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// Package cryptoutil contains friendly wrappers around the algorithms from the
// standard library's crypto package.
package cryptoutil
import (
"crypto/aes"
"crypto/cipher"
"crypto/hkdf"
"crypto/hmac"
"crypto/pbkdf2"
"crypto/rand"
"crypto/sha256"
"crypto/subtle"
"encoding/hex"
"errors"
)
const (
defaultIterations = 3749890 // from cmd/finditers
saltSize = 2 * sha256.Size
hashSize = 32
certSize = sha256.Size
aesKeySize = 32
nonceSize = aes.BlockSize
)
// A PasswordHash must be PasswordHashSize bytes.
const PasswordHashSize = saltSize + hashSize
// A PasswordHash is derived from the user's password and can be passed to
// CheckPassword to verify if two passwords match. A PasswordHash must be
// PasswordHashSize bytes.
type PasswordHash []byte
func (h PasswordHash) String() string { return hex.EncodeToString(h) }
// A RawKey is generated from a password hash and can be used to derive further
// key material.
type RawKey struct {
key []byte
}
// An HMACKey must be HMACKeySize bytes.
const HMACKeySize = sha256.Size
// An HMAC key can be used to symmetrically sign and verify messages
// using HMAC-SHA256. An HMACKey must be HMACKeyLen bytes.
type HMACKey []byte
// A SignedMessage is a message that has been cryptographically signed
// with HMAC-SHA256.
type SignedMessage []byte
// An EncryptionKey must be EncryptionKeySize bytes.
const EncryptionKeySize = aesKeySize + HMACKeySize
// An EncryptionKey is used for encrypting and decrypting data. An EncryptionKey
// must be EncryptionKeySize bytes.
type EncryptionKey []byte
// An EncryptedMessage is encrypted with AES-256-CTR-HMAC-SHA256.
type EncryptedMessage []byte
// HashIter runs PBKDF2-SHA256 for iter iterations. Useful for benchmarking.
func HashIter(password string, salt []byte, iter int) ([]byte, error) {
return pbkdf2.Key(sha256.New, password, salt, iter, sha256.Size)
}
func hashWithSalt(password string, salt []byte) (RawKey, []byte, error) {
hash, err := HashIter(password, salt[:sha256.Size], defaultIterations)
if err != nil {
return RawKey{}, nil, err
}
key, err := hkdf.Extract(sha256.New, hash, salt[sha256.Size:])
if err != nil {
return RawKey{}, nil, err
}
pwHash, err := hkdf.Expand(sha256.New, key, "pwhash", hashSize)
if err != nil {
return RawKey{}, nil, err
}
return RawKey{key}, pwHash, nil
}
// Hash hashes a user password using PBKDF2-SHA256.
func Hash(password string) (PasswordHash, error) {
salt := make([]byte, saltSize, saltSize+hashSize)
rand.Read(salt)
_, pwHash, err := hashWithSalt(password, salt)
if err != nil {
return nil, err
}
return append(salt, pwHash...), nil
}
// CheckPassword verifies password against a PasswordHash and returns an
// EncryptionKey derived from the password if successful.
func (h PasswordHash) CheckPassword(password string) (RawKey, error) {
salt, expectedHash := h[:saltSize], h[saltSize:]
key, pwHash, err := hashWithSalt(password, salt)
if err != nil {
return RawKey{}, err
}
if subtle.ConstantTimeCompare(pwHash, expectedHash) == 0 {
return RawKey{}, errors.New("incorrect password")
}
return key, nil
}
// Sign generates an HMAC-SHA256 signature and appends it to msg.
func (k HMACKey) Sign(msg []byte) SignedMessage {
mac := hmac.New(sha256.New, k)
mac.Write(msg)
return mac.Sum(msg)
}
// Verify checks whether the given message has a valid signature, and returns
// the raw message if it does.
func (k HMACKey) Verify(msg SignedMessage) ([]byte, bool) {
if len(msg) < certSize {
return nil, false
}
msg, sig := msg[:len(msg)-certSize], msg[len(msg)-certSize:]
mac := hmac.New(sha256.New, k)
mac.Write(msg)
if !hmac.Equal(sig, mac.Sum(nil)) {
return nil, false
}
return msg, true
}
// EncryptionKey derives an EncryptionKey.
func (k RawKey) EncryptionKey() (EncryptionKey, error) {
return hkdf.Expand(sha256.New, k.key, "encrypt", EncryptionKeySize)
}
// Encrypt encrypts a message with AES-256-CTR-HMAC-SHA256.
func (k EncryptionKey) Encrypt(msg []byte) (EncryptedMessage, error) {
aesKey, hmacKey := k[:aesKeySize], HMACKey(k[aesKeySize:])
block, err := aes.NewCipher(aesKey)
if err != nil {
return nil, err
}
cipherText := make([]byte, len(msg)+nonceSize, len(msg)+nonceSize+certSize)
nonce := cipherText[len(msg) : len(msg)+nonceSize]
rand.Read(nonce)
cipher.NewCTR(block, nonce).XORKeyStream(cipherText, msg)
return EncryptedMessage(hmacKey.Sign(cipherText)), nil
}
// Decrypt verifies and decrypts an encrypted message. It overwrites msg with
// the resulting plain text.
func (k EncryptionKey) Decrypt(msg EncryptedMessage) ([]byte, error) {
aesKey, hmacKey := k[:aesKeySize], HMACKey(k[aesKeySize:])
msg, ok := hmacKey.Verify(SignedMessage(msg))
if !ok {
return nil, errors.New("bad signature")
}
block, err := aes.NewCipher(aesKey)
if err != nil {
return nil, err
}
msg, nonce := msg[:len(msg)-nonceSize], msg[len(msg)-nonceSize:]
cipher.NewCTR(block, nonce).XORKeyStream(msg, msg)
return msg, nil
}
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