Question

Exercises 28-55 are problems or shortanswer questions. Describe how Claire would send a message to David using public-key encryption.

Short answer

Claire encrypts the message with David's public key, then sends it securely to David, who decrypts it with his private key.

Step-by-step solution

Understanding Public-Key Encryption

Public-key encryption involves using two keys: a public key, which can be shared with everyone, and a private key, which is kept secret. Claire would use David's public key to encrypt her message. This ensures that only David can decrypt it using his private key.

Obtaining David's Public Key

Claire must first obtain David's public key. Public keys are typically distributed through a trusted directory or directly provided by the owner. Claire ensures this key is authentic and has not been tampered with by checking a digital certificate if available.

Encrypting the Message

Once Claire has David's public key, she encrypts her message using this key. In technical terms, she takes her plaintext message and applies David's public key to transform it into ciphertext. This ciphertext is secure and unreadable to anyone who does not have the corresponding private key.

Sending the Encrypted Message

After encrypting the message, Claire sends the ciphertext to David through a communication channel like email or a messaging service. Since it's encrypted with David's public key, only David can decrypt it using his private key, ensuring the message's confidentiality.

Decryption by David

When David receives the encrypted message, he uses his private key to decrypt it. The private key is mathematically linked to the public key, enabling David to convert the ciphertext back into the original plaintext message Claire wrote.

Key concepts

Key Distribution

In the realm of public-key encryption, key distribution plays a crucial role in ensuring secure communication. The process involves sharing the public key, which anyone can access, while keeping the corresponding private key confidential.

Here's how it generally works:

• Public keys are often shared through trusted sources, such as a secure directory.
• The person receiving the key must verify its authenticity to prevent interception by malicious third parties.
• Verification can include digital certificates, ensuring that the key belongs to the intended receiver.

Before sending an encrypted message, Claire needs to obtain David's public key. This guarantees that only David will be able to decrypt the message once it is sent. Key distribution mechanisms ensure the safe exchange of these keys, preventing unauthorized access or tampering.

Message Encryption

Message encryption is a core aspect of secure communication in public-key encryption systems. Once Claire obtains David’s public key, she must encrypt her message to protect its contents from being read by unintended recipients.

Encryption transforms the message into an unreadable format known as ciphertext. This process is crucial because:

• It ensures the message content remains confidential until it reaches the intended receiver.
• Only the person with the corresponding private key can decrypt and read the message.
• Any interception by unauthorized parties leaves them with gibberish, preserving the message's secrecy.

By encrypting the message using David's public key, Claire ensures only David, with his private key, can decrypt and read it. This approach underpins the confidentiality and security inherent in public-key encryption.

Confidentiality

Confidentiality ensures that sensitive information is accessible only to those with the proper authorization. Public-key encryption provides a robust framework for maintaining confidentiality between communicators.

Here's how confidentiality is maintained:

• Messages are encrypted with the recipient's public key, making sure only the corresponding private key can decrypt them.
• This method ensures that even if the messages are intercepted during transmission, the information remains secret.
• The sender and receiver can trust that their communication is private, and information is safe from eavesdroppers.

When Claire encrypts her message with David's public key, confidentiality is assured. This system prevents any unauthorized parties from understanding the message during its journey to David.

Decryption Process

The decryption process is the final step in the journey of an encrypted message. Using his private key, David can convert the ciphertext back into its original, readable format.

Let's explore how decryption works:

• The private key is mathematically linked to the public key, making it the only way to recover the original message from the ciphertext.
• Upon receiving the encrypted message from Claire, David uses his private key to turn the ciphertext into plaintext.
• This process allows David to read Claire's message exactly as she intended, completing the secure communication loop.

Only David can perform this decryption because only he possesses his unique private key. This vital step underscores the importance of securely maintaining private keys to safeguard message integrity and confidentiality.