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Use separate in and out pointers in secp256k1_ec_pubkey_decompress

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Right now `secp256k1_ec_pubkey_decompress` takes an in/out pointer to
a public key and replaces the input key with its decompressed variant.
This forces users who store compressed keys in small (<65 byte) fixed
size buffers (for example, the Rust bindings do this) to explicitly
and wastefully copy their key to a larger buffer.

[API BREAK]
This commit is contained in:
Andrew Poelstra 2015-05-11 09:55:45 -05:00
parent 729badff14
commit 210ffed5cd
3 changed files with 28 additions and 13 deletions
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18
include/secp256k1.h
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@ -262,18 +262,20 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_create(
/** Decompress a public key. /** Decompress a public key.
* In: ctx: pointer to a context object (cannot be NULL) * In: ctx: pointer to a context object (cannot be NULL)
* In/Out: pubkey: pointer to a 65-byte array to put the decompressed public key. * In: pubkeyin: pointer to a 33-byte or 65-byte public key (cannot be NULL)
* It must contain a 33-byte or 65-byte public key already (cannot be NULL) * In/Out: pubkeyout: pointer to a 65-byte array to put the decompressed public key (cannot be NULL)
* pubkeylen: pointer to the size of the public key pointed to by pubkey (cannot be NULL) * May alias pubkeyin.
* It will be updated to reflect the new size. * pubkeylen: pointer to the size of the public key pointed to by pubkeyin (cannot be NULL)
* Returns: 0: pubkey was invalid * It will be updated to reflect the size of the public key in pubkeyout.
* 1: pubkey was valid, and was replaced with its decompressed version * Returns: 0: pubkeyin was invalid
* 1: pubkeyin was valid, and pubkeyout is its decompressed version
*/ */
SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_decompress( SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_decompress(
const secp256k1_context_t* ctx, const secp256k1_context_t* ctx,
unsigned char *pubkey, const unsigned char *pubkeyin,
unsigned char *pubkeyout,
int *pubkeylen int *pubkeylen
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3); ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
/** Export a private key in DER format. /** Export a private key in DER format.
* In: ctx: pointer to a context object, initialized for signing (cannot be NULL) * In: ctx: pointer to a context object, initialized for signing (cannot be NULL)

9
src/secp256k1.c
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@ -271,15 +271,16 @@ int secp256k1_ec_pubkey_create(const secp256k1_context_t* ctx, unsigned char *pu
return ret; return ret;
} }
int secp256k1_ec_pubkey_decompress(const secp256k1_context_t* ctx, unsigned char *pubkey, int *pubkeylen) { int secp256k1_ec_pubkey_decompress(const secp256k1_context_t* ctx, const unsigned char *pubkeyin, unsigned char *pubkeyout, int *pubkeylen) {
secp256k1_ge_t p; secp256k1_ge_t p;
int ret = 0; int ret = 0;
DEBUG_CHECK(pubkey != NULL); DEBUG_CHECK(pubkeyin != NULL);
DEBUG_CHECK(pubkeyout != NULL);
DEBUG_CHECK(pubkeylen != NULL); DEBUG_CHECK(pubkeylen != NULL);
(void)ctx; (void)ctx;
if (secp256k1_eckey_pubkey_parse(&p, pubkey, *pubkeylen)) { if (secp256k1_eckey_pubkey_parse(&p, pubkeyin, *pubkeylen)) {
ret = secp256k1_eckey_pubkey_serialize(&p, pubkey, pubkeylen, 0); ret = secp256k1_eckey_pubkey_serialize(&p, pubkeyout, pubkeylen, 0);
} }
return ret; return ret;
} }

14
src/tests.c
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@ -1511,7 +1511,19 @@ void test_ecdsa_end_to_end(void) {
CHECK(secp256k1_ec_seckey_verify(ctx, privkey) == 1); CHECK(secp256k1_ec_seckey_verify(ctx, privkey) == 1);
CHECK(secp256k1_ec_pubkey_create(ctx, pubkey, &pubkeylen, privkey, (secp256k1_rand32() & 3) != 0) == 1); CHECK(secp256k1_ec_pubkey_create(ctx, pubkey, &pubkeylen, privkey, (secp256k1_rand32() & 3) != 0) == 1);
if (secp256k1_rand32() & 1) { if (secp256k1_rand32() & 1) {
CHECK(secp256k1_ec_pubkey_decompress(ctx, pubkey, &pubkeylen)); unsigned char pubkey2[65] = {0};
int pubkey2len = pubkeylen;
/* Decompress into a new array */
CHECK(secp256k1_ec_pubkey_decompress(ctx, pubkey, pubkey2, &pubkey2len));
/* Check that the key was changed iff it was originally compressed */
if (pubkeylen == 65) {
CHECK(memcmp(pubkey, pubkey2, 65) == 0);
} else {
CHECK(memcmp(pubkey, pubkey2, 65) != 0);
}
/* Decompress in place */
CHECK(secp256k1_ec_pubkey_decompress(ctx, pubkey, pubkey, &pubkeylen));
CHECK(memcmp(pubkey, pubkey2, 65) == 0);
} }
CHECK(secp256k1_ec_pubkey_verify(ctx, pubkey, pubkeylen)); CHECK(secp256k1_ec_pubkey_verify(ctx, pubkey, pubkeylen));