How can Jack ensure the integrity of a file when sending it via email to a new recipient?

To ensure the integrity of a file when sending it via email, digitally signing the file is the most effective approach. A digital signature provides a way to verify that the file has not been altered during transmission. When Jack digitally signs the file, he uses his private key to create a unique hash of the file content, which is then attached as the signature. Upon receipt, the recipient can use Jack's public key to decrypt the signature and compare the hash with a newly generated hash of the received file. If the hashes match, it confirms that the file is intact and has not been tampered with, thereby ensuring its integrity.

Other methods, such as encrypting the file with a password or using a hashing algorithm, do provide security features, but they don't inherently guarantee integrity in the same way. Encryption protects confidentiality but does not confirm whether the file was altered. A hashing algorithm, while useful for generating a fixed representation of file data, must be paired with a mechanism that securely ties it to the sender’s identity to verify integrity effectively. Compressing the file, on the other hand, simply reduces its size for transmission and does not provide any integrity assurance.