1 files changed, 43 insertions, 0 deletions
diff --git a/identifier.go b/identifier.go
new file mode 100644
index 0000000..6da0e7c
--- /dev/null
+++ b/identifier.go
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// SPDX-FileCopyrightText: Copyright (c) 2025 Runxi Yu <https://runxiyu.org>
+
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/binary"
+	"net/http"
+	"time"
+)
+
+func makeIdentifierMAC(request *http.Request) (identifier []byte, mac []byte) {
+	identifier = make([]byte, 0, sha256.Size)
+	mac = make([]byte, 0, sha256.Size)
+
+	timeBuf := make([]byte, binary.MaxVarintLen64)
+	binary.PutVarint(timeBuf, time.Now().Unix()/604800)
+
+	remoteIP := getRemoteIP(request)
+
+	h := sha256.New()
+	h.Write(timeBuf)
+	h.Write(stringToBytes(remoteIP))
+	h.Write(stringToBytes(request.Header.Get("User-Agent")))
+	h.Write(stringToBytes(request.Header.Get("Accept-Encoding")))
+	h.Write(stringToBytes(request.Header.Get("Accept-Language")))
+	h.Write(privkeyHash)
+	identifier = h.Sum(identifier)
+	if len(identifier) != sha256.Size {
+		panic("unexpected buffer length after hashing contents")
+	}
+
+	m := hmac.New(sha256.New, privkey)
+	m.Write(identifier)
+	mac = m.Sum(mac)
+	if len(mac) != sha256.Size {
+		panic("unexpected buffer length after hmac")
+	}
+
+	return
+}