libsnark: a C++ library for zkSNARK proofs

libsnark: a C++ library for zkSNARK proofs

---

Authors

The libsnark library is developed by the SCIPR Lab project and contributors
and is released under the MIT License (see the LICENSE file).

Copyright (c) 2012-2014 SCIPR Lab and contributors (see AUTHORS file).

---

---

Overview

This library implements zkSNARK schemes, which are a cryptographic method
for proving/verifying, in zero knowledge, the integrity of computations.

A computation can be expressed as an NP statement, in forms such as the following:

• “The C program foo, when executed, returns exit code 0 if given the input bar and some additional input qux.”
• “The Boolean circuit foo is satisfiable by some input qux.”
• “The arithmetic circuit foo accepts the partial assignment bar, when extended into some full assignment qux.”
• “The set of constraints foo is satisfiable by the partial assignment bar, when extended into some full assignment qux.”

A prover who knows the witness for the NP statement (i.e., a satisfying input/assignment) can produce a short proof attesting to the truth of the NP statement. This proof can be verified by anyone, and offers the following properties.

• Zero knowledge:
  the verifier learns nothing from the proof beside the truth of the statement (i.e., the value qux, in the above examples, remains secret).
• Succinctness:
  the proof is short and easy to verify.
• Non-interactivity:
  the proof is a string (i.e. it does not require back-and-forth interaction between the prover and the verifier).
• Soundness:
  the proof is computationally sound (i.e., it is infeasible to fake a proof of a false NP statement). Such a proof system is also called an argument.
• Proof of knowledge:
  the proof attests not just that the NP statement is true, but also that the
  prover knows why (e.g., knows a valid qux).

These properties are summarized by the zkSNARK acronym, which stands for Zero-Knowledge Succinct Non-interactive ARgument of Knowledge (though zkSNARKs are also knows as
succinct non-interactive computationally-sound zero-knowledge proofs of knowledge).
For formal definitions and theoretical discussions about these, see
[BCCT12], [BCIOP13], and the references therein.

The libsnark library currently provides a C++ implementation of:

1. General-purpose proof systems:
   
   1. A preprocessing zkSNARK for the NP-complete language “R1CS”
      (Rank-1 Constraint Systems), which is a language that is similar to arithmetic
      circuit satisfiability.
   2. A preprocessing SNARK for a language of arithmetic circuits, “BACS”
      (Bilinear Arithmetic Circuit Satisfiability). This simplifies the writing
      of NP statements when the additional flexibility of R1CS is not needed.
      Internally, it reduces to R1CS.
   3. A preprocessing SNARK for the language “USCS”
      (Unitary-Square Constraint Systems). This abstracts and implements the core
      contribution of [DFGK14]
   4. A preprocessing SNARK for a language of Boolean circuits, “TBCS”
      (Two-input Boolean Circuit Satisfiability). Internally, it reduces to USCS.
      This is much more efficient than going through R1CS.
   5. ADSNARK, a preprocessing SNARKs for proving statements on authenticated
      data, as described in [BBFR15].
   6. Proof-Carrying Data (PCD). This uses recursive composition of SNARKs, as
      explained in [BCCT13] and optimized in [BCTV14b].
2. Gadget libraries (gadgetlib1 and gadgetlib2) for constructing R1CS
   instances out of modular “gadget” classes.
3. Examples of applications that use the above proof systems to prove
   statements about:
   
   1. Several toy examples.
   2. Execution of TinyRAM machine code, as explained in [BCTV14a] and
      [BCGTV13]. (Such machine code can be obtained, e.g., by compiling from C.)
      This is easily adapted to any other Random Access Machine that satisfies a
      simple load-store interface.
   3. A scalable for TinyRAM using Proof-Carrying Data, as explained in [BCTV14b]
   4. Zero-knowldge cluster MapReduce, as explained in [CTV15].

The zkSNARK construction implemented by libsnark follows, extends, and
optimizes the approach described in [BCTV14], itself an extension of
[BCGTV13], following the approach of [BCIOP13] and [GGPR13]. An alternative
implementation of the basic approach is the Pinocchio system of [PGHR13].
See these references for discussions of efficiency aspects that arise in
practical use of such constructions, as well as security and trust
considerations.

This scheme is a preprocessing zkSNARK (ppzkSNARK): before proofs can be
created and verified, one needs to first decide on a size/circuit/system
representing the NP statements to be proved, and run a generator algorithm to
create corresponding public parameters (a long proving key and a short
verification key).

Using the library involves the following high-level steps:

1. Express the statements to be proved as an R1CS (or any of the other
   languages above, such as arithmetic circuits, Boolean circuits, or TinyRAM).
   This is done by writing C++ code that constructs an R1CS, and linking this code
   together with libsnark
2. Use libsnark’s generator algorithm to create the public parameters for this
   statement (once and for all).
3. Use libsnark’s prover algorithm to create proofs of true statements about
   the satisfiability of the R1CS.
4. Use libsnark’s verifier algorithm to check proofs for alleged statements.

---

The NP-complete language R1CS

The ppzkSNARK supports proving/verifying membership in a specific NP-complete
language: R1CS (rank-1 constraint systems). An instance of the language is
specified by a set of equations over a prime field F, and each equation looks like:
< A, (1,X) > * < B , (1,X) > = < C, (1,X) >
where A,B,C are vectors over F, and X is a vector of variables.

In particular, arithmetic (as well as boolean) circuits are easily reducible to
this language by converting each gate into a rank-1 constraint. See [BCGTV13]
Appendix E (and “System of Rank 1 Quadratic Equations”) for more details about this.

---

Elliptic curve choices

The ppzkSNARK can be instantiated with different parameter choices, depending on
which elliptic curve is used. The libsnark library currently provides three
options:

• “edwards”:
  an instantiation based on an Edwards curve, providing 80 bits of security.

• “bn128”:
  an instantiation based on a Barreto-Naehrig curve, providing 128
  bits of security. The underlying curve implementation is
  [ate-pairing], which has incorporated our patch that changes the
  BN curve to one suitable for SNARK applications.

  • This implementation uses dynamically-generated machine code for the curve
    arithmetic. Some modern systems disallow execution of code on the heap, and
    will thus block this implementation.

    For example, on Fedora 20 at its default settings, you will get the error
    zmInit ERR:can't protect when running this code. To solve this,
    run sudo setsebool -P allow_execheap 1 to allow execution,
    or use make CURVE=ALT_BN128 instead.

• “alt_bn128”:
  an alternative to “bn128”, somewhat slower but avoids dynamic code generation.

Note that bn128 requires an x86-64 CPU while the other curve choices
should be architecture-independent; see portability.

---

Gadget libraries

The libsnark library currently provides two libraries for conveniently constructing
R1CS instances out of reusable “gadgets”. Both libraries provide a way to construct
gadgets on other gadgets as well as additional explicit equations. In this way,
complex R1CS instances can be built bottom up.

gadgetlib1

This is a low-level library which expose all features of the preprocessing
zkSNARK for R1CS. Its design is based on templates (as does the ppzkSNARK code)
to efficiently support working on multiple elliptic curves simultaneously. This
library is used for most of the constraint-building in libsnark, both internal
(reductions and Proof-Carrying Data) and examples applications.

gadgetlib2

This is an alternative library for constructing systems of polynomial equations
and, in particular, also R1CS instances. It is better documented and easier to
use than gadgetlib1, and its interface does not use templates. However, fewer
useful gadgets are provided.

---

Security

The theoretical security of the underlying mathematical constructions, and the
requisite assumptions, are analyzed in detailed in the aforementioned research
papers.

**
This code is a research-quality proof of concept, and has not
yet undergone extensive review or testing. It is thus not suitable,
as is, for use in critical or production systems.
**

Known issues include the following:

• The ppzkSNARK’s generator and prover exhibit data-dependent running times
  and memory usage. These form timing and cache-contention side channels,
  which may be an issue in some applications.

• Randomness is retrieved from /dev/urandom, but this should be
  changed to a carefully considered (depending on system and threat
  model) external, high-quality randomness source when creating
  long-term proving/verification keys.

---

Build instructions

The libsnark library relies on the following:

• C++ build environment
• GMP for certain bit-integer arithmetic
• libprocps for reporting memory usage
• GTest for some of the unit tests

So far we have tested these only on Linux, though we have been able to make the library work,
with some features disabled (such as memory profiling or GTest tests), on Windows via Cygwin
and on Mac OS X. (If you succeed in achieving more complete ports of the library, please
let us know!) See also the notes on portability below.

For example, on a fresh install of Ubuntu 14.04, install the following packages:

$ sudo apt-get install build-essential git libgmp3-dev libprocps3-dev libgtest-dev python-markdown libboost-all-dev libssl-dev

Or, on Fedora 20:

$ sudo yum install gcc-c++ make git gmp-devel procps-ng-devel gtest-devel python-markdown

Run the following, to fetch dependencies from their GitHub repos and compile them.
(Not required if you set CURVE to other than the default BN128 and also set NO_SUPERCOP=1.)

$ ./prepare-depends.sh

Then, to compile the library, tests, profiling harness and documentation, run:

$ make

To create just the HTML documentation, run

$ make doc

and then view the resulting README.html (which contains the very text you are reading now).

To create Doxygen documentation summarizing all files, classes and functions,
with some (currently sparse) comments, install the doxygen and graphviz packages, then run

$ make doxy

(this may take a few minutes). Then view the resulting doxygen/index.html.

Using libsnark as a library

To develop an application that uses libsnark, you could add it within the libsnark directory tree and adjust the Makefile, but it is far better to build libsnark as a (shared or static) library. You can then write your code in a separate directory tree, and link it against libsnark.

To build just the shared object library libsnark.so, run:

$ make lib

To build just the static library libsnark.a, run:

$ make lib STATIC=1

Note that static compilation requires static versions of all libraries it depends on.
It may help to minize these dependencies by appending
CURVE=ALT_BN128 NO_PROCPS=1 NO_GTEST=1 NO_SUPERCOP=1. On Fedora 21, the requisite
library RPM dependencies are then:
boost-static glibc-static gmp-static libstdc++-static openssl-static zlib-static
boost-devel glibc-devel gmp-devel gmp-devel libstdc++-devel openssl-devel openssl-devel.

To build and install the libsnark library:

$ make install PREFIX=/install/path

This will install libsnark.so into /install/path/lib; so your application should be linked using -L/install/path/lib -lsnark. It also installs the requisite headers into /install/path/include; so your application should be compiled using -I/install/path/include.

In addition, unless you use NO_SUPERCOP=1, libsupercop.a will be installed and should be linked in using -lsupercop.

Building on Windows using Cygwin

Install Cygwin using the graphical installer, including the g++, libgmp
and git packages. Then disable the dependencies not easily supported under CygWin,
using:

$ make NO_PROCPS=1 NO_GTEST=1 NO_DOCS=1

Building on Mac OS X

On Mac OS X, install GMP from MacPorts (port install gmp). Then disable the
dependencies not easily supported under CygWin, using:

$ make NO_PROCPS=1 NO_GTEST=1 NO_DOCS=1

MacPorts does not write its libraries into standard system folders, so you
might need to explicitly provide the paths to the header files and libraries by
appending CXXFLAGS=-I/opt/local/include LDFLAGS=-L/opt/local/lib to the line
above. Similarly, to pass the paths to ate-pairing you would run
INC_DIR=-I/opt/local/include LIB_DIR=-L/opt/local/lib ./prepare-depends.sh
instead of ./prepare-depends.sh above.

---

Tutorials

libsnark includes a tutorial, and some usage examples, for the high-level API.

• src/gadgetlib1/examples1 contains a simple example for constructing a
  constraint system using gadgetlib1.

• src/gadgetlib2/examples contains a tutorial for using gadgetlib2 to express
  NP statements as constraint systems. It introduces basic terminology, design
  overview, and recommended programming style. It also shows how to invoke
  ppzkSNARKs on such constraint systems. The main file, tutorial.cpp, builds
  into a standalone executable.

• src/zk_proof_systems/ppzksnark/r1cs_ppzksnark/profiling/profile_r1cs_ppzksnark.cpp
  constructs a simple constraint system and runs the ppzksnark. See below for how to
  run it.

---

Executing profiling example

The command

 $ src/zk_proof_systems/ppzksnark/r1cs_ppzksnark/profiling/profile_r1cs_ppzksnark 1000 10 Fr

exercises the ppzkSNARK (first generator, then prover, then verifier) on an
R1CS instance with 1000 equations and an input consisting of 10 field elements.

(If you get the error zmInit ERR:can't protect, see the discussion
above.)

The command

 $ src/zk_proof_systems/ppzksnark/r1cs_ppzksnark/profiling/profile_r1cs_ppzksnark 1000 10 bytes

does the same but now the input consists of 10 bytes.

---

Build options

The following flags change the behavior of the compiled code.

• make FEATUREFLAGS='-Dname1 -Dname2 ...'

  Override the active conditional #define names (you can see the default at the top of the Makefile).
  The next bullets list the most important conditionally-#defined features.
  For example, make FEATUREFLAGS='-DBINARY_OUTPUT' enables binary output and disables the default
  assembly optimizations and Montgomery-representation output.

• define BINARY_OUTPUT

  In serialization, output raw binary data (instead of decimal, when not set).

• make CURVE=choice / define CURVE_choice (where choice is one of:
  ALT_BN128, BN128, EDWARDS, MNT4, MNT6)

  Set the default curve to one of the above (see elliptic curve choices).

• make DEBUG=1 / define DEBUG

  Print additional information for debugging purposes.

• make LOWMEM=1 / define LOWMEM

  Limit the size of multi-exponentiation tables, for low-memory platforms.

• make NO_DOCS=1

  Do not generate HTML documentation, e.g. on platforms where Markdown is not easily available.

• make NO_PROCPS=1

  Do not link against libprocps. This disables memory profiling.

• make NO_GTEST=1

  Do not link against GTest. The tutorial and test suite of gadgetlib2 tutorial won’t be compiled.

• make NO_SUPERCOP=1

  Do not link against SUPERCOP for optimized crypto. The ADSNARK executables will not be built.

• make MULTICORE=1

  Enable parallelized execution of the ppzkSNARK generator and prover, using OpenMP.
  This will utilize all cores on the CPU for heavyweight parallelizabe operations such as
  FFT and multiexponentiation. The default is single-core.

  To override the maximum number of cores used, set the environment variable OMP_NUM_THREADS
  at runtime (not compile time), e.g., OMP_NUM_THREADS=8 test_r1cs_sp_ppzkpc. It defaults
  to the autodetected number of cores, but on some devices, dynamic core management confused
  OpenMP’s autodetection, so setting OMP_NUM_THREADS is necessary for full utilization.

• define NO_PT_COMPRESSION

  Do not use point compression.
  This gives much faster serialization times, at the expense of ~2x larger
  sizes for serialized keys and proofs.

• define MONTGOMERY_OUTPUT (on by default)

  Serialize Fp elements as their Montgomery representations. If this
  option is disabled then Fp elements are serialized as their
  equivalence classes, which is slower but produces human-readable
  output.

• make PROFILE_OP_COUNTS=1 / define PROFILE_OP_COUNTS

  Collect counts for field and curve operations inside static variables
  of the corresponding algebraic objects. This option works for all
  curves except bn128.

• define USE_ASM (on by default)

  Use unrolled assembly routines for F[p] arithmetic and faster heap in
  multi-exponentiation. (When not set, use GMP’s mpn_* routines instead.)

• define USE_MIXED_ADDITION

  Convert each element of the proving key and verification key to
  affine coordinates. This allows using mixed addition formulas in
  multiexponentiation and results in slightly faster prover and
  verifier runtime at expense of increased proving time.

• make PERFORMANCE=1

  Enables compiler optimizations such as link-time optimization, and disables debugging aids.
  (On some distributions this causes a plugin needed to handle lto object link error and undefined references, which can be remedied by AR=gcc-ar make ....)

Not all combinations are tested together or supported by every part of the codebase.

---

Portability

libsnark is written in fairly standard C++11.

However, having been developed on Linux on x86-64 CPUs, libsnark has some limitations
with respect to portability. Specifically:

1. libsnark’s algebraic data structures assume little-endian byte order.

2. Profiling routines use clock_gettime and readproc calls, which are Linux-specific.

3. Random-number generation is done by reading from /dev/urandom, which is
   specific to Unix-like systems.

4. libsnark binary serialization routines (see BINARY_OUTPUT above) assume
   a fixed machine word size (i.e. sizeof(mp_limb_t) for GMP’s limb data type).
   Objects serialized in binary on a 64-bit system cannot be de-serialized on
   a 32-bit system, and vice versa.
   (The decimal serialization routines have no such limitation.)

5. libsnark requires a C++ compiler with good C++11 support. It has been
   tested with g++ 4.7, g++ 4.8, and clang 3.4.

6. On x86-64, we by default use highly optimized assembly implementations for some
   operations (see USE_ASM above). On other architectures we fall back to a
   portable C++ implementation, which is slower.

Tested configurations include:

• Debian jessie with g++ 4.7 on x86-64
• Debian jessie with clang 3.4 on x86-64
• Fedora 20/21 with g++ 4.8.2/4.9.2 on x86-64 and i686
• Ubuntu 14.04 LTS with g++ 4.8 on x86-64
• Ubuntu 14.04 LTS with g++ 4.8 on x86-32, for EDWARDS and ALT_BN128 curve choices
• Debian wheezy with g++ 4.7 on ARM little endian (Debian armel port) inside QEMU, for EDWARDS and ALT_BN128 curve choices
• Windows 7 with g++ 4.8.3 under Cygwin 1.7.30 on x86-64 with NO_PROCPS=1, NO_GTEST=1 and NO_DOCS=1, for EDWARDS and ALT_BN128 curve choices
• Mac OS X 10.9.4 (Mavericks) with Apple LLVM version 5.1 (based on LLVM 3.4svn) on x86-64 with NO_PROCPS=1, NO_GTEST=1 and NO_DOCS=1

---

Directory structure

The directory structure of the libsnark library is as follows:

• src/ — main C++ source code, containing the following modules:

  • algebra/ — fields and elliptic curve groups
  • common/ — miscellaneous utilities
  • gadgetlib1/ — gadgetlib1, a library to construct R1CS instances
    
    • gadgets/ — basic gadgets for gadgetlib1
  • gadgetlib2/ — gadgetlib2, a library to construct R1CS instances
  • qap/ — quadratic arithmetic program
    
    • domains/ — support for fast interpolation/evaluation, by providing
      FFTs and Lagrange-coefficient computations for various domains
  • relations/ — interfaces for expressing statement (relations between instances and witnesses) as various NP-complete languages
    
    • constraint_satisfaction_problems/ — R1CS and USCS languages
    • circuit_satisfaction_problems/ — Boolean and arithmetic circuit satisfiability languages
    • ram_computations/ — RAM computation languages
  • zk_proof_systems — interfaces and implementations of the proof systems
  • reductions — reductions between languages (used internally, but contains many examples of building constraints)

  Some of these module directories have the following subdirectories:

  • …
    
    • examples/ — example code and tutorials for this module
    • tests/ — unit tests for this module

  In particular, the top-level API examples are at src/r1cs_ppzksnark/examples/ and src/gadgetlib2/examples/.

• depsrc/ — created by prepare_depends.sh for retrieved sourcecode and local builds of external code
  (currently: [ate-pairing], and its dependency xbyak).

• depinst/ — created by prepare_depends.sh and Makefile
  for local installation of locally-compiled dependencies.

• doxygen/ — created by make doxy and contains a Doxygen summary of all files, classes etc. in libsnark.

---

Further considerations

Multiexponentiation window size

The ppzkSNARK’s generator has to solve a fixed-base multi-exponentiation
problem. We use a window-based method in which the optimal window size depends
on the size of the multiexponentiation instance and the platform.

On our benchmarking platform (a 3.40 GHz Intel Core i7-4770 CPU), we have
computed for each curve optimal windows, provided as
“fixed_base_exp_window_table” initialization sequences, for each curve; see
X_init.cpp for X=edwards,bn128,alt_bn128.

Performance on other platforms may not be optimal (but probably not be far off).
Future releases of the libsnark library will include a tool that generates
optimal window sizes.

---

References

[BBFR15]
ADSNARK: nearly practical and privacy-preserving proofs on authenticated data
,
Michael Backes, Manuel Barbosa, Dario Fiore, Raphael M. Reischuk,
IEEE Symposium on Security and Privacy (Oakland) 2015

[BCCT12]
From extractable collision resistance to succinct non-Interactive arguments of knowledge, and back again
,
Nir Bitansky, Ran Canetti, Alessandro Chiesa, Eran Tromer,
Innovations in Computer Science (ITCS) 2012

[BCCT13]
Recursive composition and bootstrapping for SNARKs and proof-carrying data

Nir Bitansky, Ran Canetti, Alessandro Chiesa, Eran Tromer,
Symposium on Theory of Computing (STOC) 13

[BCGTV13]
SNARKs for C: Verifying Program Executions Succinctly and in Zero Knowledge
,
Eli Ben-Sasson, Alessandro Chiesa, Daniel Genkin, Eran Tromer, Madars Virza,
CRYPTO 2013

[BCIOP13]
Succinct Non-Interactive Arguments via Linear Interactive Proofs
,
Nir Bitansky, Alessandro Chiesa, Yuval Ishai, Rafail Ostrovsky, Omer Paneth,
Theory of Cryptography Conference 2013

[BCTV14a]
Succinct Non-Interactive Zero Knowledge for a von Neumann Architecture
,
Eli Ben-Sasson, Alessandro Chiesa, Eran Tromer, Madars Virza,
USENIX Security 2014

[BCTV14b]
Scalable succinct non-interactive arguments via cycles of elliptic curves
,
Eli Ben-Sasson, Alessandro Chiesa, Eran Tromer, Madars Virza,
CRYPTO 2014

[CTV15]
Cluster computing in zero knowledge
,
Alessandro Chiesa, Eran Tromer, Madars Virza,
Eurocrypt 2015

[DFGK14]
Square span programs with applications to succinct NIZK arguments
,
George Danezis, Cedric Fournet, Jens Groth, Markulf Kohlweiss,
ASIACCS 2014

[GGPR13]
Quadratic span programs and succinct NIZKs without PCPs
,
Rosario Gennaro, Craig Gentry, Bryan Parno, Mariana Raykova,
EUROCRYPT 2013

[ate-pairing]
High-Speed Software Implementation of the Optimal Ate Pairing over Barreto-Naehrig Curves
,
MITSUNARI Shigeo, TERUYA Tadanori

[PGHR13]
Pinocchio: Nearly Practical Verifiable Computation
,
Bryan Parno, Craig Gentry, Jon Howell, Mariana Raykova,
IEEE Symposium on Security and Privacy (Oakland) 2013