd error:
  mar 15 10:26:21 ubox sshd[5205]: error: userauth_pubkey: could not parse key: Invalid key length [preauth]
  and I have to provide password.

  I've found that aes256-cbc is not in the list of allowed ciphers by default, so I added
  Ciphers +aes256-cbc
  to /etc/ssh/sshd_config
  (and verified with nmap --script ssh2-enum-algos -sV -p 22 127.0.0.1)
  but the sshd error remains.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: openssh-server 1:7.6p1-4
  ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
  Uname: Linux 4.15.0-10-generic x86_64
  ApportVersion: 2.20.8-0ubuntu10
  Architecture: amd64
  Date: Thu Mar 15 10:03:14 2018
  InstallationDate: Installed on 2018-03-12 (2 days ago)
  InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180306.1)
  SourcePackage: openssh
  UpgradeStatus: No upgrade log present (probably fresh install)

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