3 files changed, 81 insertions, 1 deletions

diff --git a/src/core/crypto/xts_encryption_layer.cpp b/src/core/crypto/xts_encryption_layer.cpp
new file mode 100644
index 000000000..431099580
--- /dev/null
+++ b/src/core/crypto/xts_encryption_layer.cpp
@@ -0,0 +1,54 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <cstring>
+#include "common/assert.h"
+#include "core/crypto/xts_encryption_layer.h"
+
+namespace Core::Crypto {
+
+XTSEncryptionLayer::XTSEncryptionLayer(FileSys::VirtualFile base_, Key256 key_)
+    : EncryptionLayer(std::move(base_)), cipher(key_, Mode::XTS) {}
+
+size_t XTSEncryptionLayer::Read(u8* data, size_t length, size_t offset) const {
+    if (length == 0)
+        return 0;
+
+    const auto sector_offset = offset & 0x3FFF;
+    if (sector_offset == 0) {
+        if (length % 0x4000 == 0) {
+            std::vector<u8> raw = base->ReadBytes(length, offset);
+            cipher.XTSTranscode(raw.data(), raw.size(), data, offset / 0x4000, 0x4000, Op::Decrypt);
+            return raw.size();
+        }
+        if (length > 0x4000) {
+            const auto rem = length % 0x4000;
+            const auto read = length - rem;
+            return Read(data, read, offset) + Read(data + read, rem, offset + read);
+        }
+        std::vector<u8> buffer = base->ReadBytes(0x4000, offset);
+        if (buffer.size() < 0x4000)
+            buffer.resize(0x4000);
+        cipher.XTSTranscode(buffer.data(), buffer.size(), buffer.data(), offset / 0x4000, 0x4000,
+                            Op::Decrypt);
+        std::memcpy(data, buffer.data(), std::min(buffer.size(), length));
+        return std::min(buffer.size(), length);
+    }
+
+    // offset does not fall on block boundary (0x4000)
+    std::vector<u8> block = base->ReadBytes(0x4000, offset - sector_offset);
+    if (block.size() < 0x4000)
+        block.resize(0x4000);
+    cipher.XTSTranscode(block.data(), block.size(), block.data(), (offset - sector_offset) / 0x4000,
+                        0x4000, Op::Decrypt);
+    const size_t read = 0x4000 - sector_offset;
+
+    if (length + sector_offset < 0x4000) {
+        std::memcpy(data, block.data() + sector_offset, std::min<u64>(length, read));
+        return std::min<u64>(length, read);
+    }
+    std::memcpy(data, block.data() + sector_offset, read);
+    return read + Read(data + read, length - read, offset + read);
+}
+} // namespace Core::Crypto
diff --git a/src/core/crypto/xts_encryption_layer.h b/src/core/crypto/xts_encryption_layer.h
new file mode 100644
index 000000000..1e1acaf4a
--- /dev/null
+++ b/src/core/crypto/xts_encryption_layer.h
@@ -0,0 +1,26 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <vector>
+#include "core/crypto/aes_util.h"
+#include "core/crypto/encryption_layer.h"
+#include "core/crypto/key_manager.h"
+
+namespace Core::Crypto {
+
+// Sits on top of a VirtualFile and provides XTS-mode AES decription.
+class XTSEncryptionLayer : public EncryptionLayer {
+public:
+    XTSEncryptionLayer(FileSys::VirtualFile base, Key256 key);
+
+    size_t Read(u8* data, size_t length, size_t offset) const override;
+
+private:
+    // Must be mutable as operations modify cipher contexts.
+    mutable AESCipher<Key256> cipher;
+};
+
+} // namespace Core::Crypto
diff --git a/src/core/file_sys/content_archive.cpp b/src/core/file_sys/content_archive.cpp
index 47afcad9b..008e11d8c 100644
--- a/src/core/file_sys/content_archive.cpp
+++ b/src/core/file_sys/content_archive.cpp
@@ -178,7 +178,7 @@ VirtualFile NCA::Decrypt(NCASectionHeader s_header, VirtualFile in, u64 starting
             return std::static_pointer_cast<VfsFile>(out);
         }
     case NCASectionCryptoType::XTS:
-        // TODO(DarkLordZach): Implement XTSEncryptionLayer.
+        // TODO(DarkLordZach): Find a test case for XTS-encrypted NCAs
     default:
         LOG_ERROR(Crypto, "called with unhandled crypto type={:02X}",
                   static_cast<u8>(s_header.raw.header.crypto_type));