Warp/server/internal/matrix/signing/key.go

// Package signing handles the server's ed25519 signing keys: generating
// them, reading and writing the on-disk format, and naming them.
//
// Event signing itself (canonical JSON, hashes, signature blocks) arrives
// with the event core in Phase 2; this package deliberately stops at key
// material so that `warp generate-keys` can exist from day one.
package signing

import (
	"crypto/ed25519"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
)

// KeyIDPrefix is the only signing algorithm Matrix defines today.
const KeyIDPrefix = "ed25519"

// FileMode is the permission a freshly written key file gets. On Windows it
// is advisory; the warning in Load covers the Unix case.
const FileMode os.FileMode = 0o600

// KeyPair is a server signing key and the key ID that names it in a
// signature block ("ed25519:abc123").
type KeyPair struct {
	// ID is the part after the algorithm, e.g. "a_RVfN".
	ID string

	Private ed25519.PrivateKey
	Public  ed25519.PublicKey
}

// KeyID returns the full "ed25519:<id>" identifier.
func (k *KeyPair) KeyID() string { return KeyIDPrefix + ":" + k.ID }

// PublicBase64 returns the unpadded base64 public key, the encoding Matrix
// uses on the wire.
func (k *KeyPair) PublicBase64() string {
	return base64.RawStdEncoding.EncodeToString(k.Public)
}

// Generate creates a new key. An empty id gets a random one.
func Generate(id string) (*KeyPair, error) {
	if id == "" {
		var err error
		if id, err = RandomKeyID(); err != nil {
			return nil, err
		}
	}
	if err := ValidateKeyID(id); err != nil {
		return nil, err
	}
	pub, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, fmt.Errorf("generate ed25519 key: %w", err)
	}
	return &KeyPair{ID: id, Private: priv, Public: pub}, nil
}

// keyIDAlphabet is deliberately narrow: the spec allows [a-zA-Z0-9_] and
// mixed case key IDs are a nuisance to type.
const keyIDAlphabet = "abcdefghijklmnopqrstuvwxyz0123456789"

// RandomKeyID returns an identifier of the "a_xxxxx" shape Synapse uses, so
// operators recognise it.
func RandomKeyID() (string, error) {
	buf := make([]byte, 5)
	if _, err := io.ReadFull(rand.Reader, buf); err != nil {
		return "", fmt.Errorf("read randomness: %w", err)
	}
	out := make([]byte, 0, len(buf)+2)
	out = append(out, 'a', '_')
	for _, b := range buf {
		out = append(out, keyIDAlphabet[int(b)%len(keyIDAlphabet)])
	}
	return string(out), nil
}

// ValidateKeyID checks the identifier against the grammar in the spec.
func ValidateKeyID(id string) error {
	if id == "" {
		return errors.New("key ID must not be empty")
	}
	if strings.Contains(id, ":") {
		return fmt.Errorf("key ID %q must not contain a colon; pass the part after %q", id, KeyIDPrefix+":")
	}
	for _, r := range id {
		switch {
		case r >= 'a' && r <= 'z', r >= 'A' && r <= 'Z', r >= '0' && r <= '9', r == '_':
		default:
			return fmt.Errorf("key ID %q contains %q; only [a-zA-Z0-9_] are allowed", id, r)
		}
	}
	return nil
}

// Save writes the key to path in the one-line interchange format shared with
// Synapse:
//
//	ed25519 <key-id> <unpadded base64 seed>
//
// It refuses to clobber an existing file: a lost signing key means every
// event this server ever signed stops verifying.
func (k *KeyPair) Save(path string) error {
	if dir := filepath.Dir(path); dir != "" && dir != "." {
		if err := os.MkdirAll(dir, 0o700); err != nil {
			return fmt.Errorf("create key directory: %w", err)
		}
	}
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, FileMode)
	if err != nil {
		if errors.Is(err, os.ErrExist) {
			return fmt.Errorf("%s already exists; refusing to overwrite a signing key", path)
		}
		return err
	}
	defer f.Close()

	seed := base64.RawStdEncoding.EncodeToString(k.Private.Seed())
	if _, err := fmt.Fprintf(f, "%s %s %s\n", KeyIDPrefix, k.ID, seed); err != nil {
		return fmt.Errorf("write key: %w", err)
	}
	return f.Close()
}

// Load reads a key file written by Save. Lines starting with '#' and blank
// lines are ignored; the first key line wins. Older keys (a file with
// several entries) will matter once key rotation lands in Phase 5.
func Load(path string) (*KeyPair, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read signing key: %w", err)
	}
	for _, line := range strings.Split(string(data), "\n") {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		return parseKeyLine(line, path)
	}
	return nil, fmt.Errorf("%s contains no signing key", path)
}

func parseKeyLine(line, path string) (*KeyPair, error) {
	parts := strings.Fields(line)
	if len(parts) != 3 {
		return nil, fmt.Errorf("%s: malformed key line, want \"ed25519 <key-id> <base64-seed>\"", path)
	}
	if parts[0] != KeyIDPrefix {
		return nil, fmt.Errorf("%s: unsupported key algorithm %q", path, parts[0])
	}
	if err := ValidateKeyID(parts[1]); err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	seed, err := decodeBase64(parts[2])
	if err != nil {
		return nil, fmt.Errorf("%s: decode seed: %w", path, err)
	}
	if len(seed) != ed25519.SeedSize {
		return nil, fmt.Errorf("%s: seed is %d bytes, want %d", path, len(seed), ed25519.SeedSize)
	}
	priv := ed25519.NewKeyFromSeed(seed)
	pub, ok := priv.Public().(ed25519.PublicKey)
	if !ok {
		return nil, fmt.Errorf("%s: unexpected public key type", path)
	}
	return &KeyPair{ID: parts[1], Private: priv, Public: pub}, nil
}

// DecodeBase64 decodes a base64 value from the Matrix wire format.
//
// Matrix writes these unpadded, but the spec asks decoders to accept
// padding too wherever they can: keys, hashes and signatures get copied
// between implementations by hand, and rejecting a trailing "=" would turn
// a harmless transcription into a verification failure.
func DecodeBase64(s string) ([]byte, error) {
	return decodeBase64(s)
}

func decodeBase64(s string) ([]byte, error) {
	if strings.HasSuffix(s, "=") {
		return base64.StdEncoding.DecodeString(s)
	}
	return base64.RawStdEncoding.DecodeString(s)
}