How Is the ‘Message Digest’ Used in the ECDSA Signing Process?

The message digest is the fixed-length output (hash) of the message (e.g. a transaction or contract) that is to be signed. In ECDSA, the signing algorithm does not operate on the entire message but only on this digest.

The private key is used to generate a signature based on the digest. This process ensures that the signature is compact and that any change to the original message will result in a completely different digest, thus invalidating the signature.