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.

The strategy that would provide the greatest assurance of system quality at implementation is delivering only the core functionality on the initial target date. This strategy can help avoid compromising the quality of the system by focusing on the essential features that meet the user needs and expectations. Delivering only the core functionality can also help reduce the scope creep, complexity, and testing efforts of the system development project.
Implementing overtime pay and bonuses for all development staff, utilizing new system development tools to improve productivity, and recruiting IS staff to expedite system development are not strategies that would provide the greatest assurance of system quality at implementation. These strategies may help speed up the system development process, but they may also introduce new risks or challenges such as burnout, learning curve, integration issues, or communication gaps. These risks or challenges may adversely affect the quality of the system.