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Make randomization of a non-signing context a noop #587

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Feb 21, 2019
Merged

Make randomization of a non-signing context a noop #587

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7 changes: 6 additions & 1 deletion include/secp256k1.h
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Original file line number Diff line number Diff line change
Expand Up @@ -615,7 +615,7 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_mul(
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Updates the context randomization to protect against side-channel leakage.
* Returns: 1: randomization successfully updated
* Returns: 1: randomization successfully updated or nothing to randomize
* 0: error
* Args: ctx: pointer to a context object (cannot be NULL)
* In: seed32: pointer to a 32-byte random seed (NULL resets to initial state)
Expand All @@ -630,6 +630,11 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_mul(
* that it does not affect function results, but shields against attacks which
* rely on any input-dependent behaviour.
*
* This function has currently an effect only on contexts initialized for signing
* because randomization is currently used only for signing. However, this is not
* guaranteed and may change in the future. It is safe to call this function on
* contexts not initialized for signing; then it will have no effect and return 1.
*
* You should call this after secp256k1_context_create or
* secp256k1_context_clone, and may call this repeatedly afterwards.
*/
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6 changes: 3 additions & 3 deletions src/secp256k1.c
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Expand Up @@ -570,9 +570,9 @@ int secp256k1_ec_pubkey_tweak_mul(const secp256k1_context* ctx, secp256k1_pubkey

int secp256k1_context_randomize(secp256k1_context* ctx, const unsigned char *seed32) {
VERIFY_CHECK(ctx != NULL);
CHECK(ctx != secp256k1_context_no_precomp);
ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
secp256k1_ecmult_gen_blind(&ctx->ecmult_gen_ctx, seed32);
if (secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx)) {
secp256k1_ecmult_gen_blind(&ctx->ecmult_gen_ctx, seed32);
}
return 1;
}

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8 changes: 6 additions & 2 deletions src/tests.c
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Expand Up @@ -218,8 +218,12 @@ void run_context_tests(void) {
CHECK(ecount == 3);
CHECK(secp256k1_ec_pubkey_tweak_mul(vrfy, &pubkey, ctmp) == 1);
CHECK(ecount == 3);
CHECK(secp256k1_context_randomize(vrfy, ctmp) == 0);
CHECK(ecount == 4);
CHECK(secp256k1_context_randomize(vrfy, ctmp) == 1);
CHECK(ecount == 3);
CHECK(secp256k1_context_randomize(vrfy, NULL) == 1);
CHECK(ecount == 3);
CHECK(secp256k1_context_randomize(sign, ctmp) == 1);
CHECK(ecount2 == 14);
CHECK(secp256k1_context_randomize(sign, NULL) == 1);
CHECK(ecount2 == 14);
secp256k1_context_set_illegal_callback(vrfy, NULL, NULL);
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