Implement ECDSA verify in multichain about polling-station-app HOT 6 CLOSED

secp256k1_ecdsa_verify() (defined in include/secp256k1.h, line 381) is the verify function, see the comment in the header file.

The function secp256k1_ecdsa_verify() is used in keys/pubkey.cpp, secp256k1/src/bench_verify.c and a test file. In keys/pubkey.cpp the function CPubKey::Verify uses the secp256k1 verify function. This might be altered to a brainpool verify function.

The Verify function of the class CPubKey is used in the class TransactionSignatureChecker in function VerifySignature in the file script/interpreter.cpp, which are thus reasons to believe that this is indeed correct. Notice that the Verify function is also used in structs/alert.cpp, protocol/handshake.cpp, protocol/multichainblock.cpp, core/init.cpp and keys/key.cpp.

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A public key is stored in the class CPubKey. This class is defined in pubkey.h and implemented in pubkey.cpp. The class, contains a private variable called vch, which represents the public key and is a array of chars of length 65. This must be altered to at least 80 bytes for passports produced after 9 March 2014, and might even needs to be bigger for passports prior 9 March 2014. Note that the passport produced prior 9 March 2014 might use a different curve than BrainpoolP320r1, see this, page 11.

The static method GetLen() calculates the length of the public key by looking at the first byte. In sep256k1 curves, public keys can have different lengths. We need to alter GetLen() so it returns the brainpoolP320r1 public key size. Don't know if we can look at first byte and determine the size of the public key.

The Verify() function expects two arguments: a hash and a DER signature. The hash is probably a sha256 hash. In a comment double SHA-256 is mentioned.

Besides this, we might need to alter key.c, which holds a private key. This because of the genesis block?

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Small script to verify a passport active authentication signature using crypto++:

https://gist.github.com/landgenoot/6f9a58876ea0f03a48b9485159ad7869

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Seems that vchDefaultKey, in the file wallet/wallet.cpp line 3435, is not fully valid in pubkey-use-brainpool branch, opposite in what happens in the master branch where vchDefaultKey is fully valid. Looking in to this now

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Signing and verifying now works with brainpoolP320r1. The hash to be signed is also split up in four different parts and are signed/verified separate. Next step is to clean up the code and create a transaction from a passport.

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See pr digital-voting-pass/e-passport-chain#2

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