X25519 key agreement

Overview & Cryptographic Context

X25519 (specified in IETF RFC 7748) is an elliptic-curve Diffie-Hellman (ECDH) function built on the Montgomery curve Curve25519 (y² = x³ + 486662x² + x over the prime field 2²⁵⁵ - 19). Designed for constant-time Montgomery ladder execution and immunity to twist and invalid-curve attacks, X25519 uses compact 32-byte little-endian u-coordinates for public keys and 32-byte clamped scalars for private keys.

This workflow reproduces RFC 7748 §6.1, computing the 32-byte shared secret K = X25519(a, B) from Alice's 32-byte private key a and Bob's 32-byte public key B. In production protocols, this raw shared secret should always be passed through a key derivation function such as HKDF (see X25519 + HKDF).

Pipeline Architecture & Wiring Flow

1. Key Material Ingress (Workflow Input): Supplies Alice's 32-byte PrivateKey and Bob's 32-byte PeerPublicKey.
2. Scalar Multiplication (X25519 Key Exchange (ECDH)): Clamps the private scalar (clearing bits 0, 1, 2, 255 and setting bit 254) and evaluates the Montgomery ladder over GF(2²⁵⁵-19). Rejects low-order points that produce an all-zero shared secret.
3. Shared Secret Egress (Workflow Output): Outputs the 32-byte shared secret K.

Nodes & Configuration

Node reference →

Inputs, Outputs & Test Vector Verification

Verification Source: RFC 7748 §6.1 (Alice private key, Bob public key)

Inputs

Expected Outputs