mfer/mfer/serialize.go

package mfer

import (
	"bytes"
	"context"
	"crypto/sha256"
	"errors"
	"fmt"
	"math"
	"time"
	"uuid"

	"github.com/klauspost/compress/zstd"
	"google.golang.org/protobuf/proto"
)

// MAGIC is the file format magic bytes prefix (rot13 of "MANIFEST").
const MAGIC string = "ZNAVSRFG"

var (
	// errInnerNotSet is returned by generate when the inner manifest is
	// missing.
	errInnerNotSet = errors.New("inner message not set")
	// errInternal is returned by generateOuter for the same condition.
	errInternal = errors.New("inner message not set")
)

// nanosecondsInt32 converts t's nanosecond component to int32.
// time.Time.Nanosecond is documented to return a value in [0, 999999999],
// so the conversion cannot overflow. This sits directly in the manifest
// content path: silently substituting a default would zero every entry's
// mtime nanos and change the serialized bytes and their hash, so an
// out-of-contract value is a programming error and panics rather than
// being papered over.
func nanosecondsInt32(t time.Time) int32 {
	n := t.Nanosecond()
	if n < 0 || n > math.MaxInt32 {
		panic(fmt.Sprintf(
			"mfer: time.Time.Nanosecond out of contract: %d", n))
	}

	return int32(n)
}

func newTimestampFromTime(t time.Time) *Timestamp {
	return &Timestamp{
		Seconds: t.Unix(),
		Nanos:   nanosecondsInt32(t),
	}
}

func (m *manifest) generate(ctx context.Context) error {
	if m.pbInner == nil {
		return errInnerNotSet
	}

	if m.pbOuter == nil {
		e := m.generateOuter(ctx)
		if e != nil {
			return e
		}
	}

	dat, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbOuter)
	if err != nil {
		return fmt.Errorf("marshal outer message: %w", err)
	}

	m.output = bytes.NewBufferString(MAGIC)

	_, err = m.output.Write(dat)
	if err != nil {
		return fmt.Errorf("write outer message: %w", err)
	}

	return nil
}

func (m *manifest) generateOuter(ctx context.Context) error {
	if m.pbInner == nil {
		return errInternal
	}

	// Use fixed UUID if provided, otherwise generate a new one
	var manifestUUID uuid.UUID
	if len(m.fixedUUID) == uuidLength {
		copy(manifestUUID[:], m.fixedUUID)
	} else {
		manifestUUID = uuid.NewV4()
	}

	m.pbInner.Uuid = manifestUUID[:]

	innerData, err := proto.MarshalOptions{Deterministic: true}.Marshal(m.pbInner)
	if err != nil {
		return fmt.Errorf("marshal inner message: %w", err)
	}

	// Compress the inner data
	idc := new(bytes.Buffer)

	zw, err := zstd.NewWriter(idc, zstd.WithEncoderLevel(zstd.SpeedBestCompression))
	if err != nil {
		return fmt.Errorf("create compressor: %w", err)
	}

	_, err = zw.Write(innerData)
	if err != nil {
		return fmt.Errorf("compress inner message: %w", err)
	}

	_ = zw.Close()

	compressedData := idc.Bytes()

	// Hash the compressed data for integrity verification before decompression
	h := sha256.New()

	_, err = h.Write(compressedData)
	if err != nil {
		return fmt.Errorf("hash inner message: %w", err)
	}

	sha256Hash := h.Sum(nil)

	m.pbOuter = &MFFileOuter{
		InnerMessage:    compressedData,
		Size:            int64(len(innerData)),
		Sha256:          sha256Hash,
		Uuid:            manifestUUID[:],
		Version:         MFFileOuter_VERSION_ONE,
		CompressionType: MFFileOuter_COMPRESSION_ZSTD,
	}

	// Sign the manifest if signing options are provided
	if m.signingOptions != nil && m.signingOptions.KeyID != "" {
		return m.signOuter(ctx)
	}

	return nil
}

// signOuter signs the outer message with the configured GPG key and
// embeds the signature, signer fingerprint, and public key. The signer
// and public key are those of the key gpg reports it signed with, so that
// a key ID matching more than one key cannot name or embed another key.
func (m *manifest) signOuter(ctx context.Context) error {
	sigString, err := m.signatureString()
	if err != nil {
		return fmt.Errorf("build signature string: %w", err)
	}

	sig, signingKey, err := gpgSign(ctx, []byte(sigString), m.signingOptions.KeyID)
	if err != nil {
		return err
	}

	m.pbOuter.Signature = sig

	// Listing the signing key, a subkey's included, puts its primary key's
	// fingerprint first.
	fingerprint, err := gpgGetKeyFingerprint(ctx, GPGKeyID(signingKey))
	if err != nil {
		return err
	}

	m.pbOuter.Signer = fingerprint

	pubKey, err := gpgExportPublicKey(ctx, GPGKeyID(fingerprint))
	if err != nil {
		return err
	}

	m.pbOuter.SigningPubKey = pubKey

	return nil
}