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fastmcp.server.auth.jwt_issuer

JWT token issuance and verification for FastMCP OAuth Proxy. This module implements the token factory pattern for OAuth proxies, where the proxy issues its own JWT tokens to clients instead of forwarding upstream provider tokens. This maintains proper OAuth 2.0 token audience boundaries.

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Functions

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derive_jwt_key 

derive_jwt_key(upstream_secret: str, server_salt: str) -> bytes
Derive JWT signing key from upstream client secret and server salt. Uses HKDF (RFC 5869) to derive a cryptographically secure signing key from the upstream OAuth client secret combined with a server-specific salt. Args:
  • upstream_secret: The OAuth client secret from upstream provider
  • server_salt: Random salt unique to this server instance
Returns:
  • 32-byte key suitable for HS256 JWT signing

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derive_encryption_key 

derive_encryption_key(upstream_secret: str) -> bytes
Derive Fernet encryption key from upstream client secret. Uses HKDF to derive a cryptographically secure encryption key for encrypting upstream tokens at rest. Args:
  • upstream_secret: The OAuth client secret from upstream provider
Returns:
  • 32-byte Fernet key (base64url-encoded)

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derive_key_from_secret 

derive_key_from_secret(secret: str | bytes, salt: str, info: bytes) -> bytes
Derive 32-byte key from user-provided secret (string or bytes). Accepts any length input and derives a proper cryptographic key. Uses HKDF to stretch weak inputs into strong keys. Args:
  • secret: User-provided secret (any string or bytes)
  • salt: Application-specific salt string
  • info: Key purpose identifier
Returns:
  • 32-byte key suitable for HS256 JWT signing or Fernet encryption

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Classes

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JWTIssuer

Issues and validates FastMCP-signed JWT tokens using HS256. This issuer creates JWT tokens for MCP clients with proper audience claims, maintaining OAuth 2.0 token boundaries. Tokens are signed with HS256 using a key derived from the upstream client secret. Methods:

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issue_access_token 

issue_access_token(self, client_id: str, scopes: list[str], jti: str, expires_in: int = 3600) -> str
Issue a minimal FastMCP access token. FastMCP tokens are reference tokens containing only the minimal claims needed for validation and lookup. The JTI maps to the upstream token which contains actual user identity and authorization data. Args:
  • client_id: MCP client ID
  • scopes: Token scopes
  • jti: Unique token identifier (maps to upstream token)
  • expires_in: Token lifetime in seconds
Returns:
  • Signed JWT token

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issue_refresh_token 

issue_refresh_token(self, client_id: str, scopes: list[str], jti: str, expires_in: int) -> str
Issue a minimal FastMCP refresh token. FastMCP refresh tokens are reference tokens containing only the minimal claims needed for validation and lookup. The JTI maps to the upstream token which contains actual user identity and authorization data. Args:
  • client_id: MCP client ID
  • scopes: Token scopes
  • jti: Unique token identifier (maps to upstream token)
  • expires_in: Token lifetime in seconds (should match upstream refresh expiry)
Returns:
  • Signed JWT token

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verify_token 

verify_token(self, token: str) -> dict[str, Any]
Verify and decode a FastMCP token. Validates JWT signature, expiration, issuer, and audience. Args:
  • token: JWT token to verify
Returns:
  • Decoded token payload
Raises:
  • JoseError: If token is invalid, expired, or has wrong claims

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TokenEncryption

Handles encryption/decryption of upstream OAuth tokens at rest. Methods:

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encrypt 

encrypt(self, token: str) -> bytes
Encrypt a token for storage. Args:
  • token: Plain text token
Returns:
  • Encrypted token bytes

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decrypt 

decrypt(self, encrypted_token: bytes) -> str
Decrypt a token from storage. Args:
  • encrypted_token: Encrypted token bytes
Returns:
  • Plain text token
Raises:
  • cryptography.fernet.InvalidToken: If token is corrupted or key is wrong
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