U2F utils

This u2f toolset contains small footprint u2f tools for enrolling and signing operations as well as a PAM module for authenticating users on local services where they can physically plug in the u2f token (i.e. xdm, login, su, ...).

I wrote this u2f stack in order to get familar with u2f crypto, the shortcomings of u2f in general and weaknesses of other u2f stacks in particular. Remote tools for u2f ssh etc. are underway.

Build

Inside this dir,

$ git clone https://github.com/signal11/hidapi

to get the HIDAPI for accessing the security token. Then:

$ make
$ make install

You need to set up proper udev rules so the security token appears as /dev/hidraw* device, with the permissions you prefer or manually load the hid driver.

Install

To enroll a key you either use u2f-enroll or pam-enroll if you want to enroll a key suitable for PAM authentication:

localhost: # pam-enroll stealth
Remove token <ENTER>

Insert token of user 'stealth' and press token-button if available. Then <ENTER>

Got 631 bytes (sw=9000)

pubkey claims to be signed with cert (unchecked!):

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 18327115537361868814 (0xfe56fe7ae1ff180e)
    Signature Algorithm: ecdsa-with-SHA256
        Issuer: CN=Plug-up FIDO Internal Attestation CA #1
        Validity
            Not Before: Oct  3 08:06:48 2014 GMT
            Not After : Oct  3 08:06:48 2034 GMT
        Subject: CN=Plug-up FIDO Production Attestation #fe56fe7ae1ff180e
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub: 
                    04:75:ea:06:60:e2:90:63:74:84:37:00:00:af:aa:
                    32:25:3e:82:7b:d8:48:74:93:a6:86:a5:68:4c:65:
                    ca:ce:09:8b:e8:bf:4b:87:25:3d:ef:96:b9:40:23:
                    01:06:fc:46:06:1f:7d:65:46:c1:6f:14:b2:5a:bf:
                    30:19:d8:f4:27
                ASN1 OID: prime256v1
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                76:2B:44:6F:F2:94:ED:32:2A:E4:29:09:4F:A9:84:D8:85:3E:35:80
            X509v3 Authority Key Identifier: 
                keyid:CF:A7:44:F2:A1:62:50:F0:39:E9:92:85:E3:DA:50:E7:7D:B0:3A:A8

    Signature Algorithm: ecdsa-with-SHA256
         30:44:02:20:6b:5f:ea:7f:dd:ce:65:84:3b:25:d6:a6:fc:8a:
         4d:b7:3b:80:b1:e6:44:2e:ab:06:77:a9:3e:3d:b9:35:1f:22:
         02:20:59:5b:82:32:79:21:c2:8f:ad:20:62:b9:2a:ea:07:c4:
         37:a5:4d:46:a6:2c:8b:e6:ee:fb:69:5b:8a:b1:44:16

If /dev/hidraw0 is not the right device for you, adjust the device path inside pam-enroll. You can also enroll keys via u2f-enroll and store the keys manually.

pam-enroll stores the key handle along with the public key in /etc/u2f/keys:

localhost:# cat /etc/u2f/keys/_stealth
H=b67350 [...] 18273a626dc0743c
-----BEGIN PUBLIC KEY-----
MIIBSzCCAQMGByqGSM49AgEwgfcCAQEwLAYHKoZIzj0BAQIhAP////8AAAABAAAA
[...]

[...]
SLI/caIDeYpo3lRlEdIWUX87A1cWC3YpCPJ89G1Hc9Fb9TtELXRiP3tHSfhgyVU=
-----END PUBLIC KEY-----

You can then add pam_fido-u2f.so to any PAM service file (only local services) for example to the xdm display manager:

localhost: # cat /etc/pam.d/xdm
#%PAM-1.0
auth     include        common-auth
auth     required       pam_fido-u2f.so
account  include        common-account
password include        common-password
session  required       pam_loginuid.so
session  include        common-session

Next time someone logs in via xdm an u2f token is required, which must contain the private key belonging to the public part stored in /etc/u2f/keys. Note that users which are not enrolled via pam-enroll cannot longer login via xdm! So be sure that root is properly enrolled.

If you have an USB keyboard or any other HID devices besides the u2f token already attached, you may need to specify the device path in the PAM file, as in:

auth     required       pam_fido-u2f.so	device=/dev/hidraw1

for example. As whatever name it would show up once plugged in.

u2f limitations

Please note that 2FA tokens/mechanisms are of limited use to protect shell access, since there are many ways to plant 2FA-less backdoors once shell access has been gained by an attacker in the first place. A Proper gateway and VPN setup is mandatory in order for 2FA to provide a real security benefit. Also note that the FIDO U2F standard chose a NIST ECC curve (NIST P-256 aka NID_X9_62_prime256v1) for the crypto operations. Yes, thats the same NIST that apparently already backdoored other crypto protocols. So you can consider NID_X9_62_prime256v1 to be weak, but it might be good enough as a second factor for medium secured sites. Note again that USB tokens are subject to bad-USB style attacks. Some tokens even have an API beyond FIDO U2F that allows for easy storage of keystrokes and replay, once plugged in. So while it is in general a good idea to have 2FA, you always add an additional attack vector to your site that has not been there before.

Part of this code is (C) 2014 Google Inc. under a BSD-ish license. Please refer to the source code for details.