The scheme implemented is described below, and largely follows the paper
"SwiftEC: Shallue–van de Woestijne Indifferentiable Function To Elliptic Curves",
by Chavez-Saab, Rodriguez-Henriquez, and Tibouchi
(https://eprint.iacr.org/2022/759).
A new 64-byte public key format is introduced, with the property that *every*
64-byte array is an encoding for a non-infinite curve point. Each curve point
has roughly 2^256 distinct encodings. This permits disguising public keys as
uniformly random bytes.
The new API functions:
* secp256k1_ellswift_encode: convert a normal public key to an ellswift 64-byte
public key, using additional entropy to pick among the many possible
encodings.
* secp256k1_ellswift_decode: convert an ellswift 64-byte public key to a normal
public key.
* secp256k1_ellswift_create: a faster and safer equivalent to calling
secp256k1_ec_pubkey_create + secp256k1_ellswift_encode.
* secp256k1_ellswift_xdh: x-only ECDH directly on ellswift 64-byte public keys,
where the key encodings are fed to the hash function.
The scheme itself is documented in secp256k1_ellswift.h.
ef49a11d29 build: allow static or shared but not both (Cory Fields)
36b0adf1b9 build: remove warning until it's reproducible (Cory Fields)
Pull request description:
Continuing from here: https://github.com/bitcoin-core/secp256k1/issues/1224#issuecomment-1460438227
Unfortunately it wasn't really possible to keep a clean diff here because of the nature of the change. I suggest reviewing the lib creation stuff in its entirety, sorry about that :\
Rather than allowing for shared and static libs to be built at the same time like autotools, this PR switches to the CMake convention of allowing only 1.
A new `BUILD_SHARED_LIBS` option is added to match CMake convention, as well as a `SECP256K1_DISABLE_SHARED` option which overrides it. That way even projects which have `BUILD_SHARED_LIBS=1` can opt-into a static libsecp in particular.
Details:
Two object libraries are created: `secp256k1_asm` and `secp256k1_precomputed_objs`. Some tests/benchmarks use the object libraries directly, some link against the real lib: `secp256k1`.
Because the objs don't know what they're going to be linked into, they need to be told how to deal with PIC.
The `DEFINE_SYMBOL` property sets the `DLL_EXPORT` define as necessary (when building a shared lib)
ACKs for top commit:
hebasto:
re-ACK ef49a11d29, only [suggested](https://github.com/bitcoin-core/secp256k1/pull/1230#pullrequestreview-1388191165) changes since my recent [review](https://github.com/bitcoin-core/secp256k1/pull/1230#pullrequestreview-1352125381).
real-or-random:
ACK ef49a11d29
Tree-SHA512: 8870de305176fdb677caff0fdfc6f8c59c0e906489cb72bc9980e551002812685e59e20d731f2a82e33628bdfbb7261eafd6f228038cad3ec83bd74335959600
5bb03c2911 Replace `SECP256K1_ECMULT_TABLE_VERIFY` macro by a function (Hennadii Stepanov)
4429a8c218 Suppress `-Wunused-parameter` when building for coverage analysis (Hennadii Stepanov)
Pull request description:
ACKs for top commit:
real-or-random:
utACK 5bb03c2911
jonasnick:
ACK 5bb03c2911
Tree-SHA512: 19a395434ecefea201a03fc45b3f0b88f1520908926ac1207bbc6570034b1141b49c3c98e66819dcd9069dfdd28c7c6fbe957f13fb6bd178fd57ce65bfbb8fbd
