# EonaCat.SecureToken

**Secure, modern token library for .NET with key rotation, signing isolation, validation rules, and .NET Standard support.**

EonaCat.SecureToken provides a safer alternative to rolling your own authentication tokens. It focuses on:

- Strong cryptographic signing
- Versioned key management
- Token lifecycle validation
- Refresh/access token separation
- Extensible claims
- API-friendly validation results
- `.NET Standard 2.0` compatibility

## Features

### Cryptographic protection

- HMAC based token signing
- HKDF derived context-specific keys
- Constant-time signature verification (including binding-context comparison)
- Tamper detection
- Strong random key generation
- Bounded decoding: oversized or field-flooded tokens are rejected before signature
  verification, so malformed input cannot be used to exhaust memory

### Validation

- Issuer, audience (single or multi-audience accept-list), and token-type checks
- Expiry, not-before, and an independent `MaxTokenAge` backstop measured from issuance
- Token binding (IP, device fingerprint, TLS channel hash, etc.)
- Pluggable revocation check
- Pluggable replay-cache for one-time-use tokens (password reset, email verification, invitations)
- `OnValidated` audit hook fired for every validation attempt

### Key rotation

Rotate signing keys without invalidating existing tokens.

Example:

```csharp
var store = SigningKeyStore.CreateNew();

var service = new TokenService(store);

var oldToken = service.Issue(
    TokenDescriptor.Create()
        .ForSubject("user-123")
        .IssuedBy("my-api")
        .ForAudience("mobile")
);

// Rotate keys
store.Rotate();

// New tokens use the new key
var newToken = service.Issue(
    TokenDescriptor.Create()
        .ForSubject("user-456")
        .IssuedBy("my-api")
        .ForAudience("mobile")
);

// Old token still validates
service.Validate(
    oldToken,
    TokenValidationOptions.AccessToken("my-api", "mobile")
);
```

## Installation

Install from NuGet:

```bash
dotnet add package EonaCat.SecureToken
```

# Quick Start

## Create a token service

```csharp
using EonaCat.SecureToken.Core;
using EonaCat.SecureToken.Cryptography;

var keys = SigningKeyStore.CreateNew();

var tokens = new TokenService(keys);
```

## Issue an access token

```csharp
var token = tokens.Issue(
    TokenDescriptor.Create()
        .ForSubject("user-123")
        .IssuedBy("my-service")
        .ForAudience("api")
        .WithRole("admin")
        .WithClaim("email", "user@example.com")
);
```

The token contains:

- Subject
- Issuer
- Audience
- Roles
- Custom claims
- Token ID
- Expiration
- Key generation information


## Validate a token

```csharp
var result = tokens.Validate(
    token,
    TokenValidationOptions.AccessToken(
        issuer: "my-service",
        audience: "api"
    )
);

if (result.IsSuccess)
{
    var claims = result.UnwrapClaims();

    Console.WriteLine(claims.Subject);
}
```

# Token expiration

```csharp
var token = tokens.Issue(
    TokenDescriptor.Create()
        .ForSubject("user-1")
        .IssuedBy("api")
        .ForAudience("mobile")
        .WithLifetime(TimeSpan.FromMinutes(15))
);
```

Expired tokens are automatically rejected.

# Refresh tokens

Create a refresh token:

```csharp
var pair = tokens.IssueTokenPair(
    "user-1",
    "api",
    "mobile"
);

Console.WriteLine(pair.AccessToken);
Console.WriteLine(pair.RefreshToken);
```

Validate separately:

```csharp
tokens.Validate(
    pair.RefreshToken,
    TokenValidationOptions.RefreshToken("api")
);
```

Refresh tokens cannot be used as access tokens.

# Token binding

Bind tokens to a context such as a device or session:

```csharp
var token = tokens.Issue(
    TokenDescriptor.Create()
        .ForSubject("user-1")
        .IssuedBy("api")
        .ForAudience("web")
        .BoundTo("device-identifier")
);
```

Validation:

```csharp
new TokenValidationOptions
{
    ValidIssuer = "api",
    ValidAudience = "web",
    BindingContext = "device-identifier"
};
```

# Revocation

You can integrate your own revocation storage:

```csharp
var options = new TokenValidationOptions
{
    ValidIssuer = "api",
    ValidAudience = "web",

    RevocationCheck = async (tokenId, cancellationToken) =>
    {
        return await database.IsRevoked(tokenId);
    }
};
```

# Replay protection for one-time-use tokens

Revocation answers "has someone explicitly blocked this token?" Replay protection
answers a different question: "has this exact token already been used once?" Use it
for password-reset links, email-verification links, and invitations - anything that
should only ever be redeemed a single time, even before it expires.

```csharp
var replayCache = new InMemoryReplayCache(); // register as a singleton in DI

var options = TokenValidationOptions.OneTimeUse(
    issuer: "api",
    tokenType: TokenTypeConstants.PasswordReset,
    maxAge: TimeSpan.FromMinutes(15));

options.ReplayCache = replayCache;

var result = await tokens.ValidateAsync(token, options);
// Second call with the same token returns TokenResult.Replayed instead of Success.
```

`InMemoryReplayCache` is process-local and fine for a single instance. For multi-instance
deployments, implement `IReplayCache` against a shared store (Redis, or a database table
with a unique constraint on the token ID) so replay detection works across all instances.

Replay consumption only happens through `ValidateAsync`, since it has a side effect
(recording the token as used) - the synchronous `Validate` never touches the replay cache.

# Multiple audiences

Use `ValidAudiences` when the same access token needs to be accepted by more than one
downstream service:

```csharp
var options = TokenValidationOptions.AccessToken(
    issuer: "api",
    audiences: new[] { "service-a", "service-b", "service-c" });
```

The token is accepted if any one of its own audiences matches any one of the configured set.

# Audit and introspection hook

`OnValidated` is invoked for every validation attempt, success or failure, and is intended
for audit logging or metrics - not for authorization decisions:

```csharp
var options = TokenValidationOptions.AccessToken("api", "web");

options.OnValidated = e =>
{
    logger.LogInformation("Token validation: {Result} sub={Subject}", e.Result, e.Subject);
};
```

`Subject` and `TokenId` on the event are only populated when the result is a success,
since claims are never trusted before the signature has verified.

# ASP.NET Core dependency injection

```csharp
builder.Services.AddSecureTokens();
```

or provide your own key store:

```csharp
builder.Services.AddSecureTokens(
    store =>
    {
        return SigningKeyStore.FromKeys(
            new[]
            {
                (1, secretKeyBytes)
            });
    });
```

To enable replay protection via DI:

```csharp
builder.Services.AddSecureTokenReplayProtection();
```


# Security design

The library separates cryptographic purposes:

```
Master Key
    |
    +-- Signing Key
    |
    +-- Encryption Key
    |
    +-- Context-specific keys
```

This prevents accidental key reuse between operations.

# Supported frameworks

- .NET Standard 2.0
- .NET Standard 2.1
- .NET Framework 4.8
- .NET 8+

# When to use

Good fit for:

APIs  
Microservices  
Internal authentication  
Service-to-service tokens  
Applications needing key rotation  

# When not to use

Do not store secrets directly in source code.

Use:

- Environment variables
- Secret managers
- Hardware-backed key storage where required

# License
Apache License.