2 files changed, 33 insertions, 0 deletions
diff --git a/lib/events/encryptedfile.cpp b/lib/events/encryptedfile.cpp
new file mode 100644
index 00000000..5ec344bb
--- /dev/null
+++ b/lib/events/encryptedfile.cpp
@@ -0,0 +1,27 @@
+// SPDX-FileCopyrightText: 2021 Carl Schwan <carlschwan@kde.org>
+//
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include "encryptedfile.h"
+
+using namespace Quotient;
+
+QByteArray EncryptedFile::decryptFile(const QByteArray &ciphertext) const
+{
+    QString _key = key.k;
+    _key = QByteArray::fromBase64(_key.replace(QLatin1Char('_'), QLatin1Char('/')).replace(QLatin1Char('-'), QLatin1Char('+')).toLatin1());
+    const auto sha256 = QByteArray::fromBase64(hashes["sha256"].toLatin1());
+    if(sha256 != QCryptographicHash::hash(ciphertext, QCryptographicHash::Sha256)) {
+        qCWarning(E2EE) << "Hash verification failed for file";
+        return QByteArray();
+    }
+    QByteArray plaintext(ciphertext.size(), 0);
+    EVP_CIPHER_CTX *ctx;
+    int length;
+    ctx = EVP_CIPHER_CTX_new();
+    EVP_DecryptInit_ex(ctx, EVP_aes_256_ctr(), NULL, (const unsigned char *)_key.data(), (const unsigned char *)iv.toLatin1().data());
+    EVP_DecryptUpdate(ctx, (unsigned char *)plaintext.data(), &length, (const unsigned char *)ciphertext.data(), ciphertext.size());
+    EVP_DecryptFinal_ex(ctx, (unsigned char *)plaintext.data() + length, &length);
+    EVP_CIPHER_CTX_free(ctx);
+    return plaintext;
+}
diff --git a/lib/events/encryptedfile.h b/lib/events/encryptedfile.h
index 24ac9de1..f271d345 100644
--- a/lib/events/encryptedfile.h
+++ b/lib/events/encryptedfile.h
@@ -5,6 +5,10 @@
 #pragma once
 
 #include "converters.h"
+#include "logging.h"
+
+#include <openssl/evp.h>
+#include <QtCore/QCryptographicHash>
 
 namespace Quotient {
 /**
@@ -44,6 +48,8 @@ public:
     QString iv;
     QHash<QString, QString> hashes;
     QString v;
+
+    QByteArray decryptFile(const QByteArray &ciphertext) const;
 };
 
 template <>