diff --git a/src/signature.cpp b/src/signature.cpp
new file mode 100644
--- /dev/null
+++ b/src/signature.cpp
@@ -0,0 +1,279 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file signature.cpp Implementation of signature validation routines. */
+
+#include "stdafx.h"
+
+#include "signature.h"
+
+#include "debug.h"
+#include "fileio_func.h"
+#include "string_func.h"
+
+#include "3rdparty/monocypher/monocypher.h"
+#include "3rdparty/monocypher/monocypher-ed25519.h"
+#include "3rdparty/nlohmann/json.hpp"
+
+#include "safeguards.h"
+
+/** The public keys used for signature validation. */
+static const std::initializer_list<std::array<uint8_t, 32>> _public_keys_v1 = {
+	/* 2024-01-20 - Public key for Social Integration Plugins. */
+	{ 0xed, 0x5d, 0x57, 0x47, 0x21, 0x99, 0x8b, 0x02, 0xdf, 0x6e, 0x3d, 0x69, 0xe1, 0x87, 0xca, 0xd0, 0x0e, 0x88, 0xc3, 0xe2, 0xb2, 0xa6, 0x7b, 0xc0, 0x42, 0xc8, 0xd6, 0x4b, 0x65, 0xe6, 0x48, 0xf7 },
+};
+
+/**
+ * Calculate the 32-byte blake2b hash of a file.
+ *
+ * @param filename The filename to calculate the hash of.
+ * @return The 32-byte blake2b hash of the file, hex-encoded.
+ */
+static std::string CalculateHashV1(const std::string &filename)
+{
+	FILE *f = FioFOpenFile(filename, "rb", NO_DIRECTORY);
+	if (f == nullptr) {
+		return "";
+	}
+
+	std::array<uint8_t, 32> digest;
+	crypto_blake2b_ctx ctx;
+	crypto_blake2b_init(&ctx, digest.size());
+
+	while (!feof(f)) {
+		std::array<uint8_t, 1024> buf;
+		size_t len = fread(buf.data(), 1, buf.size(), f);
+
+		crypto_blake2b_update(&ctx, buf.data(), len);
+	}
+	fclose(f);
+
+	crypto_blake2b_final(&ctx, digest.data());
+	return FormatArrayAsHex(digest);
+}
+
+/**
+ * Validate whether the checksum of a file is the same.
+ *
+ * @param filename The filename to validate the checksum of.
+ * @param checksum The expected checksum.
+ * @return True iff the checksum of the file is the same as the expected checksum.
+ */
+static bool ValidateChecksum(const std::string &filename, const std::string &checksum)
+{
+	/* Checksums are "<version>$<hash>". Split out the version. */
+	auto pos = checksum.find('$');
+	assert(pos != std::string::npos); // Already validated by ValidateSchema().
+	const std::string version = checksum.substr(0, pos);
+	const std::string hash = checksum.substr(pos + 1);
+
+	/* Calculate the checksum over the file. */
+	std::string calculated_hash;
+	if (version == "1") {
+		calculated_hash = CalculateHashV1(filename);
+	} else {
+		Debug(misc, 0, "Failed to validate signature: unknown checksum version: {}", filename);
+		return false;
+	}
+
+	/* Validate the checksum is the same. */
+	if (calculated_hash.empty()) {
+		Debug(misc, 0, "Failed to validate signature: couldn't calculate checksum for: {}", filename);
+		return false;
+	}
+	if (calculated_hash != hash) {
+		Debug(misc, 0, "Failed to validate signature: checksum mismatch for: {}", filename);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * Validate whether the signature is valid for this set of files.
+ *
+ * @param signature The signature to validate.
+ * @param files The files to validate the signature against.
+ * @param filename The filename of the signatures file (for error-reporting).
+ * @return True iff the signature is valid for this set of files.
+ */
+static bool ValidateSignature(const std::string &signature, const nlohmann::json &files, const std::string &filename)
+{
+	/* Signatures are "<version>$<signature>". Split out the version. */
+	auto pos = signature.find('$');
+	assert(pos != std::string::npos); // Already validated by ValidateSchema().
+	const std::string version = signature.substr(0, pos);
+	const std::string sig_value = signature.substr(pos + 1);
+
+	/* Create the message we are going to validate. */
+	std::string message = files.dump(-1);
+
+	/* Validate the signature. */
+	if (version == "1") {
+		std::array<uint8_t, 64> sig;
+		if (sig_value.size() != 128 || !ConvertHexToBytes(sig_value, sig)) {
+			Debug(misc, 0, "Failed to validate signature: invalid signature: {}", filename);
+			return false;
+		}
+
+		for (auto &pk_value : _public_keys_v1) {
+			/* Check if the message is valid with this public key. */
+			auto res = crypto_ed25519_check(sig.data(), pk_value.data(), reinterpret_cast<uint8_t *>(message.data()), message.size());
+			if (res == 0) {
+				return true;
+			}
+		}
+
+		Debug(misc, 0, "Failed to validate signature: signature validation failed: {}", filename);
+		return false;
+	} else {
+		Debug(misc, 0, "Failed to validate signature: unknown signature version: {}", filename);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * Validate the signatures file complies with the JSON schema.
+ *
+ * @param signatures The signatures JSON to validate.
+ * @param filename The filename of the signatures file (for error-reporting).
+ * @return True iff the signatures file complies with the JSON schema.
+ */
+static bool ValidateSchema(const nlohmann::json &signatures, const std::string &filename)
+{
+	if (signatures["files"].is_null()) {
+		Debug(misc, 0, "Failed to validate signature: no files found: {}", filename);
+		return false;
+	}
+
+	if (signatures["signature"].is_null()) {
+		Debug(misc, 0, "Failed to validate signature: no signature found: {}", filename);
+		return false;
+	}
+
+	for (auto &signature : signatures["files"]) {
+		if (signature["filename"].is_null() || signature["checksum"].is_null()) {
+			Debug(misc, 0, "Failed to validate signature: invalid entry in files: {}", filename);
+			return false;
+		}
+
+		const std::string sig_filename = signature["filename"];
+		const std::string sig_checksum = signature["checksum"];
+
+		if (sig_filename.empty() || sig_checksum.empty()) {
+			Debug(misc, 0, "Failed to validate signature: invalid entry in files: {}", filename);
+			return false;
+		}
+
+		auto pos = sig_checksum.find('$');
+		if (pos == std::string::npos) {
+			Debug(misc, 0, "Failed to validate signature: invalid checksum format: {}", filename);
+			return false;
+		}
+	}
+
+	const std::string signature = signatures["signature"];
+	auto pos = signature.find('$');
+	if (pos == std::string::npos) {
+		Debug(misc, 0, "Failed to validate signature: invalid signature format: {}", filename);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * Validate that the signatures mentioned in the signature file are matching
+ * the files in question.
+ *
+ * @return True iff the files in the signature file passed validation.
+ */
+static bool _ValidateSignatureFile(const std::string &filename)
+{
+	size_t filesize;
+	FILE *f = FioFOpenFile(filename, "rb", NO_DIRECTORY, &filesize);
+	if (f == nullptr) {
+		Debug(misc, 0, "Failed to validate signature: file not found: {}", filename);
+		return false;
+	}
+
+	std::string text(filesize, '\0');
+	size_t len = fread(text.data(), filesize, 1, f);
+	if (len != 1) {
+		Debug(misc, 0, "Failed to validate signature: failed to read file: {}", filename);
+		return false;
+	}
+
+	nlohmann::json signatures;
+	try {
+		signatures = nlohmann::json::parse(text);
+	} catch (nlohmann::json::exception &) {
+		Debug(misc, 0, "Failed to validate signature: not a valid JSON file: {}", filename);
+		return false;
+	}
+
+	/*
+	 * The JSON file should look like:
+	 *
+	 *   {
+	 *     "files": [
+	 *       {
+	 *         "checksum": "version$hash"
+	 *         "filename": "filename",
+	 *       },
+	 *       ...
+	 *     ],
+	 *     "signature": "version$signature"
+	 *   }
+	 *
+	 * The signature is a signed message of the content of "files", dumped as
+	 * JSON without spaces / newlines, keys in the order as indicated above.
+	 */
+
+	if (!ValidateSchema(signatures, filename)) {
+		return false;
+	}
+
+	if (!ValidateSignature(signatures["signature"], signatures["files"], filename)) {
+		return false;
+	}
+
+	std::string dirname = std::filesystem::path(filename).parent_path().string();
+
+	for (auto &signature : signatures["files"]) {
+		const std::string sig_filename = dirname + PATHSEPCHAR + signature["filename"].get<std::string>();
+		const std::string sig_checksum = signature["checksum"];
+
+		if (!ValidateChecksum(sig_filename, sig_checksum)) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
+/**
+ * Validate that the signatures mentioned in the signature file are matching
+ * the files in question.
+ *
+ * @note if ALLOW_INVALID_SIGNATURE is defined, this function will always
+ * return true (but will still report any errors in the console).
+ *
+ * @return True iff the files in the signature file passed validation.
+ */
+bool ValidateSignatureFile(const std::string &filename)
+{
+	auto res = _ValidateSignatureFile(filename);;
+#if defined(ALLOW_INVALID_SIGNATURE)
+	(void)res; // Ignore the result.
+	return true;
+#else
+	return res;
+#endif
+}