07-25-2005
Quote:
Originally Posted by hadarot
The only issue with the above outlined process is that you now have a totally unencrypted private key laying around your hard drive, which is a security risk. So you should not always use this method of no password on the key, depending on other security factors, such as if your home directory is on an NFS exported directory within a large network, thus more vulnerable to attack.
While this is certainly true, sometimes using unencrypted ssh private keys is the most secure way to carry out some tasks. For example, if I wanted to copy an file from one server to another server every night at 3am, that would be a good time to use an unencrypted ssh key.
To mitiage the risk, as mentioned above, you should make 100% sure that the directory containing the keys isn't being shared or served, and that the permissions on the private key are 0400 (user read-only). In the example above, where the user is only doing file copies, you should look into using a restricted shell like rssh, so that even if the key is compromised, the scope of attacks is much more limited.
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LEARN ABOUT DEBIAN
pam_ssh
PAM_SSH(8) BSD System Manager's Manual PAM_SSH(8)
NAME
pam_ssh -- authentication and session management with SSH private keys
DESCRIPTION
The SSH authentication service module for PAM, pam_ssh provides functionality for two PAM categories: authentication and session management.
SSH Authentication Module
The SSH authentication component verifies the identity of a user by prompting the user for a passphrase and verifying that it can decrypt at
least one of the user's SSH login keys using that passphrase.
The following options may be passed to the authentication module:
debug syslog(3) debugging information at LOG_DEBUG level.
use_first_pass If the authentication module is not the first in the stack, and a previous module obtained the user's password, then
that password is used to decrypt the user's SSH login keys. If this fails, then the authentication module returns
failure without prompting the user for a passphrase.
try_first_pass Similar to the use_first_pass option, except that if the previously obtained password fails to decrypt any of the SSH
login keys, then the user is prompted for an SSH passphrase.
try_first_pass has no effect if pam_ssh is the first module on the stack, or if no previous modules obtained the
user's password.
allow_blank_passphrase Allow SSH keys with no passphrase.
If neither use_first_pass nor try_first_pass is specified, pam_ssh will unconditionally ask for an SSH passphrase.
In addition to the above authentication procedure, all standard SSH keys (identity, id_rsa, id_dsa) for which the obtained password matches
will be decrypted.
SSH Session Management Module
The SSH session management component initiates sessions by starting an SSH agent, passing it any SSH login keys it decrypted during the
authentication phase, and sets the environment variables accordingly.
The SSH session management component terminates the session by killing the previously started SSH agent by sending it a SIGTERM.
The following options may be passed to the session management module:
debug syslog(3) debugging information at LOG_DEBUG level.
INFORMATION LEAKS
Be careful with the using the try_first_pass option when pam_ssh is the first authentication module because it will then leak information
about existing users without login keys: such users will not be asked for a specific SSH passphrase, whereas non-existing users and existing
users with login keys will be asked for a passphrase.
FILES
$HOME/.ssh/identity
$HOME/.ssh/id_rsa
$HOME/.ssh/id_dsa OpenSSH DSA/RSA keys decrypted by pam_ssh.
$HOME/.ssh/login-keys.d/ Location of (possibly symbolic links to) OpenSSH DSA/RSA keys used for authentication and decrypted by pam_ssh.
/var/log/auth.log Usual log file for syslog(3)
SEE ALSO
ssh-agent(1), syslog(3), pam.conf(5), pam(8).
AUTHORS
Andrew J. Korty <ajk@iu.edu> wrote pam_ssh. Dag-Erling Smorgrav wrote the original OpenPAM support code. Mark R V Murray wrote the original
version of this manual page. Jens Peter Secher introduced the login-key concept.
BSD
November 26, 2001 BSD