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)
cd473e0 Avoid calling secp256k1_*_is_zero when secp256k1_*_set_b32 fails. (Gregory Maxwell)
Pull request description:
Most of the codebase correctly used short-cutting to avoid calling
_is_zero on possibly incompletely initialized elements, but a few
places were missed.
ACKs for commit cd473e:
sipa:
utACK cd473e02c372217c3a6608ce5afaa543ed78f891
jonasnick:
utACK cd473e02c372217c3a6608ce5afaa543ed78f891
Tree-SHA512: d6af2863f6795d2df26f2bd05a4e33085e88c45f7794601ea57e67238a2073ef1ee3ba0feab62a7fcbc0636c48dfd80eea07d0ca4f194414127f914b0478c732
dcf3920 Fix ability to compile tests without -DVERIFY. (Gregory Maxwell)
Pull request description:
Broken by 3f3964e4.
It's important that the tests are also run without -DVERIFY due to
the possibility that side-effects of a VERIFY_CHECK fix a bug that
would otherwise be detected.
Use of the verify_check macro in tests isn't sufficient.
ACKs for commit dcf392:
Tree-SHA512: ff7ca0e89e33f845656a4d7d18c0195d1378b020d67f89e900b18cf3d702aa81dd91ffd05a98953a481b83e4247eaf0c484bea12eab020efb3c966a456e8129f
14c7dbd Simplify control flow in DER parsing (Tim Ruffing)
ec8f20b Avoid out-of-bound pointers and integer overflows in size comparisons (Tim Ruffing)
01ee1b3 Parse DER-enconded length into a size_t instead of an int (Tim Ruffing)
3cb057f Fix possible integer overflow in DER parsing (Tim Ruffing)
Pull request description:
This is a result of auditing the code for overflow issues at random places. None of this is critical but I think all of it should be fixed.
I know this touches "red" code. I double-checked and triple-checked this but I can understand if some of the changes are not desirable because they change well-tested code.
Best reviewed in individual commits.
ACKs for commit 14c7db:
Tree-SHA512: 312dd3f961739752e1a861e75bd755920f634f87ee9668793e102c224434e8d21367452e114de729322c71a89f4fa82126aa5d32742f2bbbc091777c99515e10
e49f799 Add missing #(un)defines to base-config.h (Tim Ruffing)
77defd2 Add secp256k1_ prefix to default callback functions (Tim Ruffing)
908bdce Include stdio.h and stdlib.h explicitly in secp256k1.c (Tim Ruffing)
5db782e Allow usage of external default callbacks (Tim Ruffing)
6095a86 Replace CHECKs for no_precomp ctx by ARG_CHECKs without a return (Tim Ruffing)
Pull request description:
This is intended for environments without implementations for `abort()`, `fprintf()`, and `stderr`. e.g., embedded systems. Those can provide their own implementations of `default_illegal_callback_fn` and `default_error_callback_fn` at compile time.
If you want to use your own default callback, things will be somewhat inconsistent unfortunately: We cannot make the callback data `extern` too, because then the initialization lists for `default_illegal_callback` won't contain only constants. (`const` variables are not compile-time constants). So you cannot take callback data in your own default callback function.
As a more drastic/breaking alternative I suggest to remove the callback data entirely. I don't think it's a big loss and I would be surprised if anyone uses it. Additionally, we could even remove the possibility to set the callback function at runtime after this PR. This will simplify things a lot, and again I don't think it's a big loss.
Note that `abort()`, `fprintf()`, and `stderr` are also used in `CHECK`, which is still used in production code if we rely on gmp for scalar and field inversions (e.g., https://github.com/bitcoin-core/secp256k1/blob/master/src/scalar_impl.h#L240). This is not an issue for embedded system which probably don't want to use gmp anyway, but it is probably an issue for the reasons explained in https://github.com/bitcoin-core/secp256k1/pull/566#issuecomment-469111901.
(related downstream: https://github.com/rust-bitcoin/rust-secp256k1/pull/100 @elichai)
ACKs for commit e49f79:
Tree-SHA512: 4dec0821eef4156cbe162bd8cdf0531c1fae8c98cd9db8438170ff1aa0e59b199739eeab293695bb582246812bea5309959f02f1fb74bb57872da54ebc52313f
