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Diffstat (limited to 'src/secp256k1/include/secp256k1_ellswift.h')
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diff --git a/src/secp256k1/include/secp256k1_ellswift.h b/src/secp256k1/include/secp256k1_ellswift.h new file mode 100644 index 0000000000..3851f93098 --- /dev/null +++ b/src/secp256k1/include/secp256k1_ellswift.h @@ -0,0 +1,198 @@ +#ifndef SECP256K1_ELLSWIFT_H +#define SECP256K1_ELLSWIFT_H + +#include "secp256k1.h" + +#ifdef __cplusplus +extern "C" { +#endif + +/* This module provides an implementation of ElligatorSwift as well as a + * version of x-only ECDH using it (including compatibility with BIP324). + * + * ElligatorSwift is described in https://eprint.iacr.org/2022/759 by + * Chavez-Saab, Rodriguez-Henriquez, and Tibouchi. It permits encoding + * uniformly chosen public keys as 64-byte arrays which are indistinguishable + * from uniformly random arrays. + * + * Let f be the function from pairs of field elements to point X coordinates, + * defined as follows (all operations modulo p = 2^256 - 2^32 - 977) + * f(u,t): + * - Let C = 0xa2d2ba93507f1df233770c2a797962cc61f6d15da14ecd47d8d27ae1cd5f852, + * a square root of -3. + * - If u=0, set u=1 instead. + * - If t=0, set t=1 instead. + * - If u^3 + t^2 + 7 = 0, multiply t by 2. + * - Let X = (u^3 + 7 - t^2) / (2 * t) + * - Let Y = (X + t) / (C * u) + * - Return the first in [u + 4 * Y^2, (-X/Y - u) / 2, (X/Y - u) / 2] that is an + * X coordinate on the curve (at least one of them is, for any u and t). + * + * Then an ElligatorSwift encoding of x consists of the 32-byte big-endian + * encodings of field elements u and t concatenated, where f(u,t) = x. + * The encoding algorithm is described in the paper, and effectively picks a + * uniformly random pair (u,t) among those which encode x. + * + * If the Y coordinate is relevant, it is given the same parity as t. + * + * Changes w.r.t. the the paper: + * - The u=0, t=0, and u^3+t^2+7=0 conditions result in decoding to the point + * at infinity in the paper. Here they are remapped to finite points. + * - The paper uses an additional encoding bit for the parity of y. Here the + * parity of t is used (negating t does not affect the decoded x coordinate, + * so this is possible). + */ + +/** A pointer to a function used by secp256k1_ellswift_xdh to hash the shared X + * coordinate along with the encoded public keys to a uniform shared secret. + * + * Returns: 1 if a shared secret was successfully computed. + * 0 will cause secp256k1_ellswift_xdh to fail and return 0. + * Other return values are not allowed, and the behaviour of + * secp256k1_ellswift_xdh is undefined for other return values. + * Out: output: pointer to an array to be filled by the function + * In: x32: pointer to the 32-byte serialized X coordinate + * of the resulting shared point (will not be NULL) + * ell_a64: pointer to the 64-byte encoded public key of party A + * (will not be NULL) + * ell_b64: pointer to the 64-byte encoded public key of party B + * (will not be NULL) + * data: arbitrary data pointer that is passed through + */ +typedef int (*secp256k1_ellswift_xdh_hash_function)( + unsigned char *output, + const unsigned char *x32, + const unsigned char *ell_a64, + const unsigned char *ell_b64, + void *data +); + +/** An implementation of an secp256k1_ellswift_xdh_hash_function which uses + * SHA256(prefix64 || ell_a64 || ell_b64 || x32), where prefix64 is the 64-byte + * array pointed to by data. */ +SECP256K1_API_VAR const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_prefix; + +/** An implementation of an secp256k1_ellswift_xdh_hash_function compatible with + * BIP324. It returns H_tag(ell_a64 || ell_b64 || x32), where H_tag is the + * BIP340 tagged hash function with tag "bip324_ellswift_xonly_ecdh". Equivalent + * to secp256k1_ellswift_xdh_hash_function_prefix with prefix64 set to + * SHA256("bip324_ellswift_xonly_ecdh")||SHA256("bip324_ellswift_xonly_ecdh"). + * The data argument is ignored. */ +SECP256K1_API_VAR const secp256k1_ellswift_xdh_hash_function secp256k1_ellswift_xdh_hash_function_bip324; + +/** Construct a 64-byte ElligatorSwift encoding of a given pubkey. + * + * Returns: 1 always. + * Args: ctx: pointer to a context object + * Out: ell64: pointer to a 64-byte array to be filled + * In: pubkey: a pointer to a secp256k1_pubkey containing an + * initialized public key + * rnd32: pointer to 32 bytes of randomness + * + * It is recommended that rnd32 consists of 32 uniformly random bytes, not + * known to any adversary trying to detect whether public keys are being + * encoded, though 16 bytes of randomness (padded to an array of 32 bytes, + * e.g., with zeros) suffice to make the result indistinguishable from + * uniform. The randomness in rnd32 must not be a deterministic function of + * the pubkey (it can be derived from the private key, though). + * + * It is not guaranteed that the computed encoding is stable across versions + * of the library, even if all arguments to this function (including rnd32) + * are the same. + * + * This function runs in variable time. + */ +SECP256K1_API int secp256k1_ellswift_encode( + const secp256k1_context *ctx, + unsigned char *ell64, + const secp256k1_pubkey *pubkey, + const unsigned char *rnd32 +) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4); + +/** Decode a 64-bytes ElligatorSwift encoded public key. + * + * Returns: always 1 + * Args: ctx: pointer to a context object + * Out: pubkey: pointer to a secp256k1_pubkey that will be filled + * In: ell64: pointer to a 64-byte array to decode + * + * This function runs in variable time. + */ +SECP256K1_API int secp256k1_ellswift_decode( + const secp256k1_context *ctx, + secp256k1_pubkey *pubkey, + const unsigned char *ell64 +) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3); + +/** Compute an ElligatorSwift public key for a secret key. + * + * Returns: 1: secret was valid, public key was stored. + * 0: secret was invalid, try again. + * Args: ctx: pointer to a context object + * Out: ell64: pointer to a 64-byte array to receive the ElligatorSwift + * public key + * In: seckey32: pointer to a 32-byte secret key + * auxrnd32: (optional) pointer to 32 bytes of randomness + * + * Constant time in seckey and auxrnd32, but not in the resulting public key. + * + * It is recommended that auxrnd32 contains 32 uniformly random bytes, though + * it is optional (and does result in encodings that are indistinguishable from + * uniform even without any auxrnd32). It differs from the (mandatory) rnd32 + * argument to secp256k1_ellswift_encode in this regard. + * + * This function can be used instead of calling secp256k1_ec_pubkey_create + * followed by secp256k1_ellswift_encode. It is safer, as it uses the secret + * key as entropy for the encoding (supplemented with auxrnd32, if provided). + * + * Like secp256k1_ellswift_encode, this function does not guarantee that the + * computed encoding is stable across versions of the library, even if all + * arguments (including auxrnd32) are the same. + */ +SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ellswift_create( + const secp256k1_context *ctx, + unsigned char *ell64, + const unsigned char *seckey32, + const unsigned char *auxrnd32 +) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3); + +/** Given a private key, and ElligatorSwift public keys sent in both directions, + * compute a shared secret using x-only Elliptic Curve Diffie-Hellman (ECDH). + * + * Returns: 1: shared secret was succesfully computed + * 0: secret was invalid or hashfp returned 0 + * Args: ctx: pointer to a context object. + * Out: output: pointer to an array to be filled by hashfp. + * In: ell_a64: pointer to the 64-byte encoded public key of party A + * (will not be NULL) + * ell_b64: pointer to the 64-byte encoded public key of party B + * (will not be NULL) + * seckey32: a pointer to our 32-byte secret key + * party: boolean indicating which party we are: zero if we are + * party A, non-zero if we are party B. seckey32 must be + * the private key corresponding to that party's ell_?64. + * This correspondence is not checked. + * hashfp: pointer to a hash function. + * data: arbitrary data pointer passed through to hashfp. + * + * Constant time in seckey32. + * + * This function is more efficient than decoding the public keys, and performing + * ECDH on them. + */ +SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ellswift_xdh( + const secp256k1_context *ctx, + unsigned char *output, + const unsigned char *ell_a64, + const unsigned char *ell_b64, + const unsigned char *seckey32, + int party, + secp256k1_ellswift_xdh_hash_function hashfp, + void *data +) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(7); + +#ifdef __cplusplus +} +#endif + +#endif /* SECP256K1_ELLSWIFT_H */ |