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Naftuli Kay
Asked: 2013-12-05 20:26:28 +0800 CST

OpenSSH doesn't accept ECDSA keys

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I've just generated a ECDSA key with ssh-keygen:

ssh-keygen -t ecdsa -b 521

I then proceeded to copy this key to my server:

cat .ssh/id_ecdsa.pub | ssh myserver "tee -a .ssh/authorized_keys"

I've verified that my key is in the file.

However, when I attempt to connect, my connection is rejected:

ssh -v -i .ssh/id_ecdsa myserver

Logs:

OpenSSH_5.9p1 Debian-5ubuntu1.1, OpenSSL 1.0.1 14 Mar 2012
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to myserver [192.168.1.1] port 22.
debug1: Connection established.
debug1: identity file .ssh/id_ecdsa type 3
debug1: Checking blacklist file /usr/share/ssh/blacklist.ECDSA-521
debug1: Checking blacklist file /etc/ssh/blacklist.ECDSA-521
debug1: identity file .ssh/id_ecdsa-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.1
debug1: match: OpenSSH_6.1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.9p1 Debian-5ubuntu1.1
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA 10:27:b8:78:2c:e1:e3:42:8e:e3:66:c4:cc:4e:f1:c0
debug1: Host 'myserver' is known and matches the RSA host key.
debug1: Found key in /home/naftuli/.ssh/known_hosts:73
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering ECDSA public key: .ssh/id_ecdsa
debug1: Authentications that can continue: publickey
debug1: No more authentication methods to try.
Permission denied (publickey).

Found this in the server logs:

auth.info sshd[13874]: userauth_pubkey: unsupported public key algorithm: ecdsa-sha2-nistp521 [preauth]

Both my client and the server are using OpenSSH. The server's OpenSSH version is OpenSSH 6.1, my client's OpenSSH version is OpenSSH 5.9.

How can I tell which key algorithms are supported by my server?

ssh

4 Answers

  1. Like many other embedded systems, OpenWrt uses dropbear as its ssh server, not the more heavyweight OpenSSH that's commonly seen on Linux systems. Older versions of dropbear only support RSA and DSA keys; support for ECDSA was not added until version 2013.62 (which has only just been released a few days ago).

    It should show up soon in Barrier Breaker (trunk); but you will not see it in Attitude Adjustment.

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  2. ecdsa is supported from openssh-server version 5.7. What version of openssh-server are you running? run dpkg -l | grep openssh-server | awk '{print $3}' | cut -d: -f2 to find the version.

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  3. If your system is Red Hat Enterprise Linux 6.4 (or older) or Fedora 19 (or older), note ECDSA was removed from there. I don't have any details why was that (perhaps legal reasons): https://www.mail-archive.com/[email protected]/msg00755.html

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  4. Leaving this here because this happened to me:

    Day 1: Setting up a new machine, I copied keys in - mine first - and was able to log in fine.

    Day 2: I can't log in with my ed25519 key. Huh? I add an RSA key; it works. I generate a new ed25519 key and it works... but my old one doesn't. WTF?

    Turns out that after testing, I installed my key into root's .ssh/authorized_keys as a backup... and forgot to fix the permissions on that file. So openssh blacklisted my key, making me unable to log in. Fixing the perms on /root/.ssh/authorized_keys allowed me to log in as my user.

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