7 Cryptography in Secure Protocols Free Online FlashCards

What does a hash function provide in a secure protocol?

A hash maps data to a fixed-length digest. By itself, it neither encrypts data nor proves who created it.

What does a certificate establish, and what does it not do?

A certificate connects a public key to identity information through a CA signature. It does not contain the corresponding private key or encrypt application data.

What is the purpose of key agreement?

Key agreement lets parties derive shared secret material over a public channel without sending the secret directly.

Why does a protocol use a key-derivation function?

A KDF turns shared secret material into separate keys of appropriate sizes. HKDF is a widely used example.

What does AEAD protect, and what happens to associated data?

AEAD encrypts data and detects tampering. Associated data is authenticated but remains visible.

What does certificate validation do in a typical TLS 1.3 handshake?

The browser checks the certificate chain and whether the certificate is valid for the requested server name, connecting the server's public key to its claimed identity.

What does TLS CertificateVerify demonstrate?

The server uses its certificate's public key to verify the server's signature over the handshake transcript, proving possession of the corresponding private key and binding that proof to the handshake.

What does the TLS Finished message confirm?

The Finished message confirms that the peer has the same handshake state and derived keys.

What protection can ephemeral Diffie–Hellman provide after a long-term key is compromised?

With ephemeral Diffie–Hellman, later compromise of a long-term certificate-signing key does not by itself reveal previously recorded session traffic, assuming ephemeral secrets were securely erased.

How does X3DH support asynchronous key establishment, and what must users authenticate?

The sender uses the recipient's published identity and prekeys to establish shared secret material asynchronously. The recipient's identity key still needs authentication, such as by comparing fingerprints or scanning a QR code.

How does the Double Ratchet evolve message keys?

The Double Ratchet combines symmetric-key chains with new Diffie–Hellman contributions to derive fresh message keys over time. Ratchet steps can provide forward secrecy and, after fresh secret input, recovery from some compromises.

What happens to MLS group key material after a membership change?

A membership change advances the group to a new epoch with new key material. When MLS is correctly run, removed members are excluded from subsequent epoch secrets.