Question

Consider the failed attempt of Alice to get Bob's public key in Fig. 8-25. Suppose that Bob and Alice already share a secret key, but Alice still wants Bob's public key. Is there now a way to get it securely? If so, how?

Short answer

Yes, Alice can encrypt a request for Bob's public key using their shared secret key for secure retrieval.

Step-by-step solution

Understand the Problem

Alice and Bob want to establish secure communication, and Alice needs to obtain Bob's public key. They already share a secret key, which is crucial to finding a solution.

Identify the Role of the Secret Key

The shared secret key is a symmetric key that both Alice and Bob can use securely for encryption and decryption purposes. It can be used to safely exchange additional information.

Encrypt the Request Message

Alice can construct a request message asking Bob for his public key. She then encrypts this message using the shared secret key, ensuring that only Bob can decrypt it.

Send the Encrypted Request to Bob

After encrypting the request with the shared secret key, Alice sends this encrypted message to Bob over the communication channel.

Decrypt the Message by Bob

Bob receives the encrypted message from Alice. He uses the shared secret key to decrypt it, revealing Alice's request for his public key.

Sending Bob's Public Key

After decrypting the request, Bob can encrypt his public key using the shared secret key. He then sends the encrypted public key back to Alice.

Decrypt and Obtain the Public Key

Alice decrypts the message containing Bob's public key using the shared secret key. Now, she securely has Bob's public key.

Key concepts

Symmetric Key Encryption

Symmetric key encryption is a method where the same key is used for both encryption and decryption of data. This is an efficient and fast method of encryption because it relies on a single shared secret known as the symmetric key. In our example, Alice and Bob already share a symmetric key.
This key ensures that all communications between Alice and Bob remain secure and only accessible to them. This method is particularly useful for secure communication via email or messaging where the encryption and decryption processes need to be quick. However, the challenge lies in exchanging and maintaining the secrecy of this key, as anyone who retrieves it can eavesdrop on the communications.
Thus, taking proper measures like using secure channels initially to share this key is crucial. In essence, symmetric key encryption simplifies the encryption-decryption process but relies heavily on the security of the key itself.

Secure Communication

Secure communication is about ensuring that the data shared between parties remains confidential and intact. It involves protecting messages from eavesdropping and unauthorized access so that they are only understood by the intended recipient.
In the situation between Alice and Bob, secure communication ensures that their message exchange remains private, even if they are sharing information, such as encryption keys. Using symmetric key encryption, Alice safely wraps her message requesting Bob's public key, which keeps it safe from interception during transmission.
Only the intended receiver, Bob, can decipher the request using the shared secret key. This establishes a secure line through which sensitive information can be exchanged confidently.

Key Sharing Protocols

Key sharing protocols are systems designed to allow parties to securely exchange cryptographic keys. The ultimate goal is to enable the secure transfer of keys which can be used for further secure communications. In our scenario, Alice and Bob began with an existing secret key. Instead of manually exchanging Bob's public key, Alice encrypts her request with this shared key, wraps the message securely, and sends it to Bob.
Once Bob decrypts Alice's message and reads the request, he uses the same key to encrypt his public key before sending it back. This secure exchange showcases a simple protocol where trust is built on a pre-existing shared secret. These protocols are crucial for initiating encrypted communication paths and can be achieved through various methods like Diffie-Hellman or RSA, where keys are exchanged dynamically without revealing the secret.
Each approach has its advantages, but ultimately, the goal is to protect the keys throughout the transmission, establishing a foundation for secure data exchange.