X-Git-Url: http://www.average.org/gitweb/?p=pam_pcsc_cr.git;a=blobdiff_plain;f=README.md;fp=README.md;h=c12a0dc12b2e18b8a183b116033bdcdca2bac43b;hp=0000000000000000000000000000000000000000;hb=288c17d345dfd026fb9d71976275cdabdbeeaf88;hpb=bcae8a6776e8824e347752d8dc71492adef6aa57 diff --git a/README.md b/README.md new file mode 100644 index 0000000..c12a0dc --- /dev/null +++ b/README.md @@ -0,0 +1,147 @@ + +``` +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. +``` + +------------------------------------------------------------------------ + +# Challenge-Response PAM Module for HMAC-SHA1 Hardware Token(s) + +This package provides a UNIX +[PAM](http://en.wikipedia.org/wiki/Pluggable_Authentication_Modules) +module and accompanying setup program implementing +[HMAC-SHA1](http://en.wikipedia.org/wiki/HMAC-SHA1) challenge-response +user authentication with hardware crypto token supporting +[PC/SC](http://en.wikipedia.org/wiki/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](http://pcsclite.alioth.debian.org/) infrastructure (i.e. +library and a daemon) is used to communicate with the token over +[CCID](http://en.wikipedia.org/wiki/Integrated_Circuit_Card_Interface_Device) +(i.e. PC/SC over USB) or +[NFC](http://en.wikipedia.org/wiki/Near_field_communication). 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. + +## Theory of Challenge-Response Authentication + +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 used to decrypt stored encrypted challenge and +compare it with cleartext challenge. A new challenge is then sent +to the token, and response is used to encrypt it and store for the +future authentication 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 response is transferred in cleartext long before +being used, and can be eavesdropped on and used in a replay attack. This +is of particular concern when using NFC. This approach is used by the +[PAM module provided by Yubico](https://github.com/Yubico/yubico-pam). + +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 the 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 secret to an adversary, all hosts are +compromised. This is not the case with the first approach. + +## Module Operation + +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. +``` + +## Getting the Source + +[clone](git://git.average.org/git/pam_pcsc_cr.git) or +[browse](http://www.average.org/gitweb/?p=pam_pcsc_cr.git;a=summary) +the repo. + +## Author + +Eugene Crosser \ + + +---