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.
6fe5043 scratch: add stack frame support (Andrew Poelstra)
Pull request description:
Replaces the single-blob stack space ith one that internally manages multiple blobs, which are exposed to the user as "frames". Users allocate new blobs with `secp256k1_scratch_allocate_frame` and deallocate them with `secp256k1_scratch_deallocate_frame`. Then any calls to `secp256k1_scratch_alloc` use the frame at the top of the stack. This is guaranteed to succeed, assuming that the frame allocation succeeded and that the user is not requesting more memory than the frame was allocated with.
Tree-SHA512: 0b2072c5b9df8f3b40fb6d76e94fcfcc6a03a7da33e31249b5f24b02eb8a3311f282f6a4732153d6101968de8f9a568009a72735a1cc688a0f3040055799a09d
ec0a7b3 Don't touch leading zeros in wnaf_fixed. (Jonas Nick)
9e36d1b Fix bug in wnaf_fixed where the wnaf array is not completely zeroed when given a 0 scalar. (Jonas Nick)
96f68a0 Don't invert scalar in wnaf_fixed when it is even because a caller might intentionally give a scalar with many leading zeros. (Jonas Nick)
6dbb007 Increase sparsity of pippenger fixed window naf representation (Jonas Nick)
Pull request description:
Fixes#506
Tree-SHA512: 49a237a7d09c0c376ba4e6b1f522b9aff2517e420dfef9df810fd5ba920e0b98be8fe3f730b32e41b4aef475bc4cf3b13220024bd8d6f40c2744e6f392ff97a8
e7daa9b [build] Tweak JNI macro to warn instead of error for JNI not found. (Karl-Johan Alm)
5b22977 [build] Update ax_jni_include_dir.m4 macro to deal with recent versions of macOS (Karl-Johan Alm)
Pull request description:
Prior to this patch, this macro fails to find a working directory for the JNI headers, and results in compile failure when doing
```
./configure --enable-experimental --enable-module-ecdh --enable-jni
```
on more recent macOS versions. The relevant commit upstream is [here](http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commit;h=ab23d25b1f1ae544fffdaa0a94a794798695c672) from the [GNU.org page for the macro](https://www.gnu.org/software/autoconf-archive/ax_jni_include_dir.html). The original (identical to the version in this commit) is [here](http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_jni_include_dir.m4).
The compile failure:
```Bash
$ make
gcc -I. -g -O2 -Wall -Wextra -Wno-unused-function -c src/gen_context.c -o gen_context.o
gcc gen_context.o -o gen_context
./gen_context
CC src/libsecp256k1_la-secp256k1.lo
CC src/java/libsecp256k1_jni_la-org_bitcoin_NativeSecp256k1.lo
In file included from src/java/org_bitcoin_NativeSecp256k1.c:4:
In file included from src/java/org_bitcoin_NativeSecp256k1.h:2:
/Library/Java/JavaVirtualMachines/jdk1.7.0_71.jdk/Contents/Headers/jni.h:45:10: fatal error:
'jni_md.h' file not found
#include "jni_md.h"
^
1 error generated.
make: *** [src/java/libsecp256k1_jni_la-org_bitcoin_NativeSecp256k1.lo] Error 1
```
Tree-SHA512: 9a8305b3ed67eba0de728f91cf435857a676ba10507ab8481a3c03b50e1ce0469a3d79e751d0a697018789f21e2aa48b7eccca4d225520a3863fcf23f1fd487a
82a96e4 tests: Make sure we get the requested number of bytes from /dev/urandom (practicalswift)
Pull request description:
Make sure we get the requested number of bytes from `/dev/urandom`.
Tree-SHA512: 1b035942fd2a6ee2423fb2a2a0a0f294682c51434f86e5c106fb493d77f45aa8070662190aca6441fe389b8cdcc132d432517b8e826be2ac530a1511cd0c8919