From: Eugene Crosser Date: Wed, 4 Dec 2013 21:49:42 +0000 (+0400) Subject: Added README X-Git-Url: http://www.average.org/gitweb/?p=pam_pcsc_cr.git;a=commitdiff_plain;h=8624b16ffe42216463da9cf4301bb909a19a0b42;ds=sidebyside Added README --- diff --git a/README b/README new file mode 100644 index 0000000..af0bd75 --- /dev/null +++ b/README @@ -0,0 +1,114 @@ +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= template used to find the file. + backend:key=val backend options. + +