helm/pkg/provenance/sign.go

/*
Copyright 2016 The Kubernetes Authors All rights reserved.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package provenance

import (
	"bytes"
	"crypto"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"

	"github.com/ghodss/yaml"

	"golang.org/x/crypto/openpgp"
	"golang.org/x/crypto/openpgp/clearsign"
	"golang.org/x/crypto/openpgp/packet"

	"k8s.io/helm/pkg/chartutil"
	hapi "k8s.io/helm/pkg/proto/hapi/chart"
)

var defaultPGPConfig = packet.Config{
	DefaultHash: crypto.SHA512,
}

// SumCollection represents a collecton of file and image checksums.
//
// Files are of the form:
//	FILENAME: "sha256:SUM"
// Images are of the form:
//	"IMAGE:TAG": "sha256:SUM"
// Docker optionally supports sha512, and if this is the case, the hash marker
// will be 'sha512' instead of 'sha256'.
type SumCollection struct {
	Files  map[string]string `json:"files"`
	Images map[string]string `json:"images,omitempty"`
}

// Signatory signs things.
//
// Signatories can be constructed from a PGP private key file using NewFromFiles
// or they can be constructed manually by setting the Entity to a valid
// PGP entity.
//
// The same Signatory can be used to sign or validate multiple charts.
type Signatory struct {
	// The signatory for this instance of Helm. This is used for signing.
	Entity *openpgp.Entity
	// The keyring for this instance of Helm. This is used for verification.
	KeyRing openpgp.EntityList
}

// NewFromFiles constructs a new Signatory from the PGP key in the given filename.
//
// This will emit an error if it cannot find a valid GPG keyfile (entity) at the
// given location.
//
// Note that the keyfile may have just a public key, just a private key, or
// both. The Signatory methods may have different requirements of the keys. For
// example, ClearSign must have a valid `openpgp.Entity.PrivateKey` before it
// can sign something.
func NewFromFiles(keyfile, keyringfile string) (*Signatory, error) {
	e, err := loadKey(keyfile)
	if err != nil {
		return nil, err
	}

	ring, err := loadKeyRing(keyringfile)
	if err != nil {
		return nil, err
	}

	return &Signatory{
		Entity:  e,
		KeyRing: ring,
	}, nil
}

// Sign signs a chart with the given key.
//
// This takes the path to a chart archive file and a key, and it returns a clear signature.
//
// The Signatory must have a valid Entity.PrivateKey for this to work. If it does
// not, an error will be returned.
func (s *Signatory) ClearSign(chartpath string) (string, error) {
	if s.Entity.PrivateKey == nil {
		return "", errors.New("private key not found")
	}

	if fi, err := os.Stat(chartpath); err != nil {
		return "", err
	} else if fi.IsDir() {
		return "", errors.New("cannot sign a directory")
	}

	out := bytes.NewBuffer(nil)

	b, err := messageBlock(chartpath)
	if err != nil {
		return "", nil
	}

	// Sign the buffer
	w, err := clearsign.Encode(out, s.Entity.PrivateKey, &defaultPGPConfig)
	if err != nil {
		return "", err
	}
	_, err = io.Copy(w, b)
	w.Close()
	return out.String(), err
}

func (s *Signatory) Verify(chartpath, sigpath string) (bool, error) {
	for _, fname := range []string{chartpath, sigpath} {
		if fi, err := os.Stat(fname); err != nil {
			return false, err
		} else if fi.IsDir() {
			return false, fmt.Errorf("%s cannot be a directory", fname)
		}
	}

	// First verify the signature
	sig, err := s.decodeSignature(sigpath)
	if err != nil {
		return false, fmt.Errorf("failed to decode signature: %s", err)
	}

	by, err := s.verifySignature(sig)
	if err != nil {
		return false, err
	}
	for n := range by.Identities {
		log.Printf("info: %s signed by %q", sigpath, n)
	}

	// Second, verify the hash of the tarball.

	return true, nil
}

func (s *Signatory) decodeSignature(filename string) (*clearsign.Block, error) {
	data, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}

	block, _ := clearsign.Decode(data)
	if block == nil {
		// There was no sig in the file.
		return nil, errors.New("signature block not found")
	}

	return block, nil
}

// verifySignature verifies that the given block is validly signed, and returns the signer.
func (s *Signatory) verifySignature(block *clearsign.Block) (*openpgp.Entity, error) {
	return openpgp.CheckDetachedSignature(
		s.KeyRing,
		bytes.NewBuffer(block.Bytes),
		block.ArmoredSignature.Body,
	)
}

func messageBlock(chartpath string) (*bytes.Buffer, error) {
	var b *bytes.Buffer
	// Checksum the archive
	chash, err := sumArchive(chartpath)
	if err != nil {
		return b, err
	}

	base := filepath.Base(chartpath)
	sums := &SumCollection{
		Files: map[string]string{
			base: "sha256:" + chash,
		},
	}

	// Load the archive into memory.
	chart, err := chartutil.LoadFile(chartpath)
	if err != nil {
		return b, err
	}

	// Buffer a hash + checksums YAML file
	data, err := yaml.Marshal(chart.Metadata)
	if err != nil {
		return b, err
	}

	// FIXME: YAML uses ---\n as a file start indicator, but this is not legal in a PGP
	// clearsign block. So we use ...\n, which is the YAML document end marker.
	// http://yaml.org/spec/1.2/spec.html#id2800168
	b = bytes.NewBuffer(data)
	b.WriteString("\n...\n")

	data, err = yaml.Marshal(sums)
	if err != nil {
		return b, err
	}
	b.Write(data)

	return b, nil
}

// parseMessageBlock
func parseMessageBlock(data []byte) (*hapi.Metadata, *SumCollection, error) {
	// This sucks.
	parts := bytes.Split(data, []byte("\n...\n"))
	if len(parts) < 2 {
		return nil, nil, errors.New("message block must have at least two parts")
	}

	md := &hapi.Metadata{}
	sc := &SumCollection{}

	if err := yaml.Unmarshal(parts[0], md); err != nil {
		return md, sc, err
	}
	err := yaml.Unmarshal(parts[1], sc)
	return md, sc, err
}

// loadKey loads a GPG key found at a particular path.
func loadKey(keypath string) (*openpgp.Entity, error) {
	f, err := os.Open(keypath)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	pr := packet.NewReader(f)
	return openpgp.ReadEntity(pr)
}

func loadKeyRing(ringpath string) (openpgp.EntityList, error) {
	f, err := os.Open(ringpath)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	return openpgp.ReadKeyRing(f)
}

// sumArchive calculates a SHA256 hash (like Docker) for a given file.
//
// It takes the path to the archive file, and returns a string representation of
// the SHA256 sum.
//
// The intended use of this function is to generate a sum of a chart TGZ file.
func sumArchive(filename string) (string, error) {
	f, err := os.Open(filename)
	if err != nil {
		return "", err
	}
	defer f.Close()

	hash := crypto.SHA256.New()
	io.Copy(hash, f)
	return hex.EncodeToString(hash.Sum(nil)), nil
}