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Add ellswift module implementing ElligatorSwift

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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.
This commit is contained in:
Pieter Wuille
2022-11-04 16:18:40 -04:00
parent 79e5b2a8b8
commit c47917bbd6
8 changed files with 756 additions and 0 deletions
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6
CMakeLists.txt
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@@ -71,6 +71,11 @@ if(SECP256K1_ENABLE_MODULE_EXTRAKEYS)
add_compile_definitions(ENABLE_MODULE_EXTRAKEYS=1)
endif()
option(SECP256K1_ENABLE_MODULE_ELLSWIFT "Enable ElligatorSwift module." ON)
if(SECP256K1_ENABLE_MODULE_ELLSWIFT)
add_compile_definitions(ENABLE_MODULE_ELLSWIFT=1)
endif()
option(SECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS "Enable external default callback functions." OFF)
if(SECP256K1_USE_EXTERNAL_DEFAULT_CALLBACKS)
add_compile_definitions(USE_EXTERNAL_DEFAULT_CALLBACKS=1)
@@ -270,6 +275,7 @@ message(" ECDH ................................ ${SECP256K1_ENABLE_MODULE_ECDH}
message(" ECDSA pubkey recovery ............... ${SECP256K1_ENABLE_MODULE_RECOVERY}")
message(" extrakeys ........................... ${SECP256K1_ENABLE_MODULE_EXTRAKEYS}")
message(" schnorrsig .......................... ${SECP256K1_ENABLE_MODULE_SCHNORRSIG}")
message(" ElligatorSwift ...................... ${SECP256K1_ENABLE_MODULE_ELLSWIFT}")
message("Parameters:")
message(" ecmult window size .................. ${SECP256K1_ECMULT_WINDOW_SIZE}")
message(" ecmult gen precision bits ........... ${SECP256K1_ECMULT_GEN_PREC_BITS}")