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/)
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:
Configure, compile, and install as root. You may want to edit
pppd/Makefile after configuring to enable or disable optional
# 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
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
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:
or, for EAP itself or the SRP extensions to EAP, to the IETF PPP
Extensions working group: