asymmetric.go

/*-
 * Copyright 2014 Square Inc.
 *
 * 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 jose

import (
	"crypto"
	"crypto/aes"
	"crypto/ecdsa"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha1"
	"crypto/sha256"
	"errors"
	"fmt"
	"math/big"

	"gopkg.in/square/go-jose.v1/cipher"
)

// A generic RSA-based encrypter/verifier
type rsaEncrypterVerifier struct {
	publicKey *rsa.PublicKey
}

// A generic RSA-based decrypter/signer
type rsaDecrypterSigner struct {
	privateKey *rsa.PrivateKey
}

// A generic EC-based encrypter/verifier
type ecEncrypterVerifier struct {
	publicKey *ecdsa.PublicKey
}

// A key generator for ECDH-ES
type ecKeyGenerator struct {
	size      int
	algID     string
	publicKey *ecdsa.PublicKey
}

// A generic EC-based decrypter/signer
type ecDecrypterSigner struct {
	privateKey *ecdsa.PrivateKey
}

// newRSARecipient creates recipientKeyInfo based on the given key.
func newRSARecipient(keyAlg KeyAlgorithm, publicKey *rsa.PublicKey) (recipientKeyInfo, error) {
	// Verify that key management algorithm is supported by this encrypter
	switch keyAlg {
	case RSA1_5, RSA_OAEP, RSA_OAEP_256:
	default:
		return recipientKeyInfo{}, ErrUnsupportedAlgorithm
	}

	return recipientKeyInfo{
		keyAlg: keyAlg,
		keyEncrypter: &rsaEncrypterVerifier{
			publicKey: publicKey,
		},
	}, nil
}

// newRSASigner creates a recipientSigInfo based on the given key.
func newRSASigner(sigAlg SignatureAlgorithm, privateKey *rsa.PrivateKey) (recipientSigInfo, error) {
	// Verify that key management algorithm is supported by this encrypter
	switch sigAlg {
	case RS256, RS384, RS512, PS256, PS384, PS512:
	default:
		return recipientSigInfo{}, ErrUnsupportedAlgorithm
	}

	return recipientSigInfo{
		sigAlg: sigAlg,
		publicKey: &JsonWebKey{
			Key: &privateKey.PublicKey,
		},
		signer: &rsaDecrypterSigner{
			privateKey: privateKey,
		},
	}, nil
}

// newECDHRecipient creates recipientKeyInfo based on the given key.
func newECDHRecipient(keyAlg KeyAlgorithm, publicKey *ecdsa.PublicKey) (recipientKeyInfo, error) {
	// Verify that key management algorithm is supported by this encrypter
	switch keyAlg {
	case ECDH_ES, ECDH_ES_A128KW, ECDH_ES_A192KW, ECDH_ES_A256KW:
	default:
		return recipientKeyInfo{}, ErrUnsupportedAlgorithm
	}

	return recipientKeyInfo{
		keyAlg: keyAlg,
		keyEncrypter: &ecEncrypterVerifier{
			publicKey: publicKey,
		},
	}, nil
}

// newECDSASigner creates a recipientSigInfo based on the given key.
func newECDSASigner(sigAlg SignatureAlgorithm, privateKey *ecdsa.PrivateKey) (recipientSigInfo, error) {
	// Verify that key management algorithm is supported by this encrypter
	switch sigAlg {
	case ES256, ES384, ES512:
	default:
		return recipientSigInfo{}, ErrUnsupportedAlgorithm
	}

	return recipientSigInfo{
		sigAlg: sigAlg,
		publicKey: &JsonWebKey{
			Key: &privateKey.PublicKey,
		},
		signer: &ecDecrypterSigner{
			privateKey: privateKey,
		},
	}, nil
}

// Encrypt the given payload and update the object.
func (ctx rsaEncrypterVerifier) encryptKey(cek []byte, alg KeyAlgorithm) (recipientInfo, error) {
	encryptedKey, err := ctx.encrypt(cek, alg)
	if err != nil {
		return recipientInfo{}, err
	}

	return recipientInfo{
		encryptedKey: encryptedKey,
		header:       &rawHeader{},
	}, nil
}

// Encrypt the given payload. Based on the key encryption algorithm,
// this will either use RSA-PKCS1v1.5 or RSA-OAEP (with SHA-1 or SHA-256).
func (ctx rsaEncrypterVerifier) encrypt(cek []byte, alg KeyAlgorithm) ([]byte, error) {
	switch alg {
	case RSA1_5:
		return rsa.EncryptPKCS1v15(randReader, ctx.publicKey, cek)
	case RSA_OAEP:
		return rsa.EncryptOAEP(sha1.New(), randReader, ctx.publicKey, cek, []byte{})
	case RSA_OAEP_256:
		return rsa.EncryptOAEP(sha256.New(), randReader, ctx.publicKey, cek, []byte{})
	}

	return nil, ErrUnsupportedAlgorithm
}

// Decrypt the given payload and return the content encryption key.
func (ctx rsaDecrypterSigner) decryptKey(headers rawHeader, recipient *recipientInfo, generator keyGenerator) ([]byte, error) {
	return ctx.decrypt(recipient.encryptedKey, KeyAlgorithm(headers.Alg), generator)
}

// Decrypt the given payload. Based on the key encryption algorithm,
// this will either use RSA-PKCS1v1.5 or RSA-OAEP (with SHA-1 or SHA-256).
func (ctx rsaDecrypterSigner) decrypt(jek []byte, alg KeyAlgorithm, generator keyGenerator) ([]byte, error) {
	// Note: The random reader on decrypt operations is only used for blinding,
	// so stubbing is meanlingless (hence the direct use of rand.Reader).
	switch alg {
	case RSA1_5:
		defer func() {
			// DecryptPKCS1v15SessionKey sometimes panics on an invalid payload
			// because of an index out of bounds error, which we want to ignore.
			// This has been fixed in Go 1.3.1 (released 2014/08/13), the recover()
			// only exists for preventing crashes with unpatched versions.
			// See: https://groups.google.com/forum/#!topic/golang-dev/7ihX6Y6kx9k
			// See: https://code.google.com/p/go/source/detail?r=58ee390ff31602edb66af41ed10901ec95904d33
			_ = recover()
		}()

		// Perform some input validation.
		keyBytes := ctx.privateKey.PublicKey.N.BitLen() / 8
		if keyBytes != len(jek) {
			// Input size is incorrect, the encrypted payload should always match
			// the size of the public modulus (e.g. using a 2048 bit key will
			// produce 256 bytes of output). Reject this since it's invalid input.
			return nil, ErrCryptoFailure
		}

		cek, _, err := generator.genKey()
		if err != nil {
			return nil, ErrCryptoFailure
		}

		// When decrypting an RSA-PKCS1v1.5 payload, we must take precautions to
		// prevent chosen-ciphertext attacks as described in RFC 3218, "Preventing
		// the Million Message Attack on Cryptographic Message Syntax". We are
		// therefore deliberatly ignoring errors here.
		_ = rsa.DecryptPKCS1v15SessionKey(rand.Reader, ctx.privateKey, jek, cek)

		return cek, nil
	case RSA_OAEP:
		// Use rand.Reader for RSA blinding
		return rsa.DecryptOAEP(sha1.New(), rand.Reader, ctx.privateKey, jek, []byte{})
	case RSA_OAEP_256:
		// Use rand.Reader for RSA blinding
		return rsa.DecryptOAEP(sha256.New(), rand.Reader, ctx.privateKey, jek, []byte{})
	}

	return nil, ErrUnsupportedAlgorithm
}

// Sign the given payload
func (ctx rsaDecrypterSigner) signPayload(payload []byte, alg SignatureAlgorithm) (Signature, error) {
	var hash crypto.Hash

	switch alg {
	case RS256, PS256:
		hash = crypto.SHA256
	case RS384, PS384:
		hash = crypto.SHA384
	case RS512, PS512:
		hash = crypto.SHA512
	default:
		return Signature{}, ErrUnsupportedAlgorithm
	}

	hasher := hash.New()

	// According to documentation, Write() on hash never fails
	_, _ = hasher.Write(payload)
	hashed := hasher.Sum(nil)

	var out []byte
	var err error

	switch alg {
	case RS256, RS384, RS512:
		out, err = rsa.SignPKCS1v15(randReader, ctx.privateKey, hash, hashed)
	case PS256, PS384, PS512:
		out, err = rsa.SignPSS(randReader, ctx.privateKey, hash, hashed, &rsa.PSSOptions{
			SaltLength: rsa.PSSSaltLengthAuto,
		})
	}

	if err != nil {
		return Signature{}, err
	}

	return Signature{
		Signature: out,
		protected: &rawHeader{},
	}, nil
}

// Verify the given payload
func (ctx rsaEncrypterVerifier) verifyPayload(payload []byte, signature []byte, alg SignatureAlgorithm) error {
	var hash crypto.Hash

	switch alg {
	case RS256, PS256:
		hash = crypto.SHA256
	case RS384, PS384:
		hash = crypto.SHA384
	case RS512, PS512:
		hash = crypto.SHA512
	default:
		return ErrUnsupportedAlgorithm
	}

	hasher := hash.New()

	// According to documentation, Write() on hash never fails
	_, _ = hasher.Write(payload)
	hashed := hasher.Sum(nil)

	switch alg {
	case RS256, RS384, RS512:
		return rsa.VerifyPKCS1v15(ctx.publicKey, hash, hashed, signature)
	case PS256, PS384, PS512:
		return rsa.VerifyPSS(ctx.publicKey, hash, hashed, signature, nil)
	}

	return ErrUnsupportedAlgorithm
}

// Encrypt the given payload and update the object.
func (ctx ecEncrypterVerifier) encryptKey(cek []byte, alg KeyAlgorithm) (recipientInfo, error) {
	switch alg {
	case ECDH_ES:
		// ECDH-ES mode doesn't wrap a key, the shared secret is used directly as the key.
		return recipientInfo{
			header: &rawHeader{},
		}, nil
	case ECDH_ES_A128KW, ECDH_ES_A192KW, ECDH_ES_A256KW:
	default:
		return recipientInfo{}, ErrUnsupportedAlgorithm
	}

	generator := ecKeyGenerator{
		algID:     string(alg),
		publicKey: ctx.publicKey,
	}

	switch alg {
	case ECDH_ES_A128KW:
		generator.size = 16
	case ECDH_ES_A192KW:
		generator.size = 24
	case ECDH_ES_A256KW:
		generator.size = 32
	}

	kek, header, err := generator.genKey()
	if err != nil {
		return recipientInfo{}, err
	}

	block, err := aes.NewCipher(kek)
	if err != nil {
		return recipientInfo{}, err
	}

	jek, err := josecipher.KeyWrap(block, cek)
	if err != nil {
		return recipientInfo{}, err
	}

	return recipientInfo{
		encryptedKey: jek,
		header:       &header,
	}, nil
}

// Get key size for EC key generator
func (ctx ecKeyGenerator) keySize() int {
	return ctx.size
}

// Get a content encryption key for ECDH-ES
func (ctx ecKeyGenerator) genKey() ([]byte, rawHeader, error) {
	priv, err := ecdsa.GenerateKey(ctx.publicKey.Curve, randReader)
	if err != nil {
		return nil, rawHeader{}, err
	}

	out := josecipher.DeriveECDHES(ctx.algID, []byte{}, []byte{}, priv, ctx.publicKey, ctx.size)

	headers := rawHeader{
		Epk: &JsonWebKey{
			Key: &priv.PublicKey,
		},
	}

	return out, headers, nil
}

// Decrypt the given payload and return the content encryption key.
func (ctx ecDecrypterSigner) decryptKey(headers rawHeader, recipient *recipientInfo, generator keyGenerator) ([]byte, error) {
	if headers.Epk == nil {
		return nil, errors.New("square/go-jose: missing epk header")
	}

	publicKey, ok := headers.Epk.Key.(*ecdsa.PublicKey)
	if publicKey == nil || !ok {
		return nil, errors.New("square/go-jose: invalid epk header")
	}

	apuData := headers.Apu.bytes()
	apvData := headers.Apv.bytes()

	deriveKey := func(algID string, size int) []byte {
		return josecipher.DeriveECDHES(algID, apuData, apvData, ctx.privateKey, publicKey, size)
	}

	var keySize int

	switch KeyAlgorithm(headers.Alg) {
	case ECDH_ES:
		// ECDH-ES uses direct key agreement, no key unwrapping necessary.
		return deriveKey(string(headers.Enc), generator.keySize()), nil
	case ECDH_ES_A128KW:
		keySize = 16
	case ECDH_ES_A192KW:
		keySize = 24
	case ECDH_ES_A256KW:
		keySize = 32
	default:
		return nil, ErrUnsupportedAlgorithm
	}

	key := deriveKey(headers.Alg, keySize)
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	return josecipher.KeyUnwrap(block, recipient.encryptedKey)
}

// Sign the given payload
func (ctx ecDecrypterSigner) signPayload(payload []byte, alg SignatureAlgorithm) (Signature, error) {
	var expectedBitSize int
	var hash crypto.Hash

	switch alg {
	case ES256:
		expectedBitSize = 256
		hash = crypto.SHA256
	case ES384:
		expectedBitSize = 384
		hash = crypto.SHA384
	case ES512:
		expectedBitSize = 521
		hash = crypto.SHA512
	}

	curveBits := ctx.privateKey.Curve.Params().BitSize
	if expectedBitSize != curveBits {
		return Signature{}, fmt.Errorf("square/go-jose: expected %d bit key, got %d bits instead", expectedBitSize, curveBits)
	}

	hasher := hash.New()

	// According to documentation, Write() on hash never fails
	_, _ = hasher.Write(payload)
	hashed := hasher.Sum(nil)

	r, s, err := ecdsa.Sign(randReader, ctx.privateKey, hashed)
	if err != nil {
		return Signature{}, err
	}

	keyBytes := curveBits / 8
	if curveBits%8 > 0 {
		keyBytes += 1
	}

	// We serialize the outpus (r and s) into big-endian byte arrays and pad
	// them with zeros on the left to make sure the sizes work out. Both arrays
	// must be keyBytes long, and the output must be 2*keyBytes long.
	rBytes := r.Bytes()
	rBytesPadded := make([]byte, keyBytes)
	copy(rBytesPadded[keyBytes-len(rBytes):], rBytes)

	sBytes := s.Bytes()
	sBytesPadded := make([]byte, keyBytes)
	copy(sBytesPadded[keyBytes-len(sBytes):], sBytes)

	out := append(rBytesPadded, sBytesPadded...)

	return Signature{
		Signature: out,
		protected: &rawHeader{},
	}, nil
}

// Verify the given payload
func (ctx ecEncrypterVerifier) verifyPayload(payload []byte, signature []byte, alg SignatureAlgorithm) error {
	var keySize int
	var hash crypto.Hash

	switch alg {
	case ES256:
		keySize = 32
		hash = crypto.SHA256
	case ES384:
		keySize = 48
		hash = crypto.SHA384
	case ES512:
		keySize = 66
		hash = crypto.SHA512
	}

	if len(signature) != 2*keySize {
		return fmt.Errorf("square/go-jose: invalid signature size, have %d bytes, wanted %d", len(signature), 2*keySize)
	}

	hasher := hash.New()

	// According to documentation, Write() on hash never fails
	_, _ = hasher.Write(payload)
	hashed := hasher.Sum(nil)

	r := big.NewInt(0).SetBytes(signature[:keySize])
	s := big.NewInt(0).SetBytes(signature[keySize:])

	match := ecdsa.Verify(ctx.publicKey, hashed, r, s)
	if !match {
		return errors.New("square/go-jose: ecdsa signature failed to verify")
	}

	return nil
}