AK: Implement SipHash as the default hash algorithm for most use cases

SipHash is highly HashDoS-resistent, initialized with a random seed at
startup (i.e. non-deterministic) and usable for security-critical use
cases with large enough parameters. We just use it because it's
reasonably secure with parameters 1-3 while having excellent properties
and not being significantly slower than before.

AK/SipHash.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace AK {
+
+// Ported from the SipHash reference implementation, released to the public domain:
+// https://github.com/veorq/SipHash/blob/eee7d0d84dc7731df2359b243aa5e75d85f6eaef/siphash.c
+// The standard is SipHash-2-4, but we use 1-3 for a little more speed.
+// Cryptography should use 4-8 for (relative) conservative security,
+// though SipHash itself is NOT a cryptographically secure hash algorithm.
+template<size_t message_block_rounds, size_t finalization_rounds>
+u64 sip_hash_bytes(ReadonlyBytes input);
+unsigned standard_sip_hash(u64 input);
+unsigned secure_sip_hash(u64 input);
+
+}
+
+#ifdef USING_AK_GLOBALLY
+using AK::secure_sip_hash;
+using AK::sip_hash_bytes;
+using AK::standard_sip_hash;
+#endif