--- /dev/null
+Copyright (c) 2013 Eugene Crosser
+
+This software is provided 'as-is', without any express or implied
+warranty. In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+ 1. The origin of this software must not be misrepresented; you must
+ not claim that you wrote the original software. If you use this
+ software in a product, an acknowledgment in the product documentation
+ would be appreciated but is not required.
+
+ 2. Altered source versions must be plainly marked as such, and must
+ not be misrepresented as being the original software.
+
+ 3. This notice may not be removed or altered from any source
+ distribution.
+
+========================================================================
+
+This package provides a UNIX PAM module and accompanying setup program
+implementing HMAC-SHA1 challenge-response user authentication with
+hardware crypto token supporting PC/SC (Smartcard) interface.
+
+At the time of writing, I know of just one such hardware token, Yubikey
+Neo from Yubico http://www.yubico.com/. Pcsclite infrastructure (i.e.
+library and a daemon) is used to communicate with the token over CCID
+(i.e. PC/SC over USB) or NFC. It means that it works equally well when
+you plug the token in a USB slot and if you put it on an NFC reader.
+
+There are two ways to do challenge-response authentication: with shared
+secret and with pre-produced response. In pre-produced response, the
+host does not need to store the token's HMAC secret; on every session
+conversation with the token is performed twice with different challenges.
+The first response is compared with stored expected response, which is
+then replaced with the second response to be used on the next session.
+The advantage is that the secret is not kept anywhere except the token,
+so it's less chance of compromise. The drawback is that the expected
+response is transferred in cleartext long before being used, and can
+be eavesdropped on and reused in a replay attack. This is of particular
+concern when using NFC. This approach is used by the PAM module provided
+by Yubico.
+
+My module uses the second approach, under which the HMAC secret is
+stored both in the token and on the host. To minimize the danger of
+compromise, the host copy of the shared secret is encrypted by the key
+which is the expected response from the token. In the process of
+authentication, token's response is used to decrypt the secret, then
+this secret is used to compute the next expected token's response, and
+this expected response is used to encrypt the secret again. This next
+expected response is not transferred over the air, and the shared secret
+stays in unencrypted form in RAM (unless paged out) for a very short
+period. The downside is that if the token is used against multiple
+hosts, and one of them leaks the key to an adversary, all hosts are
+compromised. This is not the case with the first approach.
+
+Authentication file, containing nonce, encrypted shared secret,
+encrypted additional payload, and anciliary information, is named
+according to template that can be provided both to PAM module and to the
+setup program (and must be the same, obviously). In the template string,
+character '~' in the first position is substituted with the userid's
+home directory, '~' in a position other than first - with the userid
+itself, and character '?' - with the "tokenid". This latter is just an
+arbitrary string that is not involved in the authentication process.
+But, if the template contains '?' but not '~', login process can start
+without the knowlege of the userid. Userid will be picked from the file
+and injected into the PAM environment, given that tokenid is known from
+the start.
+
+Default template string is "~/.pam_cr/auth", i.e. the file lives in the
+user's home directory, in the subdirectory ".pam_cr".
+
+Authentication file must be initially created by the program
+'pam_cr_setup' included in this package.
+
+usage: pam_cr_setup [options] [username]
+ -h - show this help and exit
+ -o backend-option - token option "backend:key=val"
+ -f template - template for auth state filepath
+ -a secret | -A file-with-secret | -A -
+ - 40-character hexadecimal secret
+ -s token-serial - public I.D. of the token
+ -n nonce - initial nonce
+ -l payload - keyring unlock password
+ -p password - login password
+ -v - show returned data
+
+The only backend option existing is "ykneo:slot=1" or "ykneo:slot=2".
+Slot 2 is the default. Secret must be supplied when creating the file,
+and when modifying the file in the absense of the token. Password is
+used to construct the challenge. If not supplied empty string is used.
+The pam module also used empty string when given "noaskpass" argument,
+so this can be used for "one factor" authentication mode with token
+only. Payload is a string that can be optionally injected as the PAM
+authentication token after successful authentication; subsequent PAM
+modules like gnome keyring unlocker module will pick it up. Note that
+this keyring unlocker password may be different from the login
+password, and it is generally a good idea to make it so. The "returned
+data" is the userid as recorded in the file and the aforementioned
+payload string.
+
+PAM module has the following parameters:
+ verbose write more errors to syslog.
+ noaskpass do not try to ask the user for the challenge
+ password, use empty string for the password.
+ injectauth inject payload as PAM_AUTHTOK for the benefit
+ of subsequent PAM modules.
+ path=<string> template used to find the file.
+ backend:key=val backend options.
+
+
projects / pam_pcsc_cr.git / blobdiff