cookie_decrypt

cookie/keyorpassword.go

@@ -0,0 +1,67 @@
+package cookie
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"errors"
+	"hash"
+)
+
+type KeyOrPassword struct {
+	SecretType  int
+	Key         *Key
+	Password    string
+	DerivedKeys *DerivedKeys
+	Akey        []byte
+	Ekey        []byte
+}
+
+type DerivedKeys struct{ akey, ekey []byte }
+
+type Key struct{ bytes []byte }
+
+func (k *Key) GetRawBytes() []byte { return k.bytes }
+
+func (kp *KeyOrPassword) DeriveKeys(salt []byte) (*DerivedKeys, error) {
+	if len(salt) != SaltSize {
+		return nil, errors.New("bad salt")
+	}
+	if kp.SecretType == 1 {
+		akey := kp.hkdf(sha256.New, kp.Key.GetRawBytes(), KeyByteSize, AuthInfo, salt)
+		ekey := kp.hkdf(sha256.New, kp.Key.GetRawBytes(), KeyByteSize, EncInfo, salt)
+		return &DerivedKeys{akey: akey, ekey: ekey}, nil
+	}
+	return nil, errors.New("unsupported")
+}
+
+// === HKDF ===
+func (kp *KeyOrPassword) hkdf(hashFunc func() hash.Hash, ikm []byte, length int, info string, salt []byte) []byte {
+	digestLen := hashFunc().Size()
+	if salt == nil {
+		salt = make([]byte, digestLen)
+	}
+
+	// Extract
+	prkMac := hmac.New(hashFunc, salt)
+	prkMac.Write(ikm)
+	prk := prkMac.Sum(nil)
+
+	// Expand
+	var okm []byte
+	prev := []byte{}
+	counter := byte(1)
+
+	for len(okm) < length {
+		h := hmac.New(hashFunc, prk)
+		h.Write(prev)
+		h.Write([]byte(info))
+		h.Write([]byte{counter})
+		step := h.Sum(nil)
+
+		okm = append(okm, step...)
+		prev = step
+		counter++
+	}
+
+	return okm[:length]
+}