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EUC-EO: Cryptographic Protocols (SSL/TLS, IPSec, SSH, S/MIME, PGP)

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Cryptographic Protocols (SSL/TLS, IPSec, SSH, S/MIME, PGP)

Protocols combine primitives into a conversation that survives an active attacker. This domain tests the TLS handshake across versions, IPSec modes, the SSH layers, and the secure-messaging choices S/MIME and PGP.

Core rules

  • TLS: the handshake authenticates the server (optionally the client) with certificates, agrees a shared secret and derives symmetric traffic keys. TLS 1.2 allows RSA key transport or (EC)DHE; TLS 1.3 removed RSA key transport, static DH and CBC suites, requires (EC)DHE for forward secrecy, and completes in one round trip.
  • TLS 1.3 cipher suites name only AEAD and hash, for example TLS_AES_128_GCM_SHA256; key exchange is negotiated separately.
  • IPSec: AH authenticates the whole packet including the IP header but does not encrypt; ESP encrypts and authenticates the payload. Transport mode protects host-to-host payloads; tunnel mode wraps the entire original packet in a new IP header, as in site-to-site VPNs. IKE negotiates security associations.
  • SSH has three layers: transport (key exchange, host key, encryption), user authentication (publickey, password, keyboard-interactive) and connection (channels, port forwarding).
  • S/MIME uses X.509 certificates and dominates corporate mail; PGP uses a decentralised web of trust. Both sign and encrypt, and neither hides metadata such as sender, recipient and subject.

Worked example

A client connecting over TLS 1.3 sends a ClientHello with a key_share and supported cipher suites. The server replies with its certificate, its own key_share and a Finished message; both sides derive handshake and application keys from the ECDHE secret. The client verifies the certificate chain, then sends its Finished and application data. Because the ECDHE secret is ephemeral, later compromise of the server's private key cannot decrypt the recorded session, which is forward secrecy.

Traps

Downgrade to SSLv3, TLS 1.0 or export ciphers breaks the protocol. STARTTLS is opportunistic and can be stripped. Self-signed certificates in production defeat authentication. ESP in tunnel mode does not protect the outer IP header.