A digital signature is a cryptographic technique that verifies the authenticity and integrity of a message or document, by using a hash function and an asymmetric encryption algorithm. A hash function is a mathematical function that transforms any input data into a fixed-length output value called a digest, which is unique for each input. An asymmetric encryption algorithm uses two keys: a public key and a private key.
The public key can be shared with anyone, while the private key must be kept secret by the owner. To create a digital signature, the sender first applies a hash function to the plaintext message to generate a digest. Then, the sender encrypts the digest with their private key to produce the digital signature. To verify the digital signature, the receiver decrypts the digital signature with the sender's public key to obtain the digest. Then, the receiver applies the same hash function to the plaintext message to generate another digest. If the two digests match, it means that the message has not been altered and that it came from the sender. The security of a digital signature depends on the secrecy of the sender's private key. If an attacker obtains the sender's private key, they can create fake digital signatures for any message they want, thus compromising the control provided by the digital signature. Reversing the hash function using the digest is not possible, as hash functions are designed to be one-way functions that cannot be inverted. Altering the plaintext message will result in a different digest after applying the hash function, which will not match with the decrypted digest from the digital signature, thus invalidating the digital signature. Deciphering the receiver's public key is not relevant, as public keys are meant to be publicly available and do not affect the security of digital signatures.

If an auditor implemented a control, they would later be reviewing their own work, which creates a self- review threat and compromises independence. Participating in the project without operational responsibilities (B) or providing advice (C) is acceptable as long as the auditor does not take ownership of decisions.
Designing audit modules (D) is also permissible since they are for audit use and do not affect operational processes. ISACA's Code of Professional Ethics and IS Audit Standards emphasize independence and objectivity as fundamental requirements to maintain credibility and avoid conflicts of interest.
References (ISACA): ISACA Standards for IS Audit and Assurance; ISACA Code of Professional Ethics.

The most effective control to mitigate unintentional misuse of authorized access is security awareness training. This is because security awareness training can educate users on the proper use of their access rights, the potential consequences of misuse, and the best practices to protect the confidentiality, integrity, and availability of information systems. Security awareness training can also help users recognize and avoid common threats such as phishing, malware, and social engineering.
Annual sign-off of acceptable use policy, regular monitoring of user access logs, and formalized disciplinary action are not the most effective controls to mitigate unintentional misuse of authorized access. These controls may help deter or detect intentional misuse, but they do not address the root cause of unintentional misuse, which is often a lack of knowledge or awareness of security policies and procedures.