What kinds of real-world systems are typical cryptographic protocol applications?

E-voting, biometric passports, e-cash (Bitcoin), DRM on e-readers, signed software updates — each requires multiple algorithms combined into a multi-phase protocol with specific security goals.

For each system, the security review asks the same three questions:

1. What phases does the protocol have? Most non-trivial protocols have setup, operation, and revocation phases — each with different security requirements.
2. Which algorithms are used in each phase? Different parts may need different primitives — e.g. RSA for cert signing, AES for content encryption, HMAC for device binding.
3. Are any necessary primitives missing? Sometimes the protocol assumes an algorithm that doesn't exist or isn't strong enough — that's a research gap (e.g. fully-homomorphic encryption was theoretical for decades before becoming practical).

Tip: When designing a new system, look at the closest existing protocol first. E-voting alone has had 30+ years of academic research (Helios, Civitas, Belenios). Reinventing it from scratch — without a literature survey — almost guarantees you'll repeat known attacks.