Pedersen commitments, borromean ring signatures, and ZK range proofs.

This commit adds three new cryptosystems to libsecp256k1:

Pedersen commitments are a system for making blinded commitments
 to a value.  Functionally they work like:
  commit_b,v = H(blind_b || value_v),
 except they are additively homorphic, e.g.
  C(b1, v1) - C(b2, v2) = C(b1 - b2, v1 - v2) and
  C(b1, v1) - C(b1, v1) = 0, etc.
 The commitments themselves are EC points, serialized as 33 bytes.
 In addition to the commit function this implementation includes
 utility functions for verifying that a set of commitments sums
 to zero, and for picking blinding factors that sum to zero.
 If the blinding factors are uniformly random, pedersen commitments
 have information theoretic privacy.

Borromean ring signatures are a novel efficient ring signature
 construction for AND/OR admissions policies (the code here implements
 an AND of ORs, each of any size).  This construction requires
 32 bytes of signature per pubkey used plus 32 bytes of constant
 overhead. With these you can construct signatures like "Given pubkeys
 A B C D E F G, the signer knows the discrete logs
 satisifying (A || B) & (C || D || E) & (F || G)".

ZK range proofs allow someone to prove a pedersen commitment is in
 a particular range (e.g. [0..2^64)) without revealing the specific
 value.  The construction here is based on the above borromean
 ring signature and uses a radix-4 encoding and other optimizations
 to maximize efficiency.  It also supports encoding proofs with a
 non-private base-10 exponent and minimum-value to allow trading
 off secrecy for size and speed (or just avoiding wasting space
 keeping data private that was already public due to external
 constraints).

A proof for a 32-bit mantissa takes 2564 bytes, but 2048 bytes of
 this can be used to communicate a private message to a receiver
 who shares a secret random seed with the prover.

Also: get rid of precomputed H tables (Pieter Wuille)

configure.ac

@@ -134,6 +134,12 @@ AC_ARG_ENABLE(module_recovery,
     [enable_module_recovery=$enableval],
     [enable_module_recovery=no])
 
+
+AC_ARG_ENABLE(module_rangeproof,
+    AS_HELP_STRING([--enable-module-rangeproof],[enable Pedersen / zero-knowledge range proofs module (default is no)]),
+    [enable_module_rangeproof=$enableval],
+    [enable_module_rangeproof=no])
+
 AC_ARG_ENABLE(external_default_callbacks,
     AS_HELP_STRING([--enable-external-default-callbacks],[enable external default callback functions (default is no)]),
     [use_external_default_callbacks=$enableval],
@@ -498,6 +504,10 @@ if test x"$enable_module_recovery" = x"yes"; then
   AC_DEFINE(ENABLE_MODULE_RECOVERY, 1, [Define this symbol to enable the ECDSA pubkey recovery module])
 fi
 
+if test x"$enable_module_rangeproof" = x"yes"; then
+  AC_DEFINE(ENABLE_MODULE_RANGEPROOF, 1, [Define this symbol to enable the Pedersen / zero knowledge range proof module])
+fi
+
 AC_C_BIGENDIAN()
 
 if test x"$use_external_asm" = x"yes"; then
@@ -513,6 +523,7 @@ if test x"$enable_experimental" = x"yes"; then
   AC_MSG_NOTICE([WARNING: experimental build])
   AC_MSG_NOTICE([Experimental features do not have stable APIs or properties, and may not be safe for production use.])
   AC_MSG_NOTICE([Building ECDH module: $enable_module_ecdh])
+  AC_MSG_NOTICE([Building range proof module: $enable_module_rangeproof])
   AC_MSG_NOTICE([******])
 else
   if test x"$enable_module_ecdh" = x"yes"; then
@@ -521,6 +532,9 @@ else
   if test x"$set_asm" = x"arm"; then
     AC_MSG_ERROR([ARM assembly optimization is experimental. Use --enable-experimental to allow.])
   fi
+  if test x"$enable_module_rangeproof" = x"yes"; then
+    AC_MSG_ERROR([Range proof module is experimental. Use --enable-experimental to allow.])
+  fi
 fi
 
 AC_CONFIG_HEADERS([src/libsecp256k1-config.h])
@@ -537,7 +551,7 @@ AM_CONDITIONAL([USE_BENCHMARK], [test x"$use_benchmark" = x"yes"])
 AM_CONDITIONAL([USE_ECMULT_STATIC_PRECOMPUTATION], [test x"$set_precomp" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_ECDH], [test x"$enable_module_ecdh" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_RECOVERY], [test x"$enable_module_recovery" = x"yes"])
-AM_CONDITIONAL([USE_JNI], [test x"$use_jni" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_RANGEPROOF], [test x"$enable_module_rangeproof" = x"yes"])
 AM_CONDITIONAL([USE_EXTERNAL_ASM], [test x"$use_external_asm" = x"yes"])
 AM_CONDITIONAL([USE_ASM_ARM], [test x"$set_asm" = x"arm"])