Pass privkey export DER compression flag correctly #14195

The argument to ec_privkey_export_der is BOOLEAN GetPrivKey call ec_privkey_export_der to use the flag

SECP256K1_EC_COMPRESSED is true SECP256K1_EC_UNCOMPRESSED is true

Is it changing ec_privkey_export_der? Delete another branch that will never be reached

Looks like a bug to me...

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #12461 (scripted-diff: Rename key size consts to be relative to their class by Empact)

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Please make your description and title more descriptive.

@fingera Nice first-time contribution! Out of curiosity, how did you find this? :-)

Related: #10041 and #10043

You found out before, why not fix it?

@practicalswift https://github.com/fingera/fingera_libc/blob/master/tests/btc/key.cpp

@fingera Please re-open the PR: this is still confusing and hence unresolved :-)

utACK (after squash), this clearly fixes a bug, ec_privkey_export_der does not take a bit field (secp256k1_ec_pubkey_serialize is what takes SECP256K1_EC_COMPRESSED=258 and SECP256K1_EC_UNCOMPRESSED=2). I'm surprised none of the tests catches this.

utACK 67e17da34de278c0b0bab0caad0ad877a053b0d6 modulo squash and a more informative commit message :-)

suggested on IRC was, to prevent the bug as well as the conversion warning without using really contorted C++:

diff --git a/src/key.cpp b/src/key.cpp
index df452cd3302ee6aff363b8fc8ea328b69d8bfb55..80d6589a3c36b823402b81d759ff43520037c43a 100644
--- a/src/key.cpp
+++ b/src/key.cpp
@@ -89,7 +89,7 @@ static int ec_privkey_import_der(const secp256k1_context* ctx, unsigned char *ou
  * will be set to the number of bytes used in the buffer.
  * key32 must point to a 32-byte raw private key.
  */
-static int ec_privkey_export_der(const secp256k1_context *ctx, unsigned char *privkey, size_t *privkeylen, const unsigned char *key32, int compressed) {
+static int ec_privkey_export_der(const secp256k1_context *ctx, unsigned char *privkey, size_t *privkeylen, const unsigned char *key32, bool compressed) {
     assert(*privkeylen >= CKey::PRIVATE_KEY_SIZE);
     secp256k1_pubkey pubkey;
     size_t pubkeylen = 0;
@@ -170,7 +170,7 @@ CPrivKey CKey::GetPrivKey() const {
     size_t privkeylen;
     privkey.resize(PRIVATE_KEY_SIZE);
     privkeylen = PRIVATE_KEY_SIZE;
-    ret = ec_privkey_export_der(secp256k1_context_sign, privkey.data(), &privkeylen, begin(), fCompressed ? SECP256K1_EC_COMPRESSED : SECP256K1_EC_UNCOMPRESSED);
+    ret = ec_privkey_export_der(secp256k1_context_sign, privkey.data(), &privkeylen, begin(), fCompressed);
     assert(ret);
     privkey.resize(privkeylen);
     return privkey;

merged via 9a565a8282236f29334a6ec2de6a03358f5ce86e