add token framing, crypto and errors

errors.go

@@ -0,0 +1,15 @@
+package ficha
+
+import "errors"
+
+var (
+	// ErrInvalidToken means the token is malformed, has an unsupported
+	// version, fails descryption, or has been tampered with.
+	ErrInvalidToken = errors.New("ficha: invalid token")
+	// ErrExpiredToken means the token's expiry time has passed.
+	ErrExpiredToken = errors.New("ficha: token expired")
+	// ErrRevokedToken means the token's ID is in the revocation store.
+	ErrRevokedToken = errors.New("ficha: token revoked")
+	// ErrUnknownKey means the token references a key ID not in the keyring.
+	ErrUnknownKey = errors.New("ficha: unknown key id")
+)