Same problem here. Tested on regular 12.04 with 521 bits long ECDSA:
$ ssh-add
Enter passphrase for /home/user/.ssh/id_ecdsa:
Error reading response length from authentication socket.
Could not add identity: /home/user/.ssh/id_ecdsa
Claudio trick to export ssh-agent variables worked:
$ ssh-add
Enter passphrase for /home/user/.ssh/id_ecdsa:
Identity added: /home/user/.ssh/id_ecdsa (/home/user/.ssh/id_ecdsa)
Same problem here. Tested on regular 12.04 with 521 bits long ECDSA:
$ ssh-add .ssh/id_ ecdsa: .ssh/id_ ecdsa
Enter passphrase for /home/user/
Error reading response length from authentication socket.
Could not add identity: /home/user/
Claudio trick to export ssh-agent variables worked:
$ ssh-add .ssh/id_ ecdsa: .ssh/id_ ecdsa (/home/ user/.ssh/ id_ecdsa)
Enter passphrase for /home/user/
Identity added: /home/user/