mxaccessgw/docs/Authentication.md

Gateway Authentication

The gateway authentication subsystem verifies inbound API key credentials against a SQLite-backed key store, hashes secrets with a configurable pepper, and records administrative and verification events to an audit trail.

Token Format

API keys travel in the HTTP Authorization header as a bearer token shaped mxgw_<keyId>_<secret>. The mxgw_ prefix scopes parsing to gateway tokens, the <keyId> segment is the public identifier used for lookup, and <secret> is the high-entropy portion that the gateway verifies against a stored hash.

ApiKeyParser enforces the format and rejects malformed tokens before any database round-trip:

public bool TryParseAuthorizationHeader(string? authorizationHeader, out ParsedApiKey? apiKey)
{
    apiKey = null;

    if (string.IsNullOrWhiteSpace(authorizationHeader)
        || !authorizationHeader.StartsWith(BearerPrefix, StringComparison.OrdinalIgnoreCase))
    {
        return false;
    }

    string token = authorizationHeader[BearerPrefix.Length..].Trim();

    if (!token.StartsWith(TokenPrefix, StringComparison.OrdinalIgnoreCase))
    {
        return false;
    }

A successful parse produces a ParsedApiKey(KeyId, Secret) record. The IApiKeyParser interface exists so verification consumers can be tested without depending on header-format details.

Parsing and Secrets

Secret generation

ApiKeySecretGenerator.Generate() is the single source of new secret material. It uses 32 bytes from RandomNumberGenerator.Fill and encodes with URL-safe base64 (no padding) so secrets can be embedded in headers without escaping:

public static string Generate()
{
    Span<byte> bytes = stackalloc byte[32];
    RandomNumberGenerator.Fill(bytes);

    return Convert.ToBase64String(bytes)
        .TrimEnd('=')
        .Replace('+', '-')
        .Replace('/', '_');
}

Peppered hashing

ApiKeySecretHasher (registered behind IApiKeySecretHasher) hashes secrets with HMACSHA256 keyed by a server-side pepper. The pepper lives outside the database and is resolved by IConfiguration lookup against the configured PepperSecretName:

public byte[] HashSecret(string secret)
{
    string pepper = GetPepper();
    byte[] pepperBytes = Encoding.UTF8.GetBytes(pepper);
    byte[] secretBytes = Encoding.UTF8.GetBytes(secret);

    using HMACSHA256 hmac = new(pepperBytes);

    return hmac.ComputeHash(secretBytes);
}

The pepper is intentionally not stored alongside the hash: an attacker who exfiltrates only the SQLite file holds the hashes but lacks the keying material to brute-force candidate secrets, even if the stored hash algorithm and salt scheme are known. If the pepper is missing the hasher throws ApiKeyPepperUnavailableException, which the verifier converts to a distinct failure code rather than treating it as a credential mismatch.

Verification

ApiKeyVerifier (IApiKeyVerifier) implements the verification flow:

1. Parse the Authorization header into a ParsedApiKey.
2. Look up the ApiKeyRecord by KeyId through IApiKeyStore.FindByKeyIdAsync.
3. Reject revoked records (RevokedUtc is not null).
4. Hash the presented secret with the configured pepper.
5. Compare hashes with CryptographicOperations.FixedTimeEquals to avoid timing oracles.
6. Record a LastUsedUtc timestamp via MarkKeyUsedAsync and return an ApiKeyIdentity.

if (!CryptographicOperations.FixedTimeEquals(presentedHash, storedKey.SecretHash))
{
    return ApiKeyVerificationResult.Fail(ApiKeyVerificationFailure.SecretMismatch);
}

await keyStore.MarkKeyUsedAsync(storedKey.KeyId, DateTimeOffset.UtcNow, cancellationToken)
    .ConfigureAwait(false);

return ApiKeyVerificationResult.Success(new ApiKeyIdentity(
    KeyId: storedKey.KeyId,
    KeyPrefix: storedKey.KeyPrefix,
    DisplayName: storedKey.DisplayName,
    Scopes: storedKey.Scopes,
    Constraints: storedKey.Constraints));

ApiKeyVerificationResult carries either an ApiKeyIdentity or a discriminated ApiKeyVerificationFailure value. The failure enum distinguishes parse errors, missing pepper, missing or revoked keys, and secret mismatch so the calling middleware can emit precise audit detail without leaking which check failed to the client.

ApiKeyIdentity exposes only non-secret fields (KeyId, KeyPrefix, DisplayName, Scopes, and Constraints) and is the type downstream authorization code consumes.

Storage

The gateway keeps API key state in a dedicated SQLite database. SQLite is sufficient because credential volume is small, the gateway runs as a single process, and the file is straightforward to back up and rotate independently of the main application data.

Connection factory

AuthSqliteConnectionFactory reads GatewayOptions.Authentication.SqlitePath, ensures the parent directory exists, and opens the connection in ReadWriteCreate mode so first-run installations can create the file without manual provisioning:

public SqliteConnection CreateConnection()
{
    string sqlitePath = options.Value.Authentication.SqlitePath;
    string? directory = Path.GetDirectoryName(sqlitePath);

    if (!string.IsNullOrWhiteSpace(directory))
    {
        Directory.CreateDirectory(directory);
    }

    SqliteConnectionStringBuilder builder = new()
    {
        DataSource = sqlitePath,
        Mode = SqliteOpenMode.ReadWriteCreate
    };

    return new SqliteConnection(builder.ToString());
}

Schema

SqliteAuthSchema declares table names and the current schema version as constants. Three tables are involved:

• api_keys stores key_id, key_prefix, the secret_hash blob, display_name, serialized scopes, optional serialized constraints, and the created_utc, last_used_utc, and revoked_utc timestamps.
• api_key_audit is an append-only log keyed by an autoincrement audit_id with key_id, event_type, remote_address, created_utc, and details columns.
• schema_version carries a single row whose version column is matched against SqliteAuthSchema.CurrentVersion.

Read paths

SqliteApiKeyStore (IApiKeyStore) handles the two reads needed at request time: FindByKeyIdAsync returns any record (so revoked keys can be reported distinctly) and FindActiveByKeyIdAsync filters to non-revoked rows. MarkKeyUsedAsync updates last_used_utc only for non-revoked rows so a freshly revoked key cannot have its timestamp refreshed by a racing verification.

ApiKeyRecord is the in-memory projection. ApiKeyRecordReader.Read is shared by every read path so column ordering is defined in one place:

public static ApiKeyRecord Read(SqliteDataReader reader)
{
    return new ApiKeyRecord(
        KeyId: reader.GetString(0),
        KeyPrefix: reader.GetString(1),
        SecretHash: (byte[])reader["secret_hash"],
        DisplayName: reader.GetString(3),
        Scopes: ApiKeyScopeSerializer.Deserialize(reader.GetString(4)),
        Constraints: ApiKeyConstraintSerializer.Deserialize(reader.IsDBNull(5) ? null : reader.GetString(5)),
        CreatedUtc: DateTimeOffset.Parse(reader.GetString(6), System.Globalization.CultureInfo.InvariantCulture),
        LastUsedUtc: ReadNullableDateTimeOffset(reader, 7),
        RevokedUtc: ReadNullableDateTimeOffset(reader, 8));
}

Write paths

SqliteApiKeyAdminStore (IApiKeyAdminStore) implements administrative mutations: CreateAsync accepts an ApiKeyCreateRequest, RevokeAsync sets revoked_utc only when not already revoked, and RotateAsync replaces secret_hash, clears last_used_utc, and clears revoked_utc so a rotated key is immediately usable.

Audit trail

SqliteApiKeyAuditStore (IApiKeyAuditStore) appends ApiKeyAuditEntry values to the api_key_audit table and stamps each row with a UTC timestamp inside the store rather than trusting the caller. ListRecentAsync returns the most recent rows ordered by audit_id descending and projects them into ApiKeyAuditRecord. Rows are kept even after the referenced key is revoked because the audit history is the durable record of administrative action; the key_id column is nullable to accommodate non-key-scoped events such as init-db.

Migration

Schema bring-up is centralised behind IAuthStoreMigrator. SqliteAuthStoreMigrator executes the migration inside a single transaction so a partial failure leaves the database untouched, refuses to start when the on-disk schema version is newer than the binary supports, and idempotently creates the v1 schema:

if (existingVersion > SqliteAuthSchema.CurrentVersion)
{
    throw new AuthStoreMigrationException(
        $"Auth database schema version {existingVersion} is newer than supported version {SqliteAuthSchema.CurrentVersion}.");
}

await ApplyVersionOneAsync(connection, transaction, cancellationToken).ConfigureAwait(false);

await transaction.CommitAsync(cancellationToken).ConfigureAwait(false);

AuthStoreMigrationHostedService runs the migrator at startup, but only when API-key authentication is enabled and RunMigrationsOnStartup is true. Operators who manage schema out-of-band can disable the hosted run and use the admin CLI's init-db command instead.

AuthStoreMigrationException is a sealed InvalidOperationException so it can be caught precisely without swallowing unrelated failures.

Admin CLI

ApiKeyAdminCommandLineParser.Parse recognises a leading apikey argument and dispatches to one of the subcommands declared by ApiKeyAdminCommandKind. Each parsed invocation produces an ApiKeyAdminCommand (or an ApiKeyAdminParseResult carrying an error). ApiKeyAdminCliRunner then executes the command, runs the migrator first, calls the relevant store method, appends an audit row, and writes either text or JSON output via ApiKeyAdminOutput. The returned ApiKeyAdminListedKey projection deliberately omits the secret_hash so listing a database does not surface hash material.

The supported subcommands match ApiKeyAdminCommandKind exactly:

Subcommand	Required options	Behaviour
init-db	none	Runs the migrator and records an audit entry.
create-key	--key-id, --display-name	Generates a new secret, stores its peppered hash and optional constraints, and prints the assembled mxgw_<keyId>_<secret> token.
list-keys	none	Lists every stored key with its scopes, constraints, and revocation state.
revoke-key	--key-id	Sets revoked_utc if the key is currently active.
rotate-key	--key-id	Replaces the secret hash and prints the new token.

Examples:

mxgateway apikey init-db
mxgateway apikey create-key --key-id ops.alice --display-name "Alice (ops)" --scopes read,write
mxgateway apikey create-key --key-id area1.reader --display-name "Area 1 reader" --scopes invoke:read,metadata:read --read-subtree "Area1/*" --browse-subtree "Area1/*"
mxgateway apikey list-keys --json
mxgateway apikey revoke-key --key-id ops.alice
mxgateway apikey rotate-key --key-id ops.alice

Constraint flags are optional. --read-subtree, --write-subtree, --read-tag-glob, --write-tag-glob, and --browse-subtree are repeatable. --max-write-classification accepts one integer. --read-alarm-only and --read-historized-only are boolean flags. Existing rows with null constraints remain fully unconstrained after migration.

Key ids are restricted by the parser to ASCII letters, digits, periods, and hyphens so they remain safe to embed in the token format and in URL paths used by administrative tooling.

Scope Serialization

Scopes are persisted as a single TEXT column rather than a join table because the set is small, never queried by membership at the database level, and changes atomically with the owning row. ApiKeyScopeSerializer.Serialize writes a JSON array sorted with StringComparer.Ordinal so equivalent scope sets produce byte-identical column values, which makes audit diffing and database comparisons deterministic:

public static string Serialize(IReadOnlySet<string> scopes)
{
    return JsonSerializer.Serialize(scopes.Order(StringComparer.Ordinal));
}

public static IReadOnlySet<string> Deserialize(string value)
{
    if (string.IsNullOrWhiteSpace(value))
    {
        return new HashSet<string>(StringComparer.Ordinal);
    }

    string[]? scopes = JsonSerializer.Deserialize<string[]>(value);

    return new HashSet<string>(scopes ?? [], StringComparer.Ordinal);
}

Deserialize tolerates an empty column by returning an empty set so older rows or hand-edited records do not crash the verifier.

Registration

AuthStoreServiceCollectionExtensions.AddSqliteAuthStore wires every service in this subsystem as a singleton and registers the migration hosted service:

public static IServiceCollection AddSqliteAuthStore(this IServiceCollection services)
{
    services.AddSingleton<IApiKeyParser, ApiKeyParser>();
    services.AddSingleton<IApiKeySecretHasher, ApiKeySecretHasher>();
    services.AddSingleton<IApiKeyVerifier, ApiKeyVerifier>();
    services.AddSingleton<ApiKeyAdminCliRunner>();
    services.AddSingleton<AuthSqliteConnectionFactory>();
    services.AddSingleton<IAuthStoreMigrator, SqliteAuthStoreMigrator>();
    services.AddSingleton<IApiKeyStore, SqliteApiKeyStore>();
    services.AddSingleton<IApiKeyAdminStore, SqliteApiKeyAdminStore>();
    services.AddSingleton<IApiKeyAuditStore, SqliteApiKeyAuditStore>();
    services.AddHostedService<AuthStoreMigrationHostedService>();

    return services;
}

Singletons are safe because each operation opens its own short-lived SqliteConnection through the factory; there is no shared mutable state inside the services.

Related Documentation

• Gateway Configuration
• Authorization
• Diagnostics