OpenSSH doesn't accept ECDSA keys

Naftuli Kay asked:

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/ | 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


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 [] 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_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: 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?

My answer:

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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