git.stella-ops.org/src/__Libraries/StellaOps.Cryptography.Plugin.OfflineVerification/README.md

StellaOps.Cryptography.Plugin.OfflineVerification

Cryptographic provider for offline/air-gapped environments using .NET BCL cryptography.

Overview

The OfflineVerificationCryptoProvider wraps System.Security.Cryptography in the ICryptoProvider abstraction, enabling configuration-driven crypto while maintaining offline verification capabilities without external dependencies.

Supported Algorithms

Signing & Verification

• ECDSA: ES256 (P-256/SHA-256), ES384 (P-384/SHA-384), ES512 (P-521/SHA-512)
• RSA PKCS1: RS256, RS384, RS512
• RSA-PSS: PS256, PS384, PS512

Content Hashing

• SHA-2: SHA-256, SHA-384, SHA-512 (supports both SHA-256 and SHA256 formats)

When to Use

Use OfflineVerificationCryptoProvider when:

1. Offline/Air-Gapped Environments: Systems without network access or external cryptographic services
2. Default Cryptography: Standard NIST-approved algorithms without regional compliance requirements
3. Container Scanning: AirGap module, Scanner module, and other components that need deterministic signing
4. Testing: Local development and testing without hardware security modules

Do NOT use when:

• Regional compliance required (eIDAS, GOST R 34.10, SM2) - use specialized plugins instead
• FIPS 140-2 Level 3+ hardware security required - use HSM plugins
• Key management by external providers - use cloud KMS plugins (AWS KMS, Azure Key Vault, etc.)

Usage

Basic Hashing

using StellaOps.Cryptography;
using StellaOps.Cryptography.Plugin.OfflineVerification;

var provider = new OfflineVerificationCryptoProvider();
var hasher = provider.GetHasher("SHA-256");
var hash = hasher.ComputeHash(data);

Signing with Stored Keys

var provider = new OfflineVerificationCryptoProvider();

// Add key to provider
var signingKey = new CryptoSigningKey(
    reference: new CryptoKeyReference("my-key"),
    algorithmId: "ES256",
    privateParameters: ecParameters,
    createdAt: DateTimeOffset.UtcNow);

provider.UpsertSigningKey(signingKey);

// Get signer and sign data
var signer = provider.GetSigner("ES256", new CryptoKeyReference("my-key"));
var signature = await signer.SignAsync(data);

Ephemeral Verification (New in 1.0)

For scenarios where you only have a public key (e.g., DSSE verification, JWT verification):

var provider = new OfflineVerificationCryptoProvider();

// Public key in SubjectPublicKeyInfo (DER-encoded) format
byte[] publicKeyBytes = ...; // From certificate, JWKS, or inline

// Create ephemeral verifier
var verifier = provider.CreateEphemeralVerifier("PS256", publicKeyBytes);

// Verify signature
var isValid = await verifier.VerifyAsync(message, signature);

Supported Algorithms for Ephemeral Verification:

• ECDSA: ES256, ES384, ES512
• RSA PKCS1: RS256, RS384, RS512
• RSA-PSS: PS256, PS384, PS512

Key Format:

• Public keys must be in SubjectPublicKeyInfo (SPKI) format (DER-encoded)
• This is the standard format used in X.509 certificates, JWKs, and TLS

Dependency Injection

services.AddStellaOpsCryptoFromConfiguration(configuration);

Ensure etc/crypto-plugins-manifest.json includes:

{
  "id": "offline-verification",
  "name": "OfflineVerificationCryptoProvider",
  "assembly": "StellaOps.Cryptography.Plugin.OfflineVerification.dll",
  "type": "StellaOps.Cryptography.Plugin.OfflineVerification.OfflineVerificationCryptoProvider",
  "capabilities": [
    "signing:ES256", "signing:ES384", "signing:ES512",
    "signing:RS256", "signing:RS384", "signing:RS512",
    "signing:PS256", "signing:PS384", "signing:PS512",
    "hashing:SHA-256", "hashing:SHA-384", "hashing:SHA-512",
    "verification:ES256", "verification:ES384", "verification:ES512",
    "verification:RS256", "verification:RS384", "verification:RS512",
    "verification:PS256", "verification:PS384", "verification:PS512"
  ],
  "jurisdiction": "world",
  "compliance": ["NIST", "offline-airgap"],
  "platforms": ["linux", "windows", "osx"],
  "priority": 45,
  "enabledByDefault": true
}

API Reference

ICryptoProvider.CreateEphemeralVerifier

ICryptoSigner CreateEphemeralVerifier(string algorithmId, ReadOnlySpan<byte> publicKeyBytes)

Creates a verification-only signer from raw public key bytes. Useful for:

• DSSE envelope verification with inline public keys
• JWT/JWS verification without key persistence
• Ad-hoc signature verification in offline scenarios

Parameters:

• algorithmId: Algorithm identifier (ES256, RS256, PS256, etc.)
• publicKeyBytes: Public key in SubjectPublicKeyInfo format (DER-encoded)

Returns:

• ICryptoSigner instance with VerifyAsync support only
• SignAsync throws NotSupportedException
• KeyId returns "ephemeral"
• AlgorithmId returns the specified algorithm

Throws:

• NotSupportedException: If algorithm not supported or public key format invalid

Implementation Details

Internal Architecture

OfflineVerificationCryptoProvider (ICryptoProvider)
├── BclHasher (ICryptoHasher)
│   └── System.Security.Cryptography.SHA256/384/512
├── EcdsaSigner (ICryptoSigner)
│   └── System.Security.Cryptography.ECDsa
├── RsaSigner (ICryptoSigner)
│   └── System.Security.Cryptography.RSA
├── EcdsaEphemeralVerifier (ICryptoSigner)
│   └── System.Security.Cryptography.ECDsa (verification-only)
└── RsaEphemeralVerifier (ICryptoSigner)
    └── System.Security.Cryptography.RSA (verification-only)

Plugin Boundaries

Allowed within this plugin:

• Direct usage of System.Security.Cryptography (internal implementation)
• Creation of ECDsa, RSA, SHA256, SHA384, SHA512 instances
• Key import/export operations

Not allowed outside this plugin:

• Direct crypto library usage in production code
• All consumers must use ICryptoProvider abstraction

This boundary is enforced by:

• scripts/audit-crypto-usage.ps1 - Static analysis
• .gitea/workflows/crypto-compliance.yml - CI validation

Testing

Run unit tests:

dotnet test src/__Libraries/__Tests/StellaOps.Cryptography.Plugin.OfflineVerification.Tests

Test Coverage:

• 39 unit tests covering all algorithms and scenarios
• Provider capability matrix validation
• Known-answer tests for SHA-256/384/512
• ECDSA and RSA signing/verification roundtrips
• Ephemeral verifier creation and usage
• Error handling (unsupported algorithms, tampered data)

Performance

The abstraction layer is designed to be zero-cost:

• No heap allocations in hot paths
• Direct delegation to .NET BCL primitives
• ReadOnlySpan<byte> for memory efficiency

Benchmark results should match direct System.Security.Cryptography usage within measurement error.

Security Considerations

1. Key Storage: This provider stores keys in-memory only. For persistent key storage, integrate with a key management system.

2. Ephemeral Verification: Public keys for ephemeral verification are not cached or validated against a trust store. Callers must perform their own trust validation.

3. Algorithm Hardening:

   • RSA key sizes: 2048-bit minimum recommended
   • ECDSA curves: Only NIST P-256/384/521 supported
   • Hash algorithms: SHA-2 family only (SHA-1 explicitly NOT supported)

4. Offline Trust: In offline scenarios, establish trust through:

   • Pre-distributed public key fingerprints
   • Certificate chains embedded in airgap bundles
   • Out-of-band key verification

Compliance

• NIST FIPS 186-4: ECDSA with approved curves (P-256, P-384, P-521)
• NIST FIPS 180-4: SHA-256, SHA-384, SHA-512
• RFC 8017: RSA PKCS#1 v2.2 (RSASSA-PKCS1-v1_5 and RSASSA-PSS)
• RFC 6979: Deterministic ECDSA (when used with BouncyCastle fallback)

Not compliant with:

• eIDAS (European digital signature standards) - use eIDAS plugin
• GOST R 34.10-2012 (Russian cryptographic standards) - use CryptoPro plugin
• SM2/SM3/SM4 (Chinese cryptographic standards) - use SM plugin

Related Documentation

• Crypto Architecture Overview
• ICryptoProvider Interface
• Plugin Manifest Schema
• AirGap Module Architecture

License

BUSL-1.1 - See LICENSE file in repository root.