(Why should an asymmetric private key be used to encrypt the digest of an application?

Digital signing of software typically works by hashing the application (or its manifest) and then using the publisher’s private key to create a digital signature over that digest. The private key is used because it is secret and uniquely controlled by the publisher; only the publisher should be able to produce a valid signature. Verifiers (customers) use the publisher’s public key to validate the signature and confirm that the digest matches the software they received. This yields two key properties: integrity (the software hasn’t been altered; any modification changes the digest and breaks verification) and authenticity (the signature proves it came from the private-key holder). Option A incorrectly describes symmetric stream encryption. Option C incorrectly generalizes private-key behavior as “block encryption.” Option D is wrong because verification uses the public key, not a private key; also, “encrypting with private key” in this context is better understood as signing, not confidentiality encryption. Therefore, the correct rationale is that the asymmetric private key is used to sign the file’s digest so the corresponding public key can verify integrity and authenticity.