Verify compressed argument in secp256k1_eckey_pubkey_serialize #1642

1. 
   theStack commented at 5:28 pm on December 2, 2024: contributor

   Due to similarity to the public API function secp256k1_ec_pubkey_serialize, public API flags like SECP256K1_EC_COMPRESSED are sometimes mistakingly passed to secp256k1_eckey_pubkey_serialize in newly proposed code (this is currently the case for several modules in secp256k1-zkp, see https://github.com/BlockstreamResearch/secp256k1-zkp/pull/300), which is currently not detected. To avoid this in the future, a VERIFY_CHECK is added to check that the compressed argument is either 0 or 1.
2. Verify `compressed` argument in `secp256k1_eckey_pubkey_serialize`

   Due to similarity to the public API function `secp256k1_ec_pubkey_serialize`,
   public API flags like `SECP256K1_EC_COMPRESSED` are sometimes mistakingly
   passed to newly proposed code (this is currently the case for several modules in
   secp256k1-zkp, see https://github.com/BlockstreamResearch/secp256k1-zkp/pull/300).
   which is currently not detected. To avoid this in the future, a VERIFY_CHECK
   is added to check that the `compressed` argument is either 0 or 1.

   1823594761
3. real-or-random added the label assurance on Dec 2, 2024
4. real-or-random added the label refactor/smell on Dec 2, 2024
5. in src/eckey_impl.h:39 in 1823594761

   35@@ -35,6 +36,8 @@ static int secp256k1_eckey_pubkey_parse(secp256k1_ge *elem, const unsigned char
   36 }
   37 
   38 static int secp256k1_eckey_pubkey_serialize(secp256k1_ge *elem, unsigned char *pub, size_t *size, int compressed) {
   39+    VERIFY_CHECK(compressed == 0 || compressed == 1);
   

   ---

   ---

   

   real-or-random commented at 8:30 am on December 9, 2024:

   In other places, we simply rely on C’s int-as-bool convention, so I suggest adding a comment that explains why we enforce 0/1 in this case.
6. real-or-random approved
7. 
   real-or-random commented at 8:30 am on December 9, 2024: contributor

   utACK 1823594761046ae38e002781d65e9538dd211d27

Contributors
theStack real-or-random

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assurance refactor/smell