mfer/mfer/serialize.go

package mfer

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

	"github.com/google/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 = errors.New("internal error: pbInner 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; the guard makes that explicit.
func nanosecondsInt32(t time.Time) int32 {
	n := t.Nanosecond()
	if n < 0 || n > math.MaxInt32 {
		return 0
	}

	return int32(n)
}

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

func (m *Manifest) generate() error {
	if m.pbInner == nil {
		return errInnerNotSet
	}

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

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

	m.output = bytes.NewBufferString(MAGIC)

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

	return nil
}

func (m *Manifest) generateOuter() error {
	if m.pbInner == nil {
		return errInnerNotSet
	}

	// 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.New()
	}

	m.pbInner.Uuid = manifestUUID[:]

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

	_, err = zw.Write(innerData)
	if err != nil {
		return fmt.Errorf("serialize: compress: %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("serialize: hash write: %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()
	}

	return nil
}

// signOuter signs the outer message with the configured GPG key and
// embeds the signature, signer fingerprint, and public key.
func (m *Manifest) signOuter() error {
	sigString, err := m.signatureString()
	if err != nil {
		return fmt.Errorf("failed to generate signature string: %w", err)
	}

	sig, err := gpgSign([]byte(sigString), m.signingOptions.KeyID)
	if err != nil {
		return fmt.Errorf("failed to sign manifest: %w", err)
	}

	m.pbOuter.Signature = sig

	fingerprint, err := gpgGetKeyFingerprint(m.signingOptions.KeyID)
	if err != nil {
		return fmt.Errorf("failed to get key fingerprint: %w", err)
	}

	m.pbOuter.Signer = fingerprint

	pubKey, err := gpgExportPublicKey(m.signingOptions.KeyID)
	if err != nil {
		return fmt.Errorf("failed to export public key: %w", err)
	}

	m.pbOuter.SigningPubKey = pubKey

	return nil
}