VaultBridge.Shamir 1.0.0

VaultBridge.Shamir

A .NET implementation of Shamir's Secret Sharing over GF(256), ported from HashiCorp Vault's shamir package.

Byte-for-byte interoperable with shamir-secret-sharing (by Privy) for TypeScript/JavaScript.

License

The core Shamir implementation (GaloisField.cs, ShamirSecretSharing.cs, Share.cs) is ported from HashiCorp Vault, originally by IBM Corporation, and is licensed under the Mozilla Public License 2.0.

MPL-2.0 is file-level copyleft: the ported source files retain their MPL-2.0 license, but the package can be freely used in projects under any license.

Installation

dotnet add package VaultBridge.Shamir

Usage

Split a secret into shares

using VaultBridge.Shamir;

byte[] secret = GetMySecret(); // any byte array

// Create 5 shares, requiring any 3 to reconstruct
Share[] shares = ShamirSecretSharing.Split(secret, shares: 5, threshold: 3);

// Serialize for storage or transmission
string[] encoded = shares.Select(s => s.ToBase64()).ToArray();

Reconstruct from shares

// Deserialize
Share[] shares = encoded.Select(Share.FromBase64).ToArray();

// Any 3 (or more) of the 5 shares will recover the original secret
byte[] recovered = ShamirSecretSharing.Combine(shares);

Cross-language interop

Shares produced by this library are byte-for-byte compatible with Privy's TypeScript implementation. Each share is secretLength + 1 bytes: the y-values for each byte of the secret, followed by the x-coordinate as the final byte.

// TypeScript — reconstruct shares produced by VaultBridge.Shamir
import { combine } from 'shamir-secret-sharing';

const shares = encodedShares.map(s => Uint8Array.from(atob(s), c => c.charCodeAt(0)));
const secret = await combine(shares);

How it works

The implementation operates over GF(2^8) — the Galois field of 256 elements using the Rijndael (AES) irreducible polynomial x^8 + x^4 + x^3 + x + 1 (reduction constant 0x1B).

For each byte of the secret, a random polynomial of degree threshold - 1 is constructed with that byte as the intercept (constant term). The polynomial is evaluated at threshold or more unique x-coordinates (1–255) to produce shares. Reconstruction uses Lagrange interpolation at x = 0 to recover each byte.

Key properties:

• Information-theoretic security: Fewer than threshold shares reveal zero information about the secret
• No dependencies: Pure .NET, zero external packages
• Constant-time operations: Division uses constant-time selection to prevent timing side-channels

Attribution

This is a .NET port of the Shamir's Secret Sharing implementation from HashiCorp Vault, originally authored by IBM Corporation and licensed under MPL-2.0.