diff --git a/Compression.cc b/Compression.cc
index 9bedd4c0..92f56f8f 100644
--- a/Compression.cc
+++ b/Compression.cc
@@ -184,7 +184,7 @@ string prs_decompress(const string& data, size_t max_size) {
       }
       output += static_cast<char>(ch);
       if (max_size && (output.size() > max_size)) {
-        throw runtime_error("maximumoutput size exceeded");
+        throw runtime_error("maximum output size exceeded");
       }
       continue;
     }
@@ -258,3 +258,109 @@ string prs_decompress(const string& data, size_t max_size) {
     }
   }
 }
+
+size_t prs_decompress_size(const string& data, size_t max_size) {
+  size_t output_size = 0;
+  StringReader r(data.data(), data.size());
+
+  int32_t r3, r5;
+  int bitpos = 9;
+  int16_t currentbyte; // int16_t because it can be -1 when EOF occurs
+  int flag;
+  int offset;
+  unsigned long x;
+
+  currentbyte = get_u8_or_eof(r);
+  if (currentbyte == EOF) {
+    return output_size;
+  }
+
+  for (;;) {
+    bitpos--;
+    if (bitpos == 0) {
+      currentbyte = get_u8_or_eof(r);
+      if (currentbyte == EOF) {
+        return output_size;
+      }
+      bitpos = 8;
+    }
+    flag = currentbyte & 1;
+    currentbyte = currentbyte >> 1;
+    if (flag) {
+      int ch = get_u8_or_eof(r);
+      if (ch == EOF) {
+        return output_size;
+      }
+      output_size++;
+      if (max_size && (output_size > max_size)) {
+        throw runtime_error("maximum output size exceeded");
+      }
+      continue;
+    }
+    bitpos--;
+    if (bitpos == 0) {
+      currentbyte = get_u8_or_eof(r);
+      if (currentbyte == EOF) {
+        return output_size;
+      }
+      bitpos = 8;
+    }
+    flag = currentbyte & 1;
+    currentbyte = currentbyte >> 1;
+    if (flag) {
+      r3 = get_u8_or_eof(r);
+      if (r3 == EOF) {
+        return output_size;
+      }
+      int high_byte = get_u8_or_eof(r);
+      if (high_byte == EOF) {
+        return output_size;
+      }
+      offset = ((high_byte & 0xFF) << 8) | (r3 & 0xFF);
+      if (offset == 0) {
+        return output_size;
+      }
+      r3 = r3 & 0x00000007;
+      r5 = (offset >> 3) | 0xFFFFE000;
+      if (r3 == 0) {
+        flag = 0;
+        r3 = get_u8_or_eof(r);
+        if (r3 == EOF) {
+          return output_size;
+        }
+        r3 = (r3 & 0xFF) + 1;
+      } else {
+        r3 += 2;
+      }
+    } else {
+      r3 = 0;
+      for (x = 0; x < 2; x++) {
+        bitpos--;
+        if (bitpos == 0) {
+          currentbyte = get_u8_or_eof(r);
+          if (currentbyte == EOF) {
+            return output_size;
+          }
+          bitpos = 8;
+        }
+        flag = currentbyte & 1;
+        currentbyte = currentbyte >> 1;
+        offset = r3 << 1;
+        r3 = offset | flag;
+      }
+      offset = get_u8_or_eof(r);
+      if (offset == EOF) {
+        return output_size;
+      }
+      r3 += 2;
+      r5 = offset | 0xFFFFFF00;
+    }
+    if (r3 == 0) {
+      continue;
+    }
+    output_size += r3;
+    if (max_size && (output_size > max_size)) {
+      throw runtime_error("maximum output size exceeded");
+    }
+  }
+}
diff --git a/Compression.hh b/Compression.hh
index 790e86ca..7b59bcf1 100644
--- a/Compression.hh
+++ b/Compression.hh
@@ -8,3 +8,4 @@
 
 std::string prs_compress(const std::string& data);
 std::string prs_decompress(const std::string& data, size_t max_size = 0);
+size_t prs_decompress_size(const std::string& data, size_t max_size = 0);
diff --git a/Quest.cc b/Quest.cc
index 474e7aa8..f7cccc78 100644
--- a/Quest.cc
+++ b/Quest.cc
@@ -6,6 +6,7 @@
 #include <phosg/Strings.hh>
 
 #include "Compression.hh"
+#include "PSOEncryption.hh"
 #include "Text.hh"
 
 using namespace std;
@@ -56,8 +57,8 @@ struct PSOQuestHeaderDC { // same for dc v1 and v2, thankfully
   uint32_t unknown_offset1;
   uint32_t size;
   uint32_t unused;
+  uint8_t is_download;
   uint8_t unknown1;
-  uint8_t unknown2;
   uint16_t quest_number; // 0xFFFF for challenge quests
   char name[0x20];
   char short_description[0x80];
@@ -69,8 +70,8 @@ struct PSOQuestHeaderPC {
   uint32_t unknown_offset1;
   uint32_t size;
   uint32_t unused;
+  uint8_t is_download;
   uint8_t unknown1;
-  uint8_t unknown2;
   uint16_t quest_number; // 0xFFFF for challenge quests
   char16_t name[0x20];
   char16_t short_description[0x80];
@@ -82,7 +83,8 @@ struct PSOQuestHeaderGC {
   uint32_t unknown_offset1;
   uint32_t size;
   uint32_t unused;
-  uint16_t unknown1;
+  uint8_t is_download;
+  uint8_t unknown1;
   uint8_t quest_number;
   uint8_t episode; // 1 = ep2. apparently some quests have 0xFF here, which means ep1 (?)
   char name[0x20];
@@ -362,3 +364,76 @@ vector<shared_ptr<const Quest>> QuestIndex::filter(GameVersion version,
 
   return ret;
 }
+
+
+
+static string create_download_quest_file(const string& compressed_data,
+    size_t decompressed_size) {
+  struct PSODownloadQuestHeader {
+    uint32_t decompressed_size;
+    uint32_t encryption_seed; // note: use PC encryption, even for GC quests
+  };
+
+  string data(8, '\0');
+  auto* header = reinterpret_cast<PSODownloadQuestHeader*>(const_cast<char*>(
+      compressed_data.data()));
+  header->decompressed_size = decompressed_size + sizeof(PSODownloadQuestHeader);
+  header->encryption_seed = random_object<uint32_t>();
+  data += compressed_data;
+
+  // add extra bytes if necessary so encryption won't fail
+  data.resize((data.size() + 3) & (~3));
+
+  // TODO: for DC quests, do we use DC encryption?
+  PSOPCEncryption encr(header->encryption_seed);
+  encr.encrypt(const_cast<char*>(data.data() + sizeof(PSODownloadQuestHeader)),
+      data.size() - sizeof(PSODownloadQuestHeader));
+  return data;
+}
+
+shared_ptr<Quest> Quest::create_download_quest(const string& file_basename) const {
+  if (this->category == QuestCategory::Download) {
+    throw invalid_argument("quest is already a download quest");
+  }
+
+  string decompressed_bin = prs_decompress(*this->bin_contents());
+  void* data_ptr = const_cast<char*>(decompressed_bin.data());
+  switch (this->version) {
+    case GameVersion::DC:
+      reinterpret_cast<PSOQuestHeaderDC*>(data_ptr)->is_download = 0x01;
+      break;
+    case GameVersion::PC:
+      reinterpret_cast<PSOQuestHeaderDC*>(data_ptr)->is_download = 0x01;
+      break;
+    case GameVersion::GC:
+      reinterpret_cast<PSOQuestHeaderDC*>(data_ptr)->is_download = 0x01;
+      break;
+    case GameVersion::BB:
+      throw invalid_argument("PSOBB does not support download quests");
+    default:
+      throw invalid_argument("unknown game version");
+  }
+
+  shared_ptr<Quest> dlq(new Quest(file_basename));
+  dlq->quest_id = this->quest_id;
+  dlq->category = QuestCategory::Download;
+  dlq->episode = this->episode;
+  dlq->is_dcv1 = this->is_dcv1;
+  dlq->joinable = this->joinable;
+  dlq->version = this->version;
+  dlq->name = this->name;
+  dlq->short_description = this->short_description;
+  dlq->long_description = this->long_description;
+
+  dlq->bin_contents_ptr.reset(new string(create_download_quest_file(
+      prs_compress(decompressed_bin), decompressed_bin.size())));
+
+  auto dat_contents = this->dat_contents();
+  dlq->dat_contents_ptr.reset(new string(create_download_quest_file(
+      *dat_contents, prs_decompress_size(*dat_contents))));
+
+  save_file(dlq->bin_filename(), *dlq->bin_contents_ptr);
+  save_file(dlq->dat_filename(), *dlq->dat_contents_ptr);
+
+  return dlq;
+}
diff --git a/Quest.hh b/Quest.hh
index 04934eba..73c649ac 100644
--- a/Quest.hh
+++ b/Quest.hh
@@ -53,6 +53,9 @@ struct Quest {
 
   std::shared_ptr<const std::string> bin_contents() const;
   std::shared_ptr<const std::string> dat_contents() const;
+
+  std::shared_ptr<Quest> create_download_quest(
+      const std::string& file_basename) const;
 };
 
 struct QuestIndex {