README for Stanford IBE library v0.7.2
Ben Lynn <[email protected]>

INTRODUCTION

The Stanford IBE library is a C implementation of the Boneh-Franklin
identity-based encryption scheme. (See Boneh and Franklin, "Identity-Based
Encryption from the Weil Pairing", CRYPTO 2001.)

There are a few modifications and additions. The Boneh-Franklin scheme is
used as a Key Encapsulation Mechanism, and off-the-shelf ciphers and HMACs
are used for the actual encryption. (See Lynn, "Authenticated Identity-Based
Encryption", available on eprint.)

Identity-based signatures have also been implemented. The Boneh-Lynn-Shacham
scheme is used (see Boneh, Lynn and Shacham, "Short Signatures from the
Weil Pairing", ASIACRYPT 2001.) Certificates are employed to make the signatures
identity based. However, using a trick that we call "signature aggregation" (that
will be described in a forthcoming paper), there is not much overhead compared
to other ID-based signature schemes.

The library uses the GMP library (http://www.swox.com/gmp/) for low-level
arithmetic and the OpenSSL library (http://www.openssl.org/) for various
cryptographic primitives.

It is still under heavy development so the API could change dramatically
in future versions. (This already happened several times recently.)

DESCRIPTION

The lowest layer is fp2.c, which uses GMP to perform arithmetic in F_p^2.
It defines the fp2_t data type, which represents elements of F_p^2.

The next layer is curve.c, which performs the elliptic curve arithmetic.
It defines the point_t data type, which represents points on the curve.
It only supports specific curves so that certain optimizations can be
applied. (For example, it assumes p is a Solinas prime that is 11 mod 12
and that a particular curve is being used.)
Furthermore, some functions assume that the points are on E/F_p, not
E/F_p^2.

On top of this lies ibe_lib.c, which does basic IBE operations.
It defines the params_t data type, which represents IBE system parameters.
The headers are in ibe.h. Originally these were routines that performed the
four IBE operations described in the IBE paper and Authenticated IBE paper
(Setup, Extract, Encrypt, Decrypt and authenticated versions), but now the BF
IBE system is used as Key Encapsulation Mechanism instead, and off-the-shelf
encryption and HMAC algorithms (from OpenSSL) are used in place of the
construction of Fujisaki and Okamoto.

The file ibe.h is the most important one for application programmers. In fact,
one could easily build IBE applications using only the functions declared in this
file, provided that one already has access to a library that can do encryption
and decryption (such as OpenSSL, or the file crypto.c that is described next).

The code in crypto.c essentially consists of wrappers around OpenSSL functions.
This is so that another crypto library can be substituted in later, if needed.
Its role is to provide encryption, decryption and random number generation
routines.

Next, format.c uses ibe_lib.c and crypto.c to show how the library can be
used in applications. I called it "format" because it contained code for
saving and loading various file formats, and it is here these formats are
defined.

The gen and ibe programs are applications that use the above. They are
command-line tools that allow people to run IBE systems.

The file byte_string.c defines a data type called byte_string_t that holds a
string of bytes, and also contains various utility functions for it.
They were needed because ASCIIZ strings were not sufficient.

mm.* and benchmark.* are for debugging and benchmarking. By default,
the "void" versions will be used, and any functions starting with mm_ or bm_
can be ignored.

There are also a bunch of test programs, but the most important one is
torture.c, as the library cannot be trusted if "torture -m2" does not
run successfully (and without memory leaks).
Run "torture -h" to see how to use it.

The library is thread-safe except for IBE_init() and IBE_clear().

The data types were modeled on the GMP data types. For example,
to use byte_string's:

Declare with:
    byte_string_t foo;

Initialize with:
    byte_string_init(foo, n);
    
    where n = length of byte string

Now you can assign to foo->data[0] to foo->data[n-1], which hold
unsigned chars, and also foo->length = n.

Free with:
    byte_string_clear(foo);

INSTALLATION

Linux:

1.Install the GMP and OpenSSL libraries.
2.Edit the Makefile and change the directories as needed.
3.Run make.

Make ibe_test for a simple test program (ibe_test).

Cross-compiling for Windows:

I have only done this with the mingw32 compiler included with the Testing
distribution of Debian GNU/Linux (http://www.debian.org/).

1. Cross-compile GMP and OpenSSL.
2. Run "make WIN32=1"

USAGE

For users:

See ibe_help.txt.

For programmers:

Look at ibe_test.c for a specific example.

1) Include ibe.h and format.h in the program. (ibe_test.c doesn't need to
  include format.h because it doesn't load/save files.)

2) Call IBE_init() to initialize the library.

3) Now system parameters are needed. Declare a params_t type, e.g.
    params_t params;
  You can either generate new ones via IBE_setup(), or load them from a file,
  via FMT_load_params(), e.g.  FMT_load_params(params, "params.txt");

4) Then call the various FMT_* or IBE_* functions to do what you want.
  FMT_* is a wrapper around the IBE_* functions and deals with files,
  and IBE_* are the lower level functions and deals with memory buffers.

5) Call params_clear(params) to free the system parameters.
  Call IBE_clear() to free the memory used by the library.

I wrote a command-line program, called "ibe.c", which does the commonly-used
operations you'd expect. See ibe_help.txt on how to use it.

See gen.c on how to generate system parameters and save them, and also on how
to split the master secret and then save the pieces. (Each piece is then copied
to a server, and the PKG is run on that server using that piece.)

See combine.c on how to combine private key shares together to form a private
key.  (Right now it's quite insecure because each piece is encrypted with the
same password, so that's why the generate_password() function looks useless.
This'll be fixed eventually.)

See encrypt.c on how to encrypt a given message with a given ID,
and decrypt.c on how to decrypt a given message with a given private key.

REFERENCES

1. http://www.openssl.org/
    OpenSSL library.

2. http://www.swox.com/gmp/
    GMP library.

3. http://crypto.stanford.edu/ibe/
    Stanford IBE project page. Contains online version of Boneh and
Franklin's paper. Also a link to my homepage where more relevant papers may
be found.