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.
b3bf5f9 ecmult_impl: expand comment to explain how effective affine interacts with everything (Andrew Poelstra)
efa783f Store z-ratios in the 'x' coord they'll recover (Peter Dettman)
ffd3b34 add `secp256k1_ge_set_all_gej_var` test which deals with many infinite points (Andrew Poelstra)
84740ac ecmult_impl: save one fe_inv_var (Andrew Poelstra)
4704527 ecmult_impl: eliminate scratch memory used when generating context (Andrew Poelstra)
7f7a2ed ecmult_gen_impl: eliminate scratch memory used when generating context (Andrew Poelstra)
Pull request description:
Builds on #553
Tree-SHA512: 6031a601a4a476c1d21fc8db219383e7930434d2f199543c61aca0118412322dd814a0109c385ff1f83d16897170dd0c25051697b0f88f15234b0059b661af41
c8fbc3c [ECDH API change] Allow pass arbitrary data to hash function (Kirill Fomichev)
b00be65 [ECDH API change] Support custom hash function (Kirill Fomichev)
Pull request description:
Solve #352
Tree-SHA512: f5985874d03e976cdb3d59036af7720636ad1488da40fd3bd7881b1fb71b05036a952013d519baa84c4ce4b558bdef25c4ce76b384b297e4d0aece9e37e78a01
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
