func validateSnapshot(role string, oldSnap *data.SignedSnapshot, snapUpdate storage.MetaUpdate, roles map[string]storage.MetaUpdate, kdb *keys.KeyDB) error {
	s := &data.Signed{}
	err := json.Unmarshal(snapUpdate.Data, s)
	if err != nil {
		return errors.New("could not parse snapshot")
	}
	// version specifically gets validated when writing to store to
	// better handle race conditions there.
	if err := signed.Verify(s, role, 0, kdb); err != nil {
		return err
	}

	snap, err := data.SnapshotFromSigned(s)
	if err != nil {
		return errors.New("could not parse snapshot")
	}
	if !data.ValidTUFType(snap.Signed.Type, data.CanonicalSnapshotRole) {
		return errors.New("snapshot has wrong type")
	}
	err = checkSnapshotEntries(role, oldSnap, snap, roles)
	if err != nil {
		return err
	}
	return nil
}

func TestValidateTargetsModifiedHash(t *testing.T) {
	repo, cs, err := testutils.EmptyRepo("docker.com/notary")
	assert.NoError(t, err)
	store := storage.NewMemStorage()

	r, tg, sn, ts, err := testutils.Sign(repo)
	assert.NoError(t, err)

	snap, err := data.SnapshotFromSigned(sn)
	assert.NoError(t, err)
	snap.Signed.Meta["targets"].Hashes["sha256"][0] = snap.Signed.Meta["targets"].Hashes["sha256"][0] ^ 0xff

	sn, err = snap.ToSigned()
	assert.NoError(t, err)

	root, targets, snapshot, timestamp, err := getUpdates(r, tg, sn, ts)
	assert.NoError(t, err)

	updates := []storage.MetaUpdate{root, targets, snapshot, timestamp}

	copyTimestampKey(t, repo, store, "testGUN")
	_, err = validateUpdate(cs, "testGUN", updates, store)
	assert.Error(t, err)
	assert.IsType(t, validation.ErrBadSnapshot{}, err)
}

func TestValidateTargetsModifiedHash(t *testing.T) {
	gun := "docker.com/notary"
	repo, cs, err := testutils.EmptyRepo(gun)
	require.NoError(t, err)
	store := storage.NewMemStorage()

	r, tg, sn, ts, err := testutils.Sign(repo)
	require.NoError(t, err)

	snap, err := data.SnapshotFromSigned(sn)
	require.NoError(t, err)
	snap.Signed.Meta["targets"].Hashes["sha256"][0] = snap.Signed.Meta["targets"].Hashes["sha256"][0] ^ 0xff

	sn, err = snap.ToSigned()
	require.NoError(t, err)

	root, targets, snapshot, timestamp, err := getUpdates(r, tg, sn, ts)
	require.NoError(t, err)

	updates := []storage.MetaUpdate{root, targets, snapshot, timestamp}

	serverCrypto := testutils.CopyKeys(t, cs, data.CanonicalTimestampRole)
	_, err = validateUpdate(serverCrypto, gun, updates, store)
	require.Error(t, err)
	require.IsType(t, validation.ErrBadSnapshot{}, err)
}

func TestValidateTargetsModifiedHash(t *testing.T) {
	_, repo, _ := testutils.EmptyRepo()
	store := storage.NewMemStorage()

	r, tg, sn, ts, err := testutils.Sign(repo)
	assert.NoError(t, err)

	snap, err := data.SnapshotFromSigned(sn)
	assert.NoError(t, err)
	snap.Signed.Meta["targets"].Hashes["sha256"][0] = snap.Signed.Meta["targets"].Hashes["sha256"][0] ^ 0xff

	sn, err = snap.ToSigned()
	assert.NoError(t, err)

	root, targets, snapshot, timestamp, err := testutils.Serialize(r, tg, sn, ts)
	assert.NoError(t, err)

	updates := []storage.MetaUpdate{
		{
			Role:    "root",
			Version: 1,
			Data:    root,
		},
		{
			Role:    "targets",
			Version: 1,
			Data:    targets,
		},
		{
			Role:    "snapshot",
			Version: 1,
			Data:    snapshot,
		},
		{
			Role:    "timestamp",
			Version: 1,
			Data:    timestamp,
		},
	}

	err = validateUpdate("testGUN", updates, store)
	assert.Error(t, err)
	assert.IsType(t, ErrBadSnapshot{}, err)
}

func (rb *repoBuilder) loadSnapshot(content []byte, minVersion int, allowExpired bool) error {
	roleName := data.CanonicalSnapshotRole

	snapshotRole, err := rb.repo.Root.BuildBaseRole(roleName)
	if err != nil { // this should never happen, since it's already been validated
		return err
	}

	signedObj, err := rb.bytesToSignedAndValidateSigs(snapshotRole, content)
	if err != nil {
		return err
	}

	signedSnapshot, err := data.SnapshotFromSigned(signedObj)
	if err != nil {
		return err
	}

	if err := signed.VerifyVersion(&(signedSnapshot.Signed.SignedCommon), minVersion); err != nil {
		return err
	}

	if !allowExpired { // check must go at the end because all other validation should pass
		if err := signed.VerifyExpiry(&(signedSnapshot.Signed.SignedCommon), roleName); err != nil {
			return err
		}
	}

	// at this point, the only thing left to validate is existing checksums - we can use
	// this snapshot to bootstrap the next builder if needed - and we don't need to do
	// the 2-value assignment since we've already validated the signedSnapshot, which MUST
	// have root metadata
	rootMeta := signedSnapshot.Signed.Meta[data.CanonicalRootRole]
	rb.nextRootChecksum = &rootMeta

	if err := rb.validateChecksumsFromSnapshot(signedSnapshot); err != nil {
		return err
	}

	rb.repo.Snapshot = signedSnapshot
	return nil
}

func TestValidateSnapshotGenerateWithPrev(t *testing.T) {
	gun := "docker.com/notary"
	repo, cs, err := testutils.EmptyRepo(gun)
	require.NoError(t, err)
	store := storage.NewMemStorage()
	snapRole, err := repo.GetBaseRole(data.CanonicalSnapshotRole)
	require.NoError(t, err)

	for _, k := range snapRole.Keys {
		err := store.SetKey(gun, data.CanonicalSnapshotRole, k.Algorithm(), k.Public())
		require.NoError(t, err)
	}

	r, tg, sn, ts, err := testutils.Sign(repo)
	require.NoError(t, err)
	root, targets, snapshot, _, err := getUpdates(r, tg, sn, ts)
	require.NoError(t, err)

	updates := []storage.MetaUpdate{root, targets}

	// set the current snapshot in the store manually so we find it when generating
	// the next version
	store.UpdateCurrent(gun, snapshot)

	prev, err := data.SnapshotFromSigned(sn)
	require.NoError(t, err)

	serverCrypto := testutils.CopyKeys(t, cs, data.CanonicalTimestampRole, data.CanonicalSnapshotRole)
	updates, err = validateUpdate(serverCrypto, gun, updates, store)
	require.NoError(t, err)

	for _, u := range updates {
		if u.Role == data.CanonicalSnapshotRole {
			curr := &data.SignedSnapshot{}
			err = json.Unmarshal(u.Data, curr)
			require.NoError(t, err)
			require.Equal(t, prev.Signed.Version+1, curr.Signed.Version)
			require.Equal(t, u.Version, curr.Signed.Version)
		}
	}
}

func TestValidateSnapshotGenerateWithPrev(t *testing.T) {
	kdb, repo, cs, err := testutils.EmptyRepo("docker.com/notary")
	assert.NoError(t, err)
	store := storage.NewMemStorage()
	snapRole := kdb.GetRole(data.CanonicalSnapshotRole)

	for _, id := range snapRole.KeyIDs {
		k := kdb.GetKey(id)
		assert.NotNil(t, k)
		err := store.SetKey("testGUN", data.CanonicalSnapshotRole, k.Algorithm(), k.Public())
		assert.NoError(t, err)
	}

	r, tg, sn, ts, err := testutils.Sign(repo)
	assert.NoError(t, err)
	root, targets, snapshot, _, err := getUpdates(r, tg, sn, ts)
	assert.NoError(t, err)

	updates := []storage.MetaUpdate{root, targets}

	// set the current snapshot in the store manually so we find it when generating
	// the next version
	store.UpdateCurrent("testGUN", snapshot)

	prev, err := data.SnapshotFromSigned(sn)
	assert.NoError(t, err)

	copyTimestampKey(t, kdb, store, "testGUN")
	updates, err = validateUpdate(cs, "testGUN", updates, store)
	assert.NoError(t, err)

	for _, u := range updates {
		if u.Role == data.CanonicalSnapshotRole {
			curr := &data.SignedSnapshot{}
			err = json.Unmarshal(u.Data, curr)
			assert.Equal(t, prev.Signed.Version+1, curr.Signed.Version)
			assert.Equal(t, u.Version, curr.Signed.Version)
		}
	}
}

// downloadSnapshot is responsible for downloading the snapshot.json
func (c *Client) downloadSnapshot() error {
	logrus.Debug("downloadSnapshot")
	role := data.RoleName("snapshot")
	if c.local.Timestamp == nil {
		return ErrMissingMeta{role: "snapshot"}
	}
	size := c.local.Timestamp.Signed.Meta[role].Length
	expectedSha256, ok := c.local.Timestamp.Signed.Meta[role].Hashes["sha256"]
	if !ok {
		return ErrMissingMeta{role: "snapshot"}
	}

	var download bool
	old := &data.Signed{}
	version := 0
	raw, err := c.cache.GetMeta(role, size)
	if raw == nil || err != nil {
		logrus.Debug("no snapshot in cache, must download")
		download = true
	} else {
		// file may have been tampered with on disk. Always check the hash!
		genHash := sha256.Sum256(raw)
		if !bytes.Equal(genHash[:], expectedSha256) {
			logrus.Debug("hash of snapshot in cache did not match expected hash, must download")
			download = true
		}
		err := json.Unmarshal(raw, old)
		if err == nil {
			snap, err := data.TimestampFromSigned(old)
			if err == nil {
				version = snap.Signed.Version
			} else {
				logrus.Debug("Could not parse Signed part of snapshot, must download")
				download = true
			}
		} else {
			logrus.Debug("Could not parse snapshot, must download")
			download = true
		}
	}
	var s *data.Signed
	if download {
		raw, s, err = c.downloadSigned(role, size, expectedSha256)
		if err != nil {
			return err
		}
	} else {
		logrus.Debug("using cached snapshot")
		s = old
	}

	err = signed.Verify(s, role, version, c.keysDB)
	if err != nil {
		return err
	}
	logrus.Debug("successfully verified snapshot")
	snap, err := data.SnapshotFromSigned(s)
	if err != nil {
		return err
	}
	c.local.SetSnapshot(snap)
	if download {
		err = c.cache.SetMeta(role, raw)
		if err != nil {
			logrus.Errorf("Failed to write snapshot to local cache: %s", err.Error())
		}
	}
	return nil
}

// downloadSnapshot is responsible for downloading the snapshot.json
func (c *Client) downloadSnapshot() error {
	logrus.Debug("Downloading Snapshot...")
	role := data.CanonicalSnapshotRole
	if c.local.Timestamp == nil {
		return tuf.ErrNotLoaded{Role: data.CanonicalTimestampRole}
	}
	size := c.local.Timestamp.Signed.Meta[role].Length
	expectedHashes := c.local.Timestamp.Signed.Meta[role].Hashes
	if len(expectedHashes) == 0 {
		return data.ErrMissingMeta{Role: data.CanonicalSnapshotRole}
	}

	var download bool
	old := &data.Signed{}
	version := 0
	raw, err := c.cache.GetMeta(role, size)
	if raw == nil || err != nil {
		logrus.Debug("no snapshot in cache, must download")
		download = true
	} else {
		// file may have been tampered with on disk. Always check the hash!
		if err := data.CheckHashes(raw, expectedHashes); err != nil {
			logrus.Debug("hash of snapshot in cache did not match expected hash, must download")
			download = true
		}

		err := json.Unmarshal(raw, old)
		if err == nil {
			snap, err := data.SnapshotFromSigned(old)
			if err == nil {
				version = snap.Signed.Version
			} else {
				logrus.Debug("Could not parse Signed part of snapshot, must download")
				download = true
			}
		} else {
			logrus.Debug("Could not parse snapshot, must download")
			download = true
		}
	}
	var s *data.Signed
	if download {
		raw, s, err = c.downloadSigned(role, size, expectedHashes)
		if err != nil {
			return err
		}
	} else {
		logrus.Debug("using cached snapshot")
		s = old
	}

	snapshotRole, err := c.local.GetBaseRole(role)
	if err != nil {
		logrus.Debug("no snapshot role loaded")
		return err
	}
	err = signed.Verify(s, snapshotRole, version)
	if err != nil {
		return err
	}
	logrus.Debug("successfully verified snapshot")
	snap, err := data.SnapshotFromSigned(s)
	if err != nil {
		return err
	}
	c.local.SetSnapshot(snap)
	if download {
		err = c.cache.SetMeta(role, raw)
		if err != nil {
			logrus.Errorf("Failed to write snapshot to local cache: %s", err.Error())
		}
	}
	return nil
}