How to verify a signature programmatically

To learn how to verify signatures programmatically, let’s develop a small ec-verify program that can verify signatures produced by ec-sign or openssl pkeyutl.

Our program will take three command-line arguments:

1. The name of the input file containing the signed data
2. The name of the file containing the signature
3. The name of the file containing the verifying public key

Here is our high-level implementation plan for the program:

1. Load the signature.
2. Load the verifying public key.
3. Create an EVP_MD_CTX object that will be used as the verifying context.
4. Initialize the verifying context with the hash function’s name and the loaded public key.
5. Read the signed data chunk by chunk and feed it to the verifying context.
6. Finalize the verification and find out whether the verification has succeeded or failed.

Now it’s time to implement our plan.

Implementing the ec-verify...