Question

Alice wants to communicate with Bob, using public-key cryptography. She establishes a connection to someone she hopes is Bob. She asks him for his public key and he sends it to her in plaintext along with an X.509 certificate signed by the root CA. Alice already has the public key of the root CA. What steps does Alice carry out to verify that she is talking to Bob? Assume that Bob does not care who he is talking to (e.g., Bob is some kind of public service).

Short answer

Alice verifies the certificate signature with the root CA's public key, checks the certificate's validity, and matches the certificate details against her expectations of Bob.

Step-by-step solution

Verify Certificate Signature

Alice uses the public key of the root CA that she already possesses to verify the signature on the X.509 certificate. This involves checking the digital signature present on the certificate using the CA's public key to ensure that the certificate is indeed issued by the root CA.

Check Certificate Validity

Alice needs to ensure the certificate is still valid, which involves checking the validity dates within the certificate. She will confirm the current date falls within that range and that the certificate has not been revoked.

Match Details Against Expectations

Alice verifies that the subject details within the X.509 certificate, such as the common name (CN) or subject alternative name (SAN), match her expectations of Bob (for instance, a specific URL or domain name). This ensures the certificate is intended for Bob.

Key concepts

X.509 Certificate

The X.509 certificate is like a digital passport used in public-key cryptography. This electronic document helps assure Alice that she's genuinely connecting with Bob. Every X.509 certificate involves several essential components:

• Public Key: The key is used to encrypt messages or verify digital signatures.
• Information about the Entity: Details like name, organization, and domain identity (for websites).
• Issuer Information: Information about the Certificate Authority that issued the certificate.
• Validity Period: The "from" and "to" dates during which the certificate is considered valid.

By verifying these details, Alice can trust she’s communicating with the right person or service, preventing potential "man-in-the-middle" attacks. It's critical that Alice checks the X.509 certificate's features to ensure Bob is indeed the sender.

Root Certificate Authority (CA)

A Root Certificate Authority (CA) is like the parental guardian of digital certificates. When an X.509 certificate is issued, it often comes with a hierarchical chain of trust. Root CAs are at the top of this hierarchy and are trusted entities that issue security certificates, akin to a notary stamping an important document. Root CAs perform the following vital functions:

• Issuing Certificates: They sign and distribute digital certificates.
• Maintaining Trust: By being independently audited and maintaining strict security policies, Root CAs uphold the trustworthiness of issued certificates.
• Revocation Lists: They keep lists of certificates that are no longer valid due to compromise or expiration.

Alice, having the root CA's public key, needs it to verify that the certificate sent by "Bob" is both issued by a trusted entity and valid.

Digital Signature Verification

Digital signatures are like the unique seal of approval on a document. They ensure that a message hasn't been altered since the signature was applied, and they verify who sent the message. In Alice's case, after receiving Bob's X.509 certificate, she must verify the digital signature:

• Signature Validation: Using her pre-existing knowledge of the root CA’s public key, Alice checks the signature's legitimacy on the certificate to ensure it has not been tampered with.
• Hash Function: Digital signatures use a hash function to create a digest of the message, ensuring its integrity.

For Alice, the successful validation of the digital signature reassures her that Bob's certificate is indeed authentic and hasn't been compromised by any potential threats.