eden/src/core/crypto/ncz_decompressor.cpp

// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
// SPDX-License-Identifier: GPL-3.0-or-later

#include <algorithm>
#include <array>
#include <cstring>
#include <memory>
#include <mutex>
#include <optional>
#include <span>
#include <string>
#include <vector>

#include <zstd.h>

#include "common/common_types.h"
#include "common/logging.h"
#include "common/zstd_compression.h"
#include "core/crypto/aes_util.h"
#include "core/crypto/key_manager.h"
#include "core/crypto/ncz_decompressor.h"
#include "core/file_sys/vfs/vfs.h"

namespace Core::Crypto {

namespace {

constexpr u64 HeaderSize = 0x4000;
constexpr std::size_t SectionWire = 0x40;
constexpr std::size_t BlockFixed = 0x18;
constexpr std::size_t Chunk = 0x100000;
// Upper bound on NCZ section entries (64 bytes each, so the table is at most 4 MiB). A base NCA
// has a handful of sections, but nsz stores an update's patch (BKTR) data one entry per
// subsection, which is thousands of them.
constexpr u64 MaxSectionCount = 0x10000;
constexpr std::array<u8, 8> SectionsMagic{'N', 'C', 'Z', 'S', 'E', 'C', 'T', 'N'};
constexpr std::array<u8, 8> BlockMagic{'N', 'C', 'Z', 'B', 'L', 'O', 'C', 'K'};

struct Section {
    s64 offset, size;
    u64 crypto_type;
    Key128 key, counter;
    bool Ctr() const { return crypto_type == 3 || crypto_type == 4; }
};

struct BlockHeader {
    u32 exponent;
    u64 decompressed_size;
    std::vector<u32> sizes;
};

u64 Le(const u8* p, std::size_t n) {
    u64 v = 0;
    for (std::size_t i = n; i-- > 0;) v = (v << 8) | p[i];
    return v;
}

// Mirrors nsz AESCTR.seek(): upper 8 bytes = stored counter, lower 8 = absolute NCA offset / 16.
void SetIv(Key128& iv, const Key128& base, u64 off) {
    iv = base;
    u64 blk = off >> 4;
    for (std::size_t i = 0; i < 8; ++i) { iv[15 - i] = static_cast<u8>(blk & 0xFF); blk >>= 8; }
}

// Parse header + section table + optional block header. Returns payload_start.
std::optional<std::pair<std::vector<Section>, std::optional<BlockHeader>>> ParseHeader(
    const FileSys::VirtualFile& f, u64& payload_start) {
    const u64 fsize = f->GetSize();
    if (fsize < HeaderSize + 16) return std::nullopt;

    const auto head = f->ReadBytes(16, HeaderSize);
    if (head.size() != 16 || !std::equal(head.begin(), head.begin() + 8, SectionsMagic.begin()))
        return std::nullopt;
    const u64 count = Le(head.data() + 8, 8);
    if (count == 0 || count > MaxSectionCount || count * SectionWire > fsize - HeaderSize - 16)
        return std::nullopt;

    const auto table = f->ReadBytes(count * SectionWire, HeaderSize + 16);
    if (table.size() != count * SectionWire) return std::nullopt;

    std::vector<Section> sections;
    sections.reserve(count + 1);
    for (u64 i = 0; i < count; ++i) {
        const u8* e = table.data() + i * SectionWire;
        Section s{};
        s.offset = static_cast<s64>(Le(e, 8));
        s.size = static_cast<s64>(Le(e + 8, 8));
        s.crypto_type = Le(e + 16, 8);
        std::memcpy(s.key.data(), e + 32, 16);
        std::memcpy(s.counter.data(), e + 48, 16);
        sections.push_back(s);
    }

    const u64 block_start = HeaderSize + 16 + count * SectionWire;
    const auto fixed = f->ReadBytes(BlockFixed, block_start);
    std::optional<BlockHeader> bh;
    if (fixed.size() == BlockFixed &&
        std::equal(fixed.begin(), fixed.begin() + 8, BlockMagic.begin())) {
        BlockHeader b{};
        b.exponent = fixed[11];
        const s32 n = static_cast<s32>(Le(fixed.data() + 12, 4));
        if (n <= 0) return std::nullopt;
        b.decompressed_size = Le(fixed.data() + 16, 8);
        const auto sizes = f->ReadBytes(static_cast<u64>(n) * 4, block_start + BlockFixed);
        if (sizes.size() != static_cast<u64>(n) * 4) return std::nullopt;
        b.sizes.resize(n);
        for (s32 i = 0; i < n; ++i) b.sizes[i] = static_cast<u32>(Le(sizes.data() + i * 4, 4));
        payload_start = block_start + BlockFixed + static_cast<u64>(n) * 4;
        bh = std::move(b);
    } else {
        payload_start = block_start;
    }

    if (!sections.empty() && sections[0].offset > static_cast<s64>(HeaderSize)) {
        Section fake{};
        fake.offset = static_cast<s64>(HeaderSize);
        fake.size = sections[0].offset - static_cast<s64>(HeaderSize);
        sections.insert(sections.begin(), fake);
    }
    return std::make_pair(std::move(sections), std::move(bh));
}

// Sequential reader over independently-compressed blocks, one block cached (like nsz_rs).
class BlockReader {
public:
    BlockReader(FileSys::VirtualFile f, const BlockHeader& bh, u64 payload_start)
        : source{std::move(f)}, block_size{1ULL << bh.exponent}, sizes{bh.sizes},
          decompressed_size{bh.decompressed_size} {
        if (bh.exponent < 14 || bh.exponent > 32 || sizes.empty())
            throw std::runtime_error("bad NCZBLOCK header");
        u64 off = payload_start;
        offsets.reserve(sizes.size());
        for (u32 s : sizes) { offsets.push_back(off); off += s; }
    }

    // Fills `out` from `position`; returns bytes written.
    std::size_t ReadAt(u64 position, std::span<u8> out) {
        std::size_t done = 0;
        while (done < out.size() && position < decompressed_size) {
            const u64 id = position / block_size;
            if (id >= sizes.size()) break;
            if (cache_id != id && !Load(id)) break;
            const u64 in = position - id * block_size;
            const std::size_t n = static_cast<std::size_t>(
                std::min<u64>(out.size() - done, cache.size() - in));
            std::memcpy(out.data() + done, cache.data() + in, n);
            done += n;
            position += n;
        }
        return done;
    }

private:
    bool Load(u64 id) {
        u64 want = block_size;
        if (id + 1 == sizes.size()) {
            const u64 rem = decompressed_size % block_size;
            if (rem > 0) want = rem; // avoid the 0-size bug from the original nsz
        }
        try { cache.resize(static_cast<std::size_t>(want)); }
        catch (const std::bad_alloc&) { return false; }
        const u32 csize = sizes[id];
        if (csize < want) {
            const auto comp = source->ReadBytes(csize, offsets[id]);
            if (comp.size() != csize ||
                Common::Compression::DecompressDataZSTD(comp, cache) != want) return false;
        } else if (source->Read(cache.data(), cache.size(), offsets[id]) != cache.size()) {
            return false;
        }
        cache_id = id;
        return true;
    }

    FileSys::VirtualFile source;
    u64 block_size;
    std::vector<u64> offsets;
    std::vector<u32> sizes;
    u64 decompressed_size;
    u64 cache_id = ~0ULL;
    std::vector<u8> cache;
};

class BlockNczFile final : public FileSys::VfsFile {
public:
    BlockNczFile(std::string name_, std::vector<u8> hdr_, std::vector<Section> sections_,
                 std::unique_ptr<BlockReader> body_)
        : name{std::move(name_)}, hdr{std::move(hdr_)}, sections{std::move(sections_)},
          body{std::move(body_)} {
        nca_size = HeaderSize;
        for (const auto& s : sections)
            nca_size = std::max<u64>(nca_size, static_cast<u64>(s.offset + s.size));
        ciphers.reserve(sections.size());
        for (const auto& s : sections)
            ciphers.push_back(s.Ctr() ? std::make_unique<AESCipher<Key128>>(s.key, Mode::CTR)
                                      : nullptr);
    }

    std::string GetName() const override { return name; }
    std::size_t GetSize() const override { return static_cast<std::size_t>(nca_size); }
    bool Resize(std::size_t) override { return false; }
    FileSys::VirtualDir GetContainingDirectory() const override { return nullptr; }
    bool IsWritable() const override { return false; }
    bool IsReadable() const override { return true; }

    std::size_t Read(u8* data, std::size_t len, std::size_t off) const override {
        if (off >= nca_size) return 0;
        len = static_cast<std::size_t>(std::min<u64>(len, nca_size - off));
        const u64 end = off + len;
        u64 pos = off;
        if (pos < HeaderSize) {
            const u64 n = std::min<u64>(end, HeaderSize) - pos;
            std::memcpy(data, hdr.data() + pos, static_cast<std::size_t>(n));
            pos += n;
        }
        std::scoped_lock lock{mutex};
        for (std::size_t i = 0; i < sections.size() && pos < end; ++i) {
            const auto& s = sections[i];
            const u64 begin = std::max<u64>(static_cast<u64>(s.offset), HeaderSize);
            const u64 stop = std::max<u64>(static_cast<u64>(s.offset + s.size), begin);
            if (stop <= pos) continue;
            if (pos < begin) {
                const u64 g = std::min(end, begin);
                std::memset(data + (pos - off), 0, static_cast<std::size_t>(g - pos));
                pos = g;
                if (pos >= end) break;
            }
            const u64 pe = std::min(end, stop);
            const std::span<u8> piece(data + (pos - off), static_cast<std::size_t>(pe - pos));
            if (body->ReadAt(pos - HeaderSize, piece) != piece.size())
                return static_cast<std::size_t>(pos - off);
            if (ciphers[i]) {
                Key128 iv{};
                SetIv(iv, s.counter, pos);
                ciphers[i]->SetIV(iv);
                ciphers[i]->Transcode(piece.data(), piece.size(), piece.data(), Op::Encrypt);
            }
            pos = pe;
        }
        return len;
    }

    std::size_t Write(const u8*, std::size_t, std::size_t) override { return 0; }
    bool Rename(std::string_view) override { return false; }

private:
    const std::string name;
    const std::vector<u8> hdr;
    const std::vector<Section> sections;
    const std::unique_ptr<BlockReader> body;
    std::vector<std::unique_ptr<AESCipher<Key128>>> ciphers;
    mutable std::mutex mutex;
    u64 nca_size{};
};

// Solid NCZ: one zstd stream over the whole body. Decoded incrementally into memory, up to the
// requested offset, so sequential install reads are O(n) instead of O(n^2).
class SolidNczFile final : public FileSys::VfsFile {
public:
    SolidNczFile(std::string name_, FileSys::VirtualFile source_,
                 std::vector<Section> sections_, u64 payload_start_)
        : name{std::move(name_)}, source{std::move(source_)}, sections{std::move(sections_)},
          payload_start{payload_start_} {
        const auto raw_header = source->ReadBytes(HeaderSize, 0);
        data.assign(raw_header.begin(), raw_header.end());
        frontier = HeaderSize;
    }

    ~SolidNczFile() override {
        if (stream != nullptr) ZSTD_freeDStream(stream);
    }

    std::string GetName() const override { return name; }
    std::size_t GetSize() const override {
        u64 size = HeaderSize;
        for (const auto& s : sections)
            size = std::max<u64>(size, static_cast<u64>(s.offset + s.size));
        return static_cast<std::size_t>(size);
    }
    bool Resize(std::size_t) override { return false; }
    FileSys::VirtualDir GetContainingDirectory() const override { return nullptr; }
    bool IsWritable() const override { return false; }
    bool IsReadable() const override { return true; }

    std::size_t Read(u8* out, std::size_t len, std::size_t off) const override {
        std::scoped_lock lock{mutex};
        const u64 nca_size = GetSize();
        if (off >= nca_size) return 0;
        len = static_cast<std::size_t>(std::min<u64>(len, nca_size - off));
        const u64 needed = off + len;
        if (needed > frontier && !DecodeTo(needed)) return 0;
        std::memcpy(out, data.data() + off, len);
        return len;
    }

    std::size_t Write(const u8*, std::size_t, std::size_t) override { return 0; }
    bool Rename(std::string_view) override { return false; }

private:
    bool DecodeTo(u64 needed) const {
        if (stream == nullptr) {
            stream = ZSTD_createDStream();
            if (stream == nullptr) return false;
            ZSTD_DCtx_setParameter(stream, ZSTD_d_windowLogMax, 31);
            read_pos = payload_start;
        }

        const u64 file_size = source->GetSize();
        std::vector<u8> out_buf(Chunk);

        std::size_t index = 0;
        for (; index < sections.size(); ++index) {
            const auto& s = sections[index];
            const u64 begin = std::max<u64>(static_cast<u64>(s.offset), HeaderSize);
            const u64 end = std::max<u64>(static_cast<u64>(s.offset) + static_cast<u64>(s.size),
                                          begin);
            if (end > frontier) break;
        }

        for (; index < sections.size() && frontier < needed; ++index) {
            const auto& s = sections[index];
            const u64 begin = std::max<u64>(static_cast<u64>(s.offset), HeaderSize);
            const u64 end = std::max<u64>(static_cast<u64>(s.offset) + static_cast<u64>(s.size),
                                          begin);
            if (end <= frontier) continue;
            u64 pos = std::max<u64>(begin, frontier);

            std::optional<AESCipher<Key128>> cipher;
            if (s.Ctr()) cipher.emplace(s.key, Mode::CTR);

            while (pos < end && frontier < needed) {
                const u64 want = std::min<u64>(end - pos, out_buf.size());
                std::size_t produced = 0;
                while (produced < want) {
                    if (in.pos >= in.size) {
                        const u64 remaining = file_size - read_pos;
                        const std::size_t chunk = static_cast<std::size_t>(
                            std::min<u64>(Chunk, remaining));
                        if (chunk == 0) { LOG_ERROR(Crypto, "Solid NCZ: truncated"); return false; }
                        const auto bytes = source->ReadBytes(chunk, read_pos);
                        if (bytes.empty()) { LOG_ERROR(Crypto, "Solid NCZ: truncated"); return false; }
                        read_pos += bytes.size();
                        in_buf.assign(bytes.begin(), bytes.end());
                        in.src = in_buf.data();
                        in.size = in_buf.size();
                        in.pos = 0;
                    }
                    ZSTD_outBuffer outb{out_buf.data() + produced, want - produced, 0};
                    const std::size_t res = ZSTD_decompressStream(stream, &outb, &in);
                    if (ZSTD_isError(res)) {
                        LOG_ERROR(Crypto, "Solid NCZ: zstd error");
                        return false;
                    }
                    produced += outb.pos;
                }
                if (cipher) {
                    Key128 iv{};
                    SetIv(iv, s.counter, pos);
                    cipher->SetIV(iv);
                    cipher->Transcode(out_buf.data(), produced, out_buf.data(), Op::Encrypt);
                }
                data.insert(data.end(), out_buf.begin(), out_buf.begin() + produced);
                pos += produced;
                frontier = pos;
            }
        }
        return frontier >= needed;
    }

    const std::string name;
    const FileSys::VirtualFile source;
    const std::vector<Section> sections;
    const u64 payload_start;
    mutable std::mutex mutex;
    mutable std::vector<u8> data;
    mutable ZSTD_DStream* stream = nullptr;
    mutable u64 read_pos = 0;
    mutable u64 frontier = 0;
    mutable std::vector<u8> in_buf;
    mutable ZSTD_inBuffer in{nullptr, 0, 0};
};

// Shared opener: block path always; solid only when allow_solid is true.
FileSys::VirtualFile OpenNcz(const FileSys::VirtualFile& f, std::string_view name,
                             bool allow_solid) {
    if (!f) return nullptr;
    u64 ps = 0;
    auto parsed = ParseHeader(f, ps);
    if (!parsed) return nullptr;
    auto& [sections, block] = *parsed;

    if (block) {
        auto hdr = f->ReadBytes(HeaderSize, 0);
        if (hdr.size() != HeaderSize) return nullptr;
        std::unique_ptr<BlockReader> body;
        try { body = std::make_unique<BlockReader>(f, *block, ps); }
        catch (const std::exception& e) {
            LOG_ERROR(Crypto, "Failed to open NCZBLOCK: {}", e.what());
            return nullptr;
        }
        return std::make_shared<BlockNczFile>(std::string{name}, std::move(hdr),
                                              std::move(sections), std::move(body));
    }

    if (allow_solid) {
        return std::make_shared<SolidNczFile>(std::string{name}, f, std::move(sections), ps);
    }

    LOG_DEBUG(Crypto, "Solid NCZ {} skipped (only NCZBLOCK is supported for play)", name);
    return nullptr;
}

} // Anonymous namespace

bool IsBlockCompressedNCZ(const FileSys::VirtualFile& f) {
    if (!f) return false;
    u64 ps = 0;
    const auto p = ParseHeader(f, ps);
    return p && p->second.has_value();
}

FileSys::VirtualFile DecompressNCZ(const FileSys::VirtualFile& f, std::string_view name) {
    return OpenNcz(f, name, /*allow_solid=*/false);
}

FileSys::VirtualFile DecompressNCZForInstall(const FileSys::VirtualFile& f,
                                             std::string_view name) {
    return OpenNcz(f, name, /*allow_solid=*/true);
}

} // namespace Core::Crypto