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Security Fundamentals & Crypto · Advanced

Hybrid Key Exchange

Combining a classical and a post-quantum key exchange so the session is secure if either algorithm holds.

Definition

Hybrid key exchange concatenates or combines shared secrets from algorithms such as X25519 and ML-KEM-768. It protects against harvest-now-decrypt-later while hedging against undiscovered weaknesses in new PQC algorithms. X25519MLKEM768 is the default in major browsers and is OpenSSH's default key exchange as of version 10.

Example

Cloudflare reports that a large share of its TLS 1.3 traffic now uses X25519MLKEM768.

Exam tip

Hybrid = classical + PQC together for defense in depth during migration.

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