diff --git a/archive.py b/archive.py
index f0917de..a1346b6 100644
--- a/archive.py
+++ b/archive.py
@@ -117,13 +117,13 @@ def crc32(filename) -> int:
         return hash
 
 
-def zip_local_file_header(filename: str, filepath: str) -> bytes:
+def zip_local_file_header(filename: str, filepath: str, crc: int) -> bytes:
     buffer_size = 30 + len(filename)
     buffer = bytearray(buffer_size)
     stat = os.stat(filepath)
 
     # Field 1: local file header signature (buffer[0:4])
-    buffer[0:4] = b'PK\x03\x04'
+    buffer[0:4] = b'\x50\x4b\x03\x04'
 
     # Field 2: version needed to extract (minimum) (buffer[4:6])
     buffer[4:6] = b'\x0a'
@@ -138,7 +138,7 @@ def zip_local_file_header(filename: str, filepath: str) -> bytes:
     buffer[12:14] = (mtime.day | (mtime.month << 5) | ((mtime.year - 1980) << 9)).to_bytes(2, byteorder='little')
 
     # Field 6: crc-32 of uncompressed data (buffer[14:18])
-    buffer[14:18] = crc32(filepath).to_bytes(4, byteorder='little')
+    buffer[14:18] = crc.to_bytes(4, byteorder='little')
 
     # Field 7: compressed size (buffer[18:22])
     buffer[18:22] = stat.st_size.to_bytes(4, byteorder='little')
@@ -157,7 +157,7 @@ def zip_local_file_header(filename: str, filepath: str) -> bytes:
     return buffer
 
 
-def zip_central_directory_file_header(filename: str, filepath: str, offset: int) -> bytes:
+def zip_central_directory_file_header(filename: str, filepath: str, crc: int, offset: int) -> bytes:
     buffer_size = 46 + len(filename)
     buffer = bytearray(buffer_size)
     stat = os.stat(filepath)
@@ -181,7 +181,7 @@ def zip_central_directory_file_header(filename: str, filepath: str, offset: int)
     buffer[14:16] = (mtime.day | (mtime.month << 5) | ((mtime.year - 1980) << 9)).to_bytes(2, byteorder='little')
 
     # Field 6: crc-32 of uncompressed data (buffer[16:20])
-    buffer[16:20] = crc32(filepath).to_bytes(4, byteorder='little')
+    buffer[16:20] = crc.to_bytes(4, byteorder='little')
 
     # Field 7: compressed size (buffer[20:24])
     buffer[20:24] = stat.st_size.to_bytes(4, byteorder='little')
@@ -203,6 +203,7 @@ def zip_central_directory_file_header(filename: str, filepath: str, offset: int)
     # Field 14: external file attributes (buffer[38:42])
 
     # Field 15: relative offset of the local file header (buffer[42:46])
+    buffer[42:46] = offset.to_bytes(4, byteorder='little')
 
     # Field 16: filename (buffer[46:46+len(filename)])
     buffer[46:46+len(filename)] = filename.encode('ascii')
@@ -241,11 +242,14 @@ class ZipSender(ArchiveSender):
     def generator(self):
         def generate():
             local_offsets = dict()
+            crcs = dict()
             current_byte = 0
 
             for name, file in self.files.items():
+                crcs[name] = crc32(file)
+
                 local_offsets[name] = current_byte
-                chunk = zip_local_file_header(name, file)
+                chunk = zip_local_file_header(name, file, crcs[name])
                 current_byte += len(chunk)
 
                 yield chunk
@@ -264,7 +268,7 @@ class ZipSender(ArchiveSender):
             centra_directory_offset = current_byte
 
             for name, file, in self.files.items():
-                chunk = zip_central_directory_file_header(name, file, local_offsets[name])
+                chunk = zip_central_directory_file_header(name, file, crcs[name], local_offsets[name])
                 central_directory_size += len(chunk)
                 current_byte += len(chunk)
                 yield chunk