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KEM is a cleaner way to think about public-key encryption for key exchange. Traditional public-key encryption encrypts arbitrary messages. KEM is more focused: the sender generates a random symmetric key, encapsulates it (encrypts it) using the recipient's public key, and sends the ciphertext. The recipient decapsulates (decrypts) it using their private key to get the same symmetric key. Then both parties use that symmetric key for actual data encryption with AES or similar. Why is this better? Simpler security proofs, cleaner API design, better performance in some cases. More importantly, NIST designed the post-quantum competition around KEMs because they're the right primitive for key exchange. CRYSTALS-Kyber (ML-KEM, FIPS 203) is a KEM. In TLS 1.3, you use a KEM to establish session keys. In VPNs, a KEM sets up tunnel encryption. It's the post-quantum replacement for RSA-OAEP and ECDH key exchange.
KEMs simplify protocol design. Instead of worrying about padding schemes and chosen ciphertext attacks, you have one operation: encapsulate/decapsulate a random key. All NIST PQC encryption standards use the KEM interface, making them interchangeable in protocols - you can try Kyber, fall back to Classic McEliece, or run both in hybrid mode.
Every TLS connection, every VPN tunnel, every secure messaging session needs key exchange. KEMs provide the post-quantum mechanism for this. They're fast enough for real-time applications, small enough for resource-constrained devices (especially Kyber), and secure enough to protect data for decades.
TLS 1.3 session key establishment replacing ECDH, VPN tunnel setup and key negotiation, secure messaging protocols key exchange, SSH connection key establishment, encrypted email key distribution, hybrid classical-quantum key exchange
https://www.qnulabs.com/qnu-labs-hybrid-approach-blending-quantum-and-post-quantum-cryptography-for-future-proof-security/
KEM, key encapsulation mechanism, CRYSTALS-Kyber, ML-KEM, FIPS 203, post-quantum key exchange, key establishment, TLS 1.3, VPN key exchange, public key encryption, shared secret generation, NIST PQC