EAP with MD5-Challenge and SRP-SHA1 support
by James Carlson, Sun Microsystems
Version 2, September 22nd, 2002


1.  What it does

    The Extensible Authentication Protocol (EAP; RFC 2284) is a
    security protocol that can be used with PPP.  It provides a means
    to plug in multiple optional authentication methods.

    This implementation includes the required default MD5-Challenge
    method, which is similar to CHAP (RFC 1994), as well as the new
    SRP-SHA1 method.  This latter method relies on an exchange that is
    not vulnerable to dictionary attacks (as is CHAP), does not
    require the server to keep a cleartext copy of the secret (as in
    CHAP), supports identity privacy, and produces a temporary shared
    key that could be used for data encryption.

    The SRP-SHA1 method is based on draft-ietf-pppext-eap-srp-03.txt,
    a work in progress.

2.  Required libraries

    Two other packages are required first.  Download and install
    OpenSSL and Thomas Wu's SRP implementation.

	http://www.openssl.org/ (or ftp://ftp.openssl.org/source/)
	http://srp.stanford.edu/

    Follow the directions in each package to install the SSL and SRP
    libraries.  Once SRP is installed, you may run tconf as root to
    create known fields, if desired.  (This step is not required.)

3.  Installing the patch

    The EAP-SRP patch described here is integrated into this version
    of pppd.  The following patch may be used with older pppd sources:

	ftp://playground.sun.com/carlsonj/eap/ppp-2.4.1-eap-1.tar.gz

    Configure, compile, and install as root.  You may want to edit
    pppd/Makefile after configuring to enable or disable optional
    features.

	% ./configure
	% make
	% su
	# make install

    If you use csh or tcsh, run "rehash" to pick up the new commands.

    If you're using Solaris, and you run into trouble with the
    pseudonym feature on the server side ("no DES here" shows in the
    log file), make sure that you have the "domestic" versions of the
    DES libraries linked.  You should see "crypt_d" in "ldd
    /usr/local/bin/pppd".  If you see "crypt_i" instead, then make
    sure that /usr/lib/libcrypt.* links to /usr/lib/libcrypt_d.*.  (If
    you have the international version of Solaris, then you won't have
    crypt_d.  You might want to find an alternative DES library.)

4.  Adding the secrets

    On the EAP SRP-SHA1 client side, access to the cleartext secret is
    required.  This can be done in two ways:

	- Enter the client name, server name, and password in the
          /etc/ppp/srp-secrets file.  This file has the same format as
          the existing chap-secrets and pap-secrets files.

	  clientname servername "secret here"

	- Use the "password" option in any of the standard
          configuration files (or the command line) to specify the
          secret.

	  password "secret here"

    On the EAP SRP-SHA1 server side, a secret verifier is required.
    This is a one-way hash of the client's name and password.  To
    generate this value, run the srp-entry program (see srp-entry(8)).
    This program prompts for the client name and the passphrase (the
    secret).  The output will be an entry, such as the following,
    suitable for use in the server's srp-secrets file.  Note that if
    this is transferred by cut-and-paste, the entry must be a single
    line of text in the file.

pppuser srpserver 0:LFDpwg4HBLi4/kWByzbZpW6pE95/iIWBSt7L.DAkHsvwQphtiq0f6reoUy/1LC1qYqjcrV97lCDmQHQd4KIACGgtkhttLdP3KMowvS0wLXLo25FPJeG2sMAUEWu/HlJPn2/gHyh9aT.ZxUs5MsoQ1E61sJkVBc.2qze1CdZiQGTK3qtWRP6DOpM1bfhKtPoVm.g.MiCcTMWzc54xJUIA0mgKtpthE3JrqCc81cXUt4DYi5yBzeeGTqrI0z2/Gj8Jp7pS4Fkq3GmnYjMxnKfQorFXNwl3m7JSaPa8Gj9/BqnorJOsnSMlIhBe6dy4CYytuTbNb4Wv/nFkmSThK782V:2cIyMp1yKslQgE *

    The "secret" field consists of three entries separated by colons.
    The first entry is the index of the modulus and generator from
    SRP's /etc/tpasswd.conf.  If the special value 0 is used, then the
    well-known modulus/generator value is used (this is recommended,
    because it is much faster).  The second value is the verifier
    value.  The third is the password "salt."  These latter two values
    are encoded in base64 notation.

    For EAP MD5-Challenge, both client and server use the existing
    /etc/ppp/chap-secrets file.

5.  Configuration options

    There are two main options relating to EAP available for the
    client.  These are:

	refuse-eap		- refuse to authenticate with EAP
	srp-use-pseudonym	- use the identity privacy if
				  offered by server

    The second option stores a pseudonym, if offered by the EAP
    SRP-SHA1 server, in the $HOME/.ppp_pseudonym file.  The pseudonym
    is typically an encrypted version of the client identity.  During
    EAP start-up, the pseudonym stored in this file is offered to the
    peer as the identity.  If this is accepted by the peer, then
    eavesdroppers will be unable to determine the identity of the
    client.  Each time the client is authenticated, the server will
    offer a new pseudoname to the client using an obscured (reversibly
    encrypted) message.  Thus, access across successive sessions
    cannot be tracked.

    There are two main options for EAP on the server:

	require-eap		- require client to use EAP
	srp-pn-secret "string"	- set server's pseudoname secret

    The second option sets the long-term secret used on the server to
    encrypt the user's identity to produce pseudonames.  The
    pseudoname is constructed by hashing this string with the current
    date (to the nearest day) with SHA1, then using this hash as the
    key for a DES encryption of the client's name.  The date is added
    to the hash for two reasons.  First, this allows the pseudonym to
    change daily.  Second, it allows the server to decode any previous
    pseudonym by trying previous dates.

    See the pppd(8) man page for additional options.

6.  Comments welcome!

    This is still an experimental implementation.  It has been tested
    and reviewed carefully for correctness, but may still be
    incomplete or have other flaws.  All comments are welcome.  Please
    address them to the author:

		james.d.carlson@sun.com

    or, for EAP itself or the SRP extensions to EAP, to the IETF PPP
    Extensions working group:

		ietf-ppp@merit.edu