Implements basic signing and verification functions for the BIP-340 Schnorr Signature Scheme.

It passes the tests attached to the BIP (dart run example/example.dart to run that), but no guarantees are made of anything and this is not safe cryptography, do not use to store Bitcoins.

Provides these functions:

1. String sign(String privateKey, String message, String aux)

Generates a schnorr signature using the BIP-340 scheme.

• privateKey must be 32-bytes hex-encoded, i.e., 64 characters.
• message must also be 32-bytes hex-encoded (a hash of the actual message).
• aux must be 32-bytes random bytes, generated at signature time.
• Returns the signature as a string of 64 bytes hex-encoded, i.e., 128 characters.

2. bool verify(String publicKey, String message, String signature)

Verifies a schnorr signature using the BIP-340 scheme.

• publicKey must be 32-bytes hex-encoded, i.e., 64 characters (if you have a pubkey with 33 bytes just remove the first one).
• message must also be 32-bytes hex-encoded (a hash of the actual message).
• signature must be 64-bytes hex-encoded, i.e., 128 characters.
• Returns true if the signature is valid, false otherwise.

3. String getPublicKey(String privateKey)

Produces the public key from a private key

• privateKey must be a 32-bytes hex-encoded string, i.e. 64 characters.
• Returns a public key as also 32-bytes hex-encoded.

Made for integration with Nostr.