Linux: allow using GnuTLS instead of OpenSSL for computing SHA256 hashes

Prevents issues with often changing OpenSSL ABI.

Ref #429

src/acceleratedcryptographichash_gnutls.cpp

@@ -0,0 +1,39 @@
+/*
+ * Use GnuTLS for hashing as their code is more optimized than Qt's
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2022 Raspberry Pi Ltd
+ */
+
+#include "acceleratedcryptographichash.h"
+
+AcceleratedCryptographicHash::AcceleratedCryptographicHash(QCryptographicHash::Algorithm method)
+{
+    if (method != QCryptographicHash::Sha256)
+        throw std::runtime_error("Only sha256 implemented");
+
+    gnutls_hash_init(&_sha256, GNUTLS_DIG_SHA256);
+
+}
+
+AcceleratedCryptographicHash::~AcceleratedCryptographicHash()
+{
+    gnutls_hash_deinit(_sha256, NULL);
+}
+
+void AcceleratedCryptographicHash::addData(const char *data, int length)
+{
+    gnutls_hash(_sha256, data, length);
+}
+
+void AcceleratedCryptographicHash::addData(const QByteArray &data)
+{
+    addData(data.constData(), data.size());
+}
+
+QByteArray AcceleratedCryptographicHash::result()
+{
+    unsigned char binhash[gnutls_hash_get_len(GNUTLS_DIG_SHA256)];
+    gnutls_hash_output(_sha256, binhash);
+    return QByteArray((char *) binhash, sizeof binhash);
+}