Protocol Demo P-256 ECDH → MAC Comparison Part 3 of 4 · Server Verification

DID Auth: server

Server side: recomputes the MAC from its own private key and compares it with the MAC sent by the client.

For explaining a protocol only. Do not use this in a real service. The keys shown are public demo keys.

Overview & Cryptographic Context

This workflow is Part 3 of the four-part DID Auth learning suite (Key Pairs → Client → Server → End-to-End Demo). Upon receiving the client's 65-byte P-256 public key and 8-byte response Received MAC, the server performs ECDH using its own 32-byte Server Private Key and the Client Public Key.

By the commutativity of scalar multiplication on elliptic curves ([d_server]Q_client = [d_server · d_client]G = [d_client]Q_server), the server derives the exact same 32-byte shared secret, splits SHA-256(Z) into the same 16-byte AES key and 16-byte IV, encrypts its 32-byte challenge Nonce, slices the expected 8-byte MAC, and compares it against Received MAC using a Value Comparator.

Pipeline Architecture & Wiring Flow

ECDH (P-256) ──► SHA-256 → Key / IV ──► AES-128-CBC → Expected MAC (8B) ──► Value Comparator (== Received MAC) ──► Verified (true)

Shared Secret Reconstruction (ECDH Key Agreement): Computes the 32-byte P-256 shared secret from Server Private Key and Client Public Key.
Expected MAC Recomputation (SHA-256 → Byte Slicer × 2 → AES-128-CBC → Byte Slicer): Reconstructs the 8-byte MAC 39b415c1dfeb8f52 over the 32-byte challenge Nonce.
Constant-Time Verdict (Value Comparator): Compares the recomputed 8-byte slice (a) with Received MAC (b) using operator == and outputs Verified = true.

Nodes & Configuration

Node	Node Role & Key Parameters	Count
Workflow Input	Supplies `Server Private Key`, `Client Public Key`, `Nonce`, and `Received MAC`	1
ECDH Key Agreement	`curve = "p256"`, `format = "raw_x"`	1
Cryptographic Hash	`algorithm = "SHA-256"`	1
Byte Slicer	Three instances: `Key` (`0, 16`), `IV` (`16, 16`), and `MAC` (`-16, 8`)	3
AES Encrypt	Labeled `AES-128-CBC`; configured with `mode = "CBC"`, `padding = "None"`	1
Value Comparator	Labeled `MAC matches`; configured with `operator = "=="`	1
Workflow Output	Emits boolean verdict `Verified`	1

Node reference →

Inputs, Outputs & Test Vector Verification

Verification Source: independent implementation: Node crypto (reference/did-auth/did-auth.mjs), no published vector

Inputs

Name	Type	Value (Hex)
`Server Private Key`	`bytes` (32 B)	`a4b87f7821ada85b4e7f63618924b4c3f3bbb78cddcab485a7ee6f15fdfc7261`
`Client Public Key`	`bytes` (65 B)	`0415dc193b8e2a688d603303e85ec8fcc15dcba44ef2156a128c386b66b4f5e1d2188e7ca9d0385c02e72c6899f2771e86bb017a2d2903d89a2892c2199aec8f4d`
`Nonce`	`bytes` (32 B)	`000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f`
`Received MAC`	`bytes` (8 B)	`39b415c1dfeb8f52`

Expected Outputs

Name	Type	Value
`Verified`	`bool`	`true`