Hybrid KEM Encapsulate

Category: Post-Quantum Cryptography · Node Type ID: pqc-hybridkem-encaps

Encapsulates shared secret using hybrid X25519 + ML-KEM-768 recipient public key

Overview

Performs hybrid key encapsulation against a recipient's 1,216-byte X25519 + ML-KEM-768 public key: generates an ephemeral X25519 keypair to compute the X25519 ECDH shared secret, encapsulates an ML-KEM-768 shared secret, and combines both shared secrets via SHA-256 / HKDF into a single 32-byte Shared Secret.

Emits the 32-byte combined Shared Secret alongside the 1,120-byte Hybrid Ciphertext (32B ephemeral X25519 public key || 1,088B ML-KEM-768 ciphertext).

When to Use

Encapsulating a quantum-resilient 32-byte session key for a recipient holding a hybrid secret key from Hybrid KEM KeyGen.

Parameter Configuration

Hybrid KEM Encapsulate has no configurable parameters.

Port I/O Specification

See Ports & Connections for general wiring rules, data type colors, and dynamic port creation.

Input Ports

Port Nature: Fixed Input Ports (1 total).

Port Name	Port ID	Data Type	Required	Visibility Condition	Description
`Recipient Public Key`	`publicKey`	`Bytes`	Required	Always visible	Recipient's 1,216-byte `Hybrid Public Key`.

Output Ports

Port Nature: Fixed Output Ports (2 total).

Port Name	Port ID	Data Type	Visibility Condition	Description
`Shared Secret`	`sharedSecret`	`Bytes` (Combined)	Always visible	Combined 32-byte shared secret derived from both X25519 and ML-KEM-768 components.
`Hybrid Ciphertext`	`ciphertext`	`Bytes`	Always visible	1,120-byte composite ciphertext (32B X25519 ephemeral public key \|\| 1,088B ML-KEM-768 ciphertext).

Error Cases

When any of the following conditions occurs at runtime, this node transitions to an ERROR state and downstream nodes connected to its outputs do not execute.

Error Code	Default Engine Message	Trigger Condition
`INVALID_KEY_MATERIAL`	Invalid key length or format	Raised when `Recipient Public Key` is not 1,216 bytes or contains an invalid ML-KEM-768 / X25519 public key component.

Examples

Hybrid Key Encapsulation for AES-256-GCM

Encapsulate a 32-byte hybrid key and use it to encrypt a message.

Connect Hybrid Public Key from Hybrid KEM KeyGen to Recipient Public Key.
Wire Shared Secret to AES Encrypt and Hybrid Ciphertext to Hybrid KEM Decapsulate.