Applications of Cryptography
This domain tests how primitives are assembled into systems: PKI and certificates, TLS, secure email, code signing, SSH/IPsec and disk encryption. The exam favours questions about what a component actually proves, and what it does not.
PKI and certificates
A certificate authority (CA) signs an X.509 certificate that binds a subject identity to a public key. Browsers and operating systems trust a set of root CAs; intermediates form a chain. Revocation is handled by CRLs or OCSP. A certificate does not encrypt anything by itself; it distributes a public key and asserts an identity. Self-signed certificates are not trusted unless explicitly installed.
TLS and secure channels
In a TLS handshake, the server proves possession of its private key and presents a certificate chain. The client validates the chain, hostname, validity dates, key usage and revocation status. Ephemeral Diffie-Hellman (DHE/ECDHE) provides forward secrecy; the handshake then derives symmetric keys and uses an AEAD cipher such as AES-GCM or ChaCha20-Poly1305 for records. TLS protects data in transit and authenticates the server, but it does not by itself provide non-repudiation.
Other applications
- S/MIME and PGP sign and/or encrypt email; signing and encryption are separate operations.
- Code signing binds a publisher identity to a hash of software.
- SSH uses host keys and user keys; IPsec protects network traffic at layer 3.
- Full-disk encryption (BitLocker, LUKS) protects data at rest, not a running system.
- Blockchain relies on hashes and digital signatures; it is not “encrypted” data storage.
Worked example
A client connects to bank.example. The server sends its certificate and a signature over the handshake transcript. The client checks that the certificate chains to a trusted root, that the name matches bank.example, that it is inside its validity period and not revoked. It verifies the signature with the server's public key. If all checks pass, both sides derive the same ECDHE secret; if any check fails, the client aborts. The certificate authenticates the server; ECDHE establishes fresh keys.
Traps
- Believing a certificate encrypts traffic; the session keys do.
- Assuming HTTPS proves a site is honest; it proves control of the named domain.
- Forgetting that signing does not provide confidentiality.
- Confusing CRL (list) with OCSP (real-time status).
- Treating TLS as non-repudiation.