Question

Alice was a heavy user of a type 1 anonymous remailer. She would post many messages to her favorite newsgroup, alt fanclub alice, and everyone would know they all came from Alice because they all bore the same pseudonym. Assuming that the remailer worked correctly, Trudy could not impersonate Alice. After type 1 remailers were all shut down, Alice switched to a cypherpunk remailer and started a new thread in her newsgroup. Devise a way for her to prevent Trudy from posting new messages to the newsgroup, impersonating Alice.

Short answer

Alice can use a cypherpunk remailer with public-key cryptography, signing her messages with a private key to prevent impersonation by Trudy.

Step-by-step solution

Introduction to Remailers

Alice was using a type 1 anonymous remailer, which allowed her to post messages under a pseudonym. The remailer anonymizes the message, meaning it doesn't retain identifying information about the sender. After these remailers were shut down, Alice began using a cypherpunk remailer, which provides a different method of anonymity.

Understanding Cypherpunk Remailers

Cypherpunk remailers use public-key cryptography to ensure that messages can't be traced back to the sender and can authenticate the identity of the pseudonym used. This cryptographic method prevents impersonation by allowing messages to be verified.

Setting Up a Public-Private Key Pair

Alice needs to generate a public-private key pair. The private key will be used to sign her messages before posting them to the newsgroup. The public key can be distributed to the newsgroup so others can verify that messages indeed come from Alice.

Signing Messages

Before sending a message to the newsgroup, Alice will sign her message with her private key. This involves creating a digital signature that is unique to both her private key and the message contents.

Verifying Message Authenticity

Recipients in Alice's newsgroup will use her public key to verify the digital signature of each message. This process ensures that the message was indeed signed by Alice's private key and hasn't been tampered with.

Preventing Trudy's Impersonation

Since only Alice has access to her private key, Trudy cannot impersonate Alice, because Trudy wouldn't be able to create a valid signature. Any message not signed with Alice's private key will not be recognized as legitimate by the newsgroup participants.

Key concepts

Anonymous Remailer

An anonymous remailer is a service that receives email messages from users and forwards them to the intended destination, removing any identifying information in the process. These services provide anonymity by stripping away metadata such as the sender's IP address. By using a pseudonym, users can engage in discussions or actions without revealing their true identity.

Originally, Alice used a type 1 anonymous remailer which allowed her to maintain anonymity while communicating. This type of remailer replaces Alice's real email address with a pseudonym, ensuring her identity remains shielded. However, the problem with type 1 remailers is that they use consistent pseudonyms, making it evident to others that messages under that pseudonym originate from the same user, without revealing who that user is in reality. This at least maintains consistency but not secrecy.

When type 1 remailers ceased operation, Alice switched to a cypherpunk remailer. These remailers offer enhanced privacy by employing cryptographic techniques that not only protect the sender's identity but also improve reliability in communication. Unlike type 1 remailers, cypherpunk remailers can better guard against attempts by outside parties to link a pseudonym with the real identity of the author.

Digital Signature

Digital signatures are a vital component of public-key cryptography, used to verify the authenticity and integrity of a message or document. Before sending her messages to the newsgroup, Alice signs them with her private key. This signature is essentially a cryptographic hash generated from the message content and Alice's private key.

The process works like this:

• Alice writes her message and uses a hashing algorithm to produce a unique digest of the message.
• This digest is encrypted with her private key, creating a digital signature.
• The signed message is sent to the newsgroup along with the digital signature.

When other users in the newsgroup receive the message, they utilize Alice's public key to decrypt the digital signature and verify that the hash matches the message content. If it does, they can be confident that the message was not altered and indeed came from Alice.

By implementing digital signatures, Alice prevents others like Trudy from posing as her since only Alice's private key can create a valid signature. This method secures the identity of the sender while maintaining message integrity.

Message Authentication

Message authentication is crucial in ensuring that a message is legitimate and hasn't been tampered with. In public-key cryptography, message authentication primarily relies on digital signatures and the use of public and private keys.

Alice's cypherpunk remailer system adds a layer of security by requiring each message to be signed with her private key. On receiving a message:

• Recipients use Alice's public key to verify the digital signature attached to the message. This verification confirms the message's source and ensures it's unaltered.
• If the signature matches the message, it assures the recipients that Alice is indeed the sender.
• If not, the message is likely a forgery or has been modified, and it should be treated with suspicion.

This authentication process effectively prevents malicious actors, like Trudy, from sending messages purportedly from Alice. Since Trudy does not have access to Alice's private key, she cannot forge the digital signature needed for message authentication. This protective measure upholds the newsgroup's trust in the legitimacy of Alice's communications.