diff --git a/pkg2zip.c b/pkg2zip.c
index 970b7b9..ce2c796 100644
--- a/pkg2zip.c
+++ b/pkg2zip.c
@@ -275,6 +275,7 @@ int main(int argc, char* argv[])
     printf("pkg2zip v1.8\n");
 
     int zipped = 1;
+    int cso = 5;
     const char* pkg_arg = NULL;
     const char* zrif_arg = NULL;
     for (int i = 1; i < argc; i++)
@@ -283,6 +284,14 @@ int main(int argc, char* argv[])
         {
             zipped = 0;
         }
+        else if (strncmp(argv[i], "-c", 2) == 0)
+        {
+            if (argv[i][2] != 0)
+            {
+                cso = atoi(argv[i] + 2);
+                cso = cso > 9 ? 9 : cso < 0 ? 0 : cso;
+            }
+        }
         else
         {
             if (pkg_arg != NULL)
@@ -300,7 +309,7 @@ int main(int argc, char* argv[])
     if (pkg_arg == NULL)
     {
         fprintf(stderr, "ERROR: no pkg file specified\n");
-        fatal("Usage: %s [-x] file.pkg [zRIF]\n", argv[0]);
+        fatal("Usage: %s [-x] [-c[N]] file.pkg [zRIF]\n", argv[0]);
     }
 
     printf("[*] loading...\n");
@@ -706,8 +715,8 @@ int main(int argc, char* argv[])
             {
                 if (strcmp("USRDIR/CONTENT/EBOOT.PBP", name) == 0)
                 {
-                    snprintf(path, sizeof(path), "pspemu/ISO/%s [%.9s].iso", title, id);
-                    unpack_psp_eboot(path, item_key, iv, pkg, enc_offset, data_offset, data_size);
+                    snprintf(path, sizeof(path), "pspemu/ISO/%s [%.9s].%s", title, id, cso ? "cso" : "iso");
+                    unpack_psp_eboot(path, item_key, iv, pkg, enc_offset, data_offset, data_size, cso);
                     continue;
                 }
                 else if (strcmp("USRDIR/CONTENT/PSP-KEY.EDAT", name) == 0)
diff --git a/pkg2zip_crc32.c b/pkg2zip_crc32.c
index 27b95be..3722c28 100644
--- a/pkg2zip_crc32.c
+++ b/pkg2zip_crc32.c
@@ -258,3 +258,70 @@ uint32_t crc32_done(crc32_ctx* ctx)
 
     return ~ctx->crc[0];
 }
+
+static uint32_t crc32_gfmulv(const uint32_t* m, uint32_t v)
+{
+    uint32_t r = 0;
+    while (v)
+    {
+        if (v & 1)
+        {
+            r ^= *m;
+        }
+        v >>= 1;
+        m++;
+    }
+    return r;
+}
+
+static void crc32_gfsq(const uint32_t* m, uint32_t* r)
+{
+    for (size_t i = 0; i < 32; i++)
+    {
+        r[i] = crc32_gfmulv(m, m[i]);
+    }
+}
+
+uint32_t crc32_combine(uint32_t a, uint32_t b, uint32_t blen)
+{
+    uint32_t m1[32];
+    uint32_t m2[32];
+
+    m1[0] = 0xedb88320;
+    for (uint32_t i = 1; i < 32; i++)
+    {
+        m1[i] = 1 << (i - 1);
+    }
+
+    crc32_gfsq(m1, m2);
+    crc32_gfsq(m2, m1);
+
+    for (;;)
+    {
+        crc32_gfsq(m1, m2);
+        if (blen & 1)
+        {
+            a = crc32_gfmulv(m2, a);
+        }
+        blen >>= 1;
+
+        if (blen == 0)
+        {
+            break;
+        }
+
+        crc32_gfsq(m2, m1);
+        if (blen & 1)
+        {
+            a = crc32_gfmulv(m1, a);
+        }
+        blen >>= 1;
+
+        if (blen == 0)
+        {
+            break;
+        }
+    }
+
+    return a ^ b;
+}
diff --git a/pkg2zip_crc32.h b/pkg2zip_crc32.h
index 977cc0e..d8f9dea 100644
--- a/pkg2zip_crc32.h
+++ b/pkg2zip_crc32.h
@@ -9,3 +9,8 @@ typedef struct {
 void crc32_init(crc32_ctx* ctx);
 void crc32_update(crc32_ctx* ctx, const void* buffer, size_t size);
 uint32_t crc32_done(crc32_ctx* ctx);
+
+// returns crc32(x||y)
+// where a=crc32(x) and b=crc32(y)
+// crc32(x||y) = crc32(x||z) ^ crc32(y), where z=00...00 (same length as y)
+uint32_t crc32_combine(uint32_t a, uint32_t b, uint32_t blen);
diff --git a/pkg2zip_out.c b/pkg2zip_out.c
index 8fbe0fb..e548b26 100644
--- a/pkg2zip_out.c
+++ b/pkg2zip_out.c
@@ -36,16 +36,17 @@ void out_add_folder(const char* path)
     }
 }
 
-void out_begin_file(const char* name, int compress)
+uint64_t out_begin_file(const char* name, int compress)
 {
     if (out_zipped)
     {
-        zip_begin_file(&out_zip, name, compress);
+        return zip_begin_file(&out_zip, name, compress);
     }
     else
     {
         out_file = sys_create(name);
         out_file_offset = 0;
+        return 0;
     }
 }
 
@@ -73,3 +74,45 @@ void out_write(const void* buffer, uint32_t size)
         out_file_offset += size;
     }
 }
+
+void out_write_at(uint64_t offset, const void* buffer, uint32_t size)
+{
+    if (out_zipped)
+    {
+        zip_write_file_at(&out_zip, offset, buffer, size);
+    }
+    else
+    {
+        sys_write(out_file, offset, buffer, size);
+    }
+}
+
+void out_set_offset(uint64_t offset)
+{
+    if (out_zipped)
+    {
+        zip_set_offset(&out_zip, offset);
+    }
+    else
+    {
+        out_file_offset = offset;
+
+    }
+}
+
+uint32_t out_zip_get_crc32(void)
+{
+    if (out_zipped)
+    {
+        return zip_get_crc32(&out_zip);
+    }
+    return 0;
+}
+
+void out_zip_set_crc32(uint32_t crc)
+{
+    if (out_zipped)
+    {
+        zip_set_crc32(&out_zip, crc);
+    }
+}
diff --git a/pkg2zip_out.h b/pkg2zip_out.h
index 5e37244..0d3156b 100644
--- a/pkg2zip_out.h
+++ b/pkg2zip_out.h
@@ -5,6 +5,12 @@
 void out_begin(const char* name, int zipped);
 void out_end(void);
 void out_add_folder(const char* path);
-void out_begin_file(const char* name, int compress);
+uint64_t out_begin_file(const char* name, int compress);
 void out_end_file(void);
 void out_write(const void* buffer, uint32_t size);
+
+// hacky solution to be able to write cso header after the data is written
+void out_write_at(uint64_t offset, const void* buffer, uint32_t size);
+void out_set_offset(uint64_t offset);
+uint32_t out_zip_get_crc32(void);
+void out_zip_set_crc32(uint32_t crc);
diff --git a/pkg2zip_psp.c b/pkg2zip_psp.c
index 7ae3f58..c718c95 100644
--- a/pkg2zip_psp.c
+++ b/pkg2zip_psp.c
@@ -1,12 +1,15 @@
 #include "pkg2zip_psp.h"
 #include "pkg2zip_out.h"
+#include "pkg2zip_crc32.h"
 #include "pkg2zip_utils.h"
+#include "miniz_tdef.h"
 
 #include <assert.h>
 #include <string.h>
 
-#define PSP_ISO_BLOCK_SIZE 16
-#define PSP_ISO_SECTOR_SIZE 2048
+#define ISO_SECTOR_SIZE 2048
+
+#define CSO_HEADER_SIZE 24
 
 // https://vitadevwiki.com/vita/Keys_NonVita#PSPAESKirk4.2F7
 static const uint8_t kirk7_key38[] = { 0x12, 0x46, 0x8d, 0x7e, 0x1c, 0x42, 0x20, 0x9b, 0xba, 0x54, 0x26, 0x83, 0x5e, 0xb0, 0x33, 0x03 };
@@ -308,7 +311,7 @@ static void init_psp_decrypt(aes128_key* key, uint8_t* iv, int eboot, const uint
     }
 }
 
-void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t* pkg_iv, sys_file* pkg, uint64_t enc_offset, uint64_t item_offset, uint64_t item_size)
+void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t* pkg_iv, sys_file* pkg, uint64_t enc_offset, uint64_t item_offset, uint64_t item_size, int cso)
 {
     if (item_size < 0x28)
     {
@@ -341,9 +344,9 @@ void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t
     }
 
     uint32_t iso_block = get32le(psar_header + 0x0c);
-    if (iso_block != PSP_ISO_BLOCK_SIZE)
+    if (iso_block > 16)
     {
-        fatal("ERROR: unsupported data.psar block size %u, only %u supported!", iso_block, PSP_ISO_BLOCK_SIZE);
+        fatal("ERROR: unsupported data.psar block size %u, max %u supported!\b", iso_block, 16);
     }
 
     uint8_t mac[16];
@@ -357,16 +360,37 @@ void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t
     uint32_t iso_start = get32le(psar_header + 0x54);
     uint32_t iso_end = get32le(psar_header + 0x64);
     uint32_t iso_total = iso_end - iso_start - 1;
-    uint32_t block_count = (iso_total + PSP_ISO_BLOCK_SIZE - 1) / PSP_ISO_BLOCK_SIZE;
+    uint32_t block_count = (iso_total + iso_block - 1) / iso_block;
 
     uint32_t iso_table = get32le(psar_header + 0x6c);
 
     if (iso_table + block_count * 32 > item_size)
     {
-        fatal("ERROR: offset table in data.psar file is too large!");
+        fatal("ERROR: offset table in data.psar file is too large!\n");
+    }
+
+    mz_uint cso_compress_flags = 0;
+    uint32_t cso_index = 0;
+    uint32_t cso_offset = 0;
+    uint64_t cso_size = 0;
+    uint32_t* cso_block = NULL;
+    uint32_t initial_size = 0;
+
+    uint64_t file_offset = out_begin_file(path, !cso);
+    if (cso)
+    {
+        cso_size = block_count * iso_block * ISO_SECTOR_SIZE;
+        cso_compress_flags = tdefl_create_comp_flags_from_zip_params(cso, -MZ_DEFAULT_WINDOW_BITS, MZ_DEFAULT_STRATEGY);
+
+        uint32_t cso_block_count = (uint32_t)(1 + (cso_size + ISO_SECTOR_SIZE - 1) / ISO_SECTOR_SIZE);
+        cso_block = sys_realloc(NULL, cso_block_count * sizeof(uint32_t));
+
+        initial_size = CSO_HEADER_SIZE + cso_block_count * sizeof(uint32_t);
+        out_set_offset(file_offset + initial_size);
+
+        cso_offset = initial_size;
     }
 
-    out_begin_file(path, 1);
     for (uint32_t i = 0; i < block_count; i++)
     {
         uint64_t table_offset = item_offset + psar_offset + iso_table + 32 * i;
@@ -390,7 +414,7 @@ void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t
             fatal("ERROR: iso block size/offset is to large!");
         }
 
-        uint8_t data[PSP_ISO_BLOCK_SIZE * PSP_ISO_SECTOR_SIZE];
+        uint8_t data[16 * ISO_SECTOR_SIZE];
 
         uint64_t abs_offset = item_offset + psar_offset + block_offset;
         sys_read(pkg, enc_offset + abs_offset, data, block_size);
@@ -402,22 +426,114 @@ void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t
         }
 
         uint32_t out_size;
-        if (block_size == sizeof(data))
+        if (block_size == iso_block * ISO_SECTOR_SIZE)
         {
-            out_write(data, (uint32_t)block_size);
+            if (cso)
+            {
+                for (size_t n = 0; n < iso_block * ISO_SECTOR_SIZE; n += ISO_SECTOR_SIZE)
+                {
+                    cso_block[cso_index] = cso_offset;
+
+                    uint8_t PKG_ALIGN(16) output[ISO_SECTOR_SIZE];
+                    size_t insize = ISO_SECTOR_SIZE;
+                    size_t outsize = sizeof(output);
+
+                    tdefl_compressor c;
+                    tdefl_init(&c, cso_compress_flags);
+                    tdefl_status st = tdefl_compress(&c, data + n, &insize, output, &outsize, TDEFL_FINISH);
+                    if (st == TDEFL_STATUS_DONE)
+                    {
+                        out_write(output, (uint32_t)outsize);
+                        cso_offset += (uint32_t)outsize;
+                    }
+                    else
+                    {
+                        cso_block[cso_index] |= 0x80000000;
+                        out_write(data + n, ISO_SECTOR_SIZE);
+                        cso_offset += ISO_SECTOR_SIZE;
+                    }
+                    cso_index++;
+                }
+            }
+            else
+            {
+                out_write(data, (uint32_t)block_size);
+            }
         }
         else
         {
-            uint8_t uncompressed[PSP_ISO_BLOCK_SIZE * PSP_ISO_SECTOR_SIZE];
+            uint8_t PKG_ALIGN(16) uncompressed[16 * ISO_SECTOR_SIZE];
             out_size = lzrc_decompress(uncompressed, sizeof(uncompressed), data, block_size);
-            if (out_size != sizeof(uncompressed))
+            if (out_size != iso_block * ISO_SECTOR_SIZE)
             {
-                fatal("ERROR: internal error - lzrc decompression failed! pkg may be corrupted?");
+                fatal("ERROR: internal error - lzrc decompression failed! pkg may be corrupted?\n");
+            }
+            if (cso)
+            {
+                for (size_t n = 0; n < iso_block * ISO_SECTOR_SIZE; n += ISO_SECTOR_SIZE)
+                {
+                    cso_block[cso_index] = cso_offset;
+
+                    uint8_t output[ISO_SECTOR_SIZE];
+                    size_t insize = ISO_SECTOR_SIZE;
+                    size_t outsize = sizeof(output);
+
+                    tdefl_compressor c;
+                    tdefl_init(&c, cso_compress_flags);
+                    tdefl_status st = tdefl_compress(&c, uncompressed + n, &insize, output, &outsize, TDEFL_FINISH);
+                    if (st == TDEFL_STATUS_DONE)
+                    {
+                        out_write(output, (uint32_t)outsize);
+                        cso_offset += (uint32_t)outsize;
+                    }
+                    else
+                    {
+                        cso_block[cso_index] |= 0x80000000;
+                        out_write(uncompressed + n, ISO_SECTOR_SIZE);
+                        cso_offset += ISO_SECTOR_SIZE;
+                    }
+                    cso_index++;
+                }
+            }
+            else
+            {
+                out_write(uncompressed, (uint32_t)out_size);
             }
-            out_write(uncompressed, (uint32_t)out_size);
         }
     }
 
+    if (cso)
+    {
+        cso_block[cso_index++] = cso_offset;
+
+        uint8_t cso_header[CSO_HEADER_SIZE] = { 0x43, 0x49, 0x53, 0x4f };
+        // header size
+        set32le(cso_header + 4, sizeof(cso_header));
+        // original size
+        set64le(cso_header + 8, cso_size);
+        // block size
+        set32le(cso_header + 16, ISO_SECTOR_SIZE);
+        // version
+        cso_header[20] = 1;
+
+        out_write_at(file_offset, cso_header, sizeof(cso_header));
+        out_write_at(file_offset + sizeof(cso_header), cso_block, cso_index * sizeof(uint32_t));
+
+        crc32_ctx cheader;
+        crc32_init(&cheader);
+        crc32_update(&cheader, cso_header, sizeof(cso_header));
+        crc32_update(&cheader, cso_block, cso_index * sizeof(uint32_t));
+
+        uint32_t header_crc32 = crc32_done(&cheader);
+        uint32_t data_crc32 = out_zip_get_crc32();
+        uint32_t data_len = (uint32_t)(cso_offset - initial_size);
+
+        uint32_t crc32 = crc32_combine(header_crc32, data_crc32, data_len);
+        out_zip_set_crc32(crc32);
+
+        sys_realloc(cso_block, 0);
+    }
+
     out_end_file();
 }
 
diff --git a/pkg2zip_psp.h b/pkg2zip_psp.h
index d6d14a1..e2183be 100644
--- a/pkg2zip_psp.h
+++ b/pkg2zip_psp.h
@@ -1,5 +1,5 @@
 #include "pkg2zip_aes.h"
 #include "pkg2zip_sys.h"
 
-void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t* pkg_iv, sys_file* pkg, uint64_t enc_offset, uint64_t item_offset, uint64_t item_size);
+void unpack_psp_eboot(const char* path, const aes128_key* pkg_key, const uint8_t* pkg_iv, sys_file* pkg, uint64_t enc_offset, uint64_t item_offset, uint64_t item_size, int cso);
 void unpack_psp_key(const char* path, const aes128_key* pkg_key, const uint8_t* pkg_iv, sys_file* pkg, uint64_t enc_offset, uint64_t item_offset, uint64_t item_size);
diff --git a/pkg2zip_zip.c b/pkg2zip_zip.c
index b7c8d7c..d5f9edb 100644
--- a/pkg2zip_zip.c
+++ b/pkg2zip_zip.c
@@ -108,7 +108,7 @@ void zip_add_folder(zip* z, const char* name)
     z->total += 1;
 }
 
-void zip_begin_file(zip* z, const char* name, int compress)
+uint64_t zip_begin_file(zip* z, const char* name, int compress)
 {
     size_t name_length = strlen(name);
     if (name_length > ZIP_MAX_FILENAME)
@@ -124,6 +124,7 @@ void zip_begin_file(zip* z, const char* name, int compress)
     z->current = f;
 
     crc32_init(&z->crc32);
+    z->crc32_set = 0;
 
     uint8_t header[ZIP_LOCAL_HEADER_SIZE] = { 0x50, 0x4b, 0x03, 0x04 };
     // version needed to extract
@@ -150,6 +151,8 @@ void zip_begin_file(zip* z, const char* name, int compress)
         int flags = tdefl_create_comp_flags_from_zip_params(MZ_BEST_SPEED, -MZ_DEFAULT_WINDOW_BITS, MZ_DEFAULT_STRATEGY);
         tdefl_init(&z->tdefl, flags);
     }
+
+    return z->total - f->offset;
 }
 
 void zip_write_file(zip* z, const void* data, uint32_t size)
@@ -211,7 +214,10 @@ void zip_end_file(zip* z)
         }
     }
 
-    z->current->crc32 = crc32_done(&z->crc32);
+    if (!z->crc32_set)
+    {
+        z->current->crc32 = crc32_done(&z->crc32);
+    }
 
     if (z->current->size != 0)
     {
@@ -400,3 +406,31 @@ void zip_close(zip* z)
 
     sys_realloc(z->files, 0);
 }
+
+void zip_write_file_at(zip* z, uint64_t offset, const void* data, uint32_t size)
+{
+    if (z->current->compress)
+    {
+        fatal("ERROR: cannot write at specific offset for compressed files\n");
+    }
+
+    sys_write(z->file, z->current->offset + offset, data, size);
+    z->current->size += size;
+    z->current->compressed += size;
+}
+
+void zip_set_offset(zip* z, uint64_t offset)
+{
+    z->total = z->current->offset + offset;
+}
+
+void zip_set_crc32(zip* z, uint32_t crc)
+{
+    z->current->crc32 = crc;
+    z->crc32_set = 1;
+}
+
+uint32_t zip_get_crc32(zip* z)
+{
+    return crc32_done(&z->crc32);
+}
diff --git a/pkg2zip_zip.h b/pkg2zip_zip.h
index 5c189e8..3e6e04e 100644
--- a/pkg2zip_zip.h
+++ b/pkg2zip_zip.h
@@ -20,6 +20,7 @@ typedef struct {
     uint16_t date;
     tdefl_compressor tdefl;
     crc32_ctx crc32;
+    int crc32_set;
     uint32_t allocated; // bytes
     zip_file* files;
     zip_file* current;
@@ -27,7 +28,13 @@ typedef struct {
 
 void zip_create(zip* z, const char* name);
 void zip_add_folder(zip* z, const char* name);
-void zip_begin_file(zip* z, const char* name, int compress);
+uint64_t zip_begin_file(zip* z, const char* name, int compress);
 void zip_write_file(zip* z, const void* data, uint32_t size);
 void zip_end_file(zip* z);
 void zip_close(zip* z);
+
+// hacky solution to be able to write cso header after the data is written
+void zip_write_file_at(zip* z, uint64_t offset, const void* data, uint32_t size);
+void zip_set_offset(zip* z, uint64_t offset);
+void zip_set_crc32(zip* z, uint32_t crc);
+uint32_t zip_get_crc32(zip* z);