keys.go

// Copyright © 2018 Playground Global, LLC
//
// 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 apksigner

import (
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"encoding/hex"
	"encoding/pem"
	"errors"
	"io/ioutil"
	"os"
	"path/filepath"
)

// SigningKey wraps a private key disk file with functions that know how to parse the key, and sign
// things with it. Currently only RSA keys and SHA-2/256 and SHA-2/512 digests are supported.
type SigningKey struct {
	KeyPath string
	Type    KeyAlgorithm
	Hash    HashAlgorithm
	Key     *rsa.PrivateKey
}

// Resolve loads the private key from disk and parses it. A non-nil error is returned if the parsing
// fails for any reason, or if the key type is unsupported.
func (sk *SigningKey) Resolve() error {
	var key *rsa.PrivateKey
	if sk.Type != RSA {
		// TODO: support EC
		return errors.New("elliptic curve support not currently implemented")
	}

	switch sk.Hash {
	case SHA256:
	case SHA512:
	default:
		return errors.New("unsupported hash algorithm was specified")
	}

	if sk.KeyPath == "" && sk.Key != nil {
		return nil
	}

	// parse private key
	someBytes, err := safeLoad(sk.KeyPath)
	if err != nil {
		return err
	}
	block, _ := pem.Decode(someBytes) // require cert to be in first block in file; ignore rest
	if block == nil {
		return errors.New("key does not decode as PEM")
	}
	switch sk.Type {
	case RSA:
		if block.Type != "RSA PRIVATE KEY" && block.Type != "PRIVATE KEY" {
			return errors.New("type set as RSA but PEM block does not look like a 'PRIVATE KEY'")
		}
		key, err = x509.ParsePKCS1PrivateKey(block.Bytes) // assumes ASN1 DER representation of a PKCS1 key
		if err != nil {
			//log.Debug("SigningKey.Resolve", "error parsing PKCS1 private key, retrying with PKCS8", err)

			keyPKCS8, err := x509.ParsePKCS8PrivateKey(block.Bytes)
			if err != nil {
				//log.Debug("SigningKey.Resolve", "error parsing PKCS8 private key", err)
				return err
			}
			key = keyPKCS8.(*rsa.PrivateKey)
		}
		sk.Key = key
		return nil

	case EC:
		// TODO: support EC
		return errors.New("EC not currently supported")

	default:
		return errors.New("unknown signing key type")
	}
}

// Sign returns the input bytes signed using the private key and the provided hash function. A
// non-nil error indicates that the signing operation failed for some reason, usually do to
// incorrect use of the configured cryptosystem.
//
// It is an error to call this function before Resolve(). Note again that currently only RSA is
// supported; the returned bytes will specifically be in binary DER-encoded PKCS#1v1.5 format.
func (sk *SigningKey) Sign(data []byte, hash crypto.Hash) ([]byte, error) {
	h := hash.New()
	h.Write(data)
	sum := h.Sum(nil)
	return sk.SignPrehashed(sum, hash)
}

// SignPrehashed is the same as Sign, except that its input bytes must be pre-hashed (or at least
// the same length as a digest under the provided crypto.Hash scheme.)
func (sk *SigningKey) SignPrehashed(data []byte, hash crypto.Hash) ([]byte, error) {
	res, err := rsa.SignPKCS1v15(rand.Reader, sk.Key, hash, data)
	if err != nil {
		//log.Debug("SigningKey.SignPrehashed", "error during sign", err)
	}
	return res, err
}

// SigningCert is a SigningKey that adds a public key Certificate.
type SigningCert struct {
	SigningKey
	CertPath    string
	Certificate *x509.Certificate
	CertHash    string
	CertBytes   []byte
}

// Resolve parses the PEM-encoded DER/ASN.1 X.509 certificate, as well as the private key (by
// calling SigningKey.Resolve() on itself.) A non-nil error is returned if the parsing fails for any
// reason, or on I/O errors.
func (sc *SigningCert) Resolve() error {
	err := sc.SigningKey.Resolve()
	if err != nil {
		return err
	}

	// parse Certificate
	var someBytes []byte
	if sc.CertPath != "" && sc.CertBytes == nil {
		someBytes, err = safeLoad(sc.CertPath)
	} else {
		someBytes = sc.CertBytes
	}
	if err != nil {
		return err
	}
	block, _ := pem.Decode(someBytes) // require cert to be in first block in file; ignore rest
	if block == nil {
		return errors.New("certificate does not decode as PEM")
	}
	cert, err := x509.ParseCertificate(block.Bytes) // assumes ASN1 DER
	if err != nil {
		return err
	}
	b := sha256.Sum256(cert.Raw)
	certHash := hex.EncodeToString(b[:]) // cert.Raw == block.Bytes

	switch sc.Type {
	case RSA:
		switch cert.PublicKey.(type) {
		case *rsa.PublicKey:
		default:
			return errors.New("type set as RSA but certificate doesn't contain RSA public key")
		}
		certPubKey := cert.PublicKey.(*rsa.PublicKey)
		if sc.Key.N.Cmp(certPubKey.N) != 0 || sc.Key.E != certPubKey.E {
			//log.Debug("SigningCert.Resolve", "certificate public key does not match private key's copy", sc.Key.N, certPubKey.N, sc.Key.E, certPubKey.E)
			return errors.New("certificate public key does not match private key's copy")
		}
		sc.Certificate, sc.CertHash = cert, certHash
		return nil

	case EC:
		// TODO: support EC
		return errors.New("EC not currently supported")

	default:
		return errors.New("unknown signing key type")
	}
}

func safeLoad(path string) ([]byte, error) {
	var err error
	/*
		myPath := ""
		if myPath, err = filepath.Abs(filepath.Dir(os.Args[0])); err != nil {
			log.Error("android.safeLoad", "could not locate executable directory", err)
			return nil, err
		}
	*/
	if path, err = filepath.Abs(path); err != nil {
		//log.Error("android.safeLoad", "file '"+path+"' does not resolve")
		return nil, err
	}
	/* This turns out to be spammy and restrictive. Revisit this later.
	if !strings.HasPrefix(path, myPath) {
		log.Warn("android.safeLoad", "path '"+path+"' is not under executable pwd '" + myPath + "'")
	}
	*/
	if stat, err := os.Stat(path); err != nil || (stat != nil && stat.IsDir()) {
		//log.Error("android.safeLoad", "file '"+path+"' does not stat or is a directory", err)
		return nil, err
	}
	fileBytes, err := ioutil.ReadFile(path)
	if err != nil {
		//log.Error("android.safeLoad", "file '"+path+"' failed to load", err)
		return nil, err
	}
	return fileBytes, nil
}