mesh-controller/internal/token/token.go

// Package token is the thing a person carries to a machine that is joining.
//
// novox/hq ADR 0004: it carries four things, and it is the only thing a joining node needs —
// where the broker is, what certificate to expect there, whose signature to believe afterwards,
// and a one-time right to join.
//
// Its authenticity comes from the channel it travelled, not from anything the node can check
// afterwards: trust on first use, with the first use moved out of band. Which makes the token
// security-critical, because it carries the pin. Tampering with it substitutes the mesh — still
// better than the alternative, where there is nothing to tamper with and a node trusts the first
// answer it gets.
package token

import (
	"crypto/ed25519"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"strings"
)

// Token is the four things, and it is assembled by whatever holds all four.
//
// Two contexts contribute: `inventory` owns the node record and mints the secret, `identity` owns
// the signing key. Neither reads the other's store — the process holding both grants asks each
// for its part (novox/hq ADR 0008).
type Token struct {
	Version int `json:"v"`

	// Broker is an address and not a name. There is no resolution before joining, which is why
	// this is the one place in the mesh where an address is carried deliberately.
	Broker string `json:"broker,omitempty"`

	// Fingerprint is the broker certificate's, checked before anything is sent.
	Fingerprint string `json:"fingerprint,omitempty"`

	// Signer is the control plane's public signing key: whose declarations to believe.
	Signer []byte `json:"signer,omitempty"`

	// Secret is the one-time right to join. Useless once used, useless after it expires.
	Secret string `json:"secret"`
}

// Missing names the parts that are not filled in.
//
// Returned as a list rather than a bool, because "this token cannot be used" is not an answer
// anybody can act on and "it has no broker address" is. Everything here is required: a token
// missing the fingerprint cannot verify what it connects to, and one missing the signer makes
// the control plane's authority transitive through the broker — which ADR 0004 rejects, because
// a compromised broker could then forge declarations, and that is the whole machine.
func (t Token) Missing() []string {
	var missing []string
	if strings.TrimSpace(t.Broker) == "" {
		missing = append(missing, "the broker's address — there is nowhere to connect to")
	}
	if strings.TrimSpace(t.Fingerprint) == "" {
		missing = append(missing,
			"the broker certificate's fingerprint — nothing to check the connection against")
	}
	if len(t.Signer) != ed25519.PublicKeySize {
		missing = append(missing,
			"the control plane's signing key — declarations could not be told from forgeries")
	}
	if strings.TrimSpace(t.Secret) == "" {
		missing = append(missing, "the one-time secret — nothing to present")
	}
	return missing
}

// Complete reports whether this token could actually be used to join.
func (t Token) Complete() bool { return len(t.Missing()) == 0 }

// Encode renders the token as one line a person can carry.
//
// Base64 of JSON: self-describing, so a token from an older control plane says what it is rather
// than being misread by a newer one; and one line, because it is copied by hand between a
// terminal and a machine.
func (t Token) Encode() (string, error) {
	t.Version = 1
	raw, err := json.Marshal(t)
	if err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(raw), nil
}

// Decode reads a token a person pasted.
func Decode(encoded string) (Token, error) {
	raw, err := base64.RawURLEncoding.DecodeString(strings.TrimSpace(encoded))
	if err != nil {
		return Token{}, fmt.Errorf("this is not a token: %w", err)
	}
	var t Token
	if err := json.Unmarshal(raw, &t); err != nil {
		return Token{}, fmt.Errorf("this is not a token: %w", err)
	}
	if t.Version != 1 {
		return Token{}, fmt.Errorf(
			"this token says it is version %d, and this host understands version 1. It was made "+
				"by a different control plane than the one this was built against", t.Version)
	}
	return t, nil
}

// base64Decode and encodeBase64 exist for the tests, which construct a token by hand to prove a
// version this build does not understand is refused. Kept beside the encoding they mirror.
func base64Decode(s string) ([]byte, error) {
	return base64.RawURLEncoding.DecodeString(strings.TrimSpace(s))
}

func encodeBase64(s string) string {
	return base64.RawURLEncoding.EncodeToString([]byte(s))
}