Remember secrets for the duration of a login session
Keysafe is a command line application that is used to remember passwords, or any other kind of secret, for a login session. Once remembered, the password can be recalled for reuse during the session. The password is stored in memory, and does not persist beyond the login session.
Typically secrets such as passwords are required during a login session to access secured services, and normally this requires the user to provide the password each time the service is accessed. Doing so repeatedly can be tiresome and errorprone.
The Keysafe application can be used to remember the secret in a secure way, and to provide that secret each time the secured service is accessed for the duration of the login session.
The remembered secret is discarded once the login session terminates, or when the secret expires. Once the secret is discarded, reuse requires the user to re-enter the secret.
- Linux keyctl(2)
- Linux splice(2)
- bash(1), sh(1), ksh(1) etc
The Keysafe application can be run directly from the cloned git repository. To install the application in this way:
- git clone https://github.com/earlchew/keysafe.git
- cd keysafe && ./install.sh
- Optionally: cd keysafe && ln -s ./keysafe /usr/local/bin/
Alternatively the application can be installed in a target directory using the package manager:
- git clone https://github.com/earlchew/keysafe.git
- cd keysafe && pip install -r requirements.txt .
The following three concepts are required to understand and use Keysafe:
- The application that requires the password
- The key used to identify the application
- The means by which the password will be supplied to the application
For the purposes of illustration, the openssl passwd command can represent a useful application, and the -noverify -salt xx options used to ensure repeatable results for the password Pa55w0rd:
$ openssl passwd -noverify -salt xx Password: xxU4b0XBMjadY
To keep secrets as secure possible, the Keysafe application uses the following techniques:
- Avoid writing secrets to files, and reading secrets from files
- Avoid writing secrets as command line arguments
- Avoid writing secrets to shell history
- Limit the lifetime of memorised secrets
- Memorise encrypted secrets
While these techniques do much to limit the exposure of the password in plaintext, it remains possible for programs run by the root user, and other programs that are running under the user’s own account, from eavesdropping on channels used to transmit the password to the secured service, or examining the memory of the client used to access the secured service.
For illustration, the name EXAMPLE will be used to identify this application, and as described in the manual page for sslpasswd, the password can be supplied as a command line argument, from a file, from stdin, or from the terminal.
In these examples, the Keysafe application will be invoked three times:
- Revoking the password from the previous example, if any
- Remember the new password on th initial invocation of the openssl instance
- Recall the password when invoking a new openssl instance
Using a File to Send Secrets
This first example illustrates a couple of main points:
- The use of an unexported intermediate shell variable $_KEYSAFE_hCYju to prevents the key salt being recorded in shell history. Inspection of shell history will only show the use of the shell variable, but will not show the actual value being used.
- The use of process substitution at the shell <($_KEYSAFE_hCYju) prevents the key salt being recorded as part of the title of the process. Inspection of the process using ps(1) will only show that the salt is being read from /dev/fd/N or similar.
- The key name is augmented with a suffix to disambiguate it from instances used in other shell instances in the same login session.
- While the key provides a useful mnemonic to make it easy to remember the its purpose, the addition of of salt allows secrets to be stored securely. The revised command available at the command line (shown in bold in the example). If the revised command is acceptable, the user presses <RETURN> to proceed, otherwise the command line is available for further editing.
- If no value was previously recorded for the key, the Keysafe application prompts the user with the word Memento: to type the initial value.
- The command line argument @@ is replaced by the name of the pipe over which the application can read the secret.
$ keyctl show @s Keyring 1069142298 --alswrv 1021 1021 keyring: _ses 1030239571 --alswrv 1021 1021 \_ user: keysafe:EXAMPLE-2804 $ cat /proc/keys ... d683553 I--Q--- 1 59m 3f230000 1021 1021 user keysafe:EXAMPLE-2804: 100 $ keyctl print $((0x3d683553)) gAAAAABZfNOm5ymrvBaxRwdHPICfJ7XDXcf95-UwYUhCIhekhR7RPoCZjc5hmKSWS1pekSGtMVY8ePH6_OT_bl4dB4ZtBpeHzA==
The following points are noteworthy:
- The name of the key is plain to see and can be helpful to remember its purpose.
- Without further use, the key will expire after a configurable time, or when the session terminates.
- The keyring records an encrypted version of the value that requires the salt to decrypt. In this example the salt is stored in the shell variable $_KEYSAFE_hCYju and is not plainly visible.
If the wrong secret value was stored, the keyring can be updated with the correct value:
Using Command Arguments to Send Secrets
Under normal circumstances this is the worst of all ways to provide a password to a program:
- The password would be present on the command line in plaintext
- The password is visible on the terminal
- The password is visible process listings (eg ps(1), /proc/pid/cmdline, etc)
- The password is visible in shell histories
Using Keysafe mitigates these risks because the secret is tunnelled to the application program. The secret is available to the process via its in-process command line (ie (argc, argv)), but the secret is not visible on the terminal, in processing listings, nor present in shell histories.
The following demonstrates that while the program can read the password directly from its command line argument, casual inspection by an external observer does not reveal the password:
$ CMD='echo args: "$@" ; echo cmdline: ; tr "\0" "\n" < /proc/$$/cmdline' $ keysafe -a -s <($_KEYSAFE_hn3nf) EXAMPLE-2804 -- sh -c "$CMD" 0 1 @@ 2 args: 1 Pa55w0rd 2 cmdline: sh -c echo args: "$@" ; echo cmdline: ; tr "\0" "\n" < /proc/$$/cmdline 0 1 @@ 2
Using a Pipeline to Send Secrets
This next example shows sslpasswd reading the password from stdin:
The final example shows sslpasswd reading the password from the controlling terminal: