Question 189 - CISA discussion

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.