diff --git a/Lib/zipfile.py b/Lib/zipfile.py
index d99c0d76977750..ebeb6100c1887d 100644
--- a/Lib/zipfile.py
+++ b/Lib/zipfile.py
@@ -787,19 +787,12 @@ class ZipExtFile(io.BufferedIOBase):
 
     def __init__(self, fileobj, mode, zipinfo, pwd=None,
                  close_fileobj=False):
+        self._zinfo = zipinfo
         self._fileobj = fileobj
         self._pwd = pwd
         self._close_fileobj = close_fileobj
 
         self._compress_type = zipinfo.compress_type
-        self._compress_left = zipinfo.compress_size
-        self._left = zipinfo.file_size
-
-        self._decompressor = _get_decompressor(self._compress_type)
-
-        self._eof = False
-        self._readbuffer = b''
-        self._offset = 0
 
         self.newlines = None
 
@@ -808,33 +801,37 @@ def __init__(self, fileobj, mode, zipinfo, pwd=None,
 
         if hasattr(zipinfo, 'CRC'):
             self._expected_crc = zipinfo.CRC
-            self._running_crc = crc32(b'')
         else:
             self._expected_crc = None
 
         self._seekable = False
         try:
             if fileobj.seekable():
-                self._orig_compress_start = fileobj.tell()
-                self._orig_compress_size = zipinfo.compress_size
                 self._orig_file_size = zipinfo.file_size
-                self._orig_start_crc = self._running_crc
                 self._seekable = True
         except AttributeError:
             pass
 
         self._decrypter = None
-        if pwd:
-            if zipinfo.flag_bits & 0x8:
-                # compare against the file type from extended local headers
-                check_byte = (zipinfo._raw_time >> 8) & 0xff
-            else:
-                # compare against the CRC otherwise
-                check_byte = (zipinfo.CRC >> 24) & 0xff
-            h = self._init_decrypter()
-            if h != check_byte:
-                raise RuntimeError("Bad password for file %r" % zipinfo.orig_filename)
 
+        # Compress start is the byte after the 'local file header' ie. the
+        # start of 'encryption header' section, if present, or
+        # 'file data' otherwise.
+        self._compress_start = fileobj.tell()
+        self.read_init()
+
+    def read_init(self):
+        """Set or reset this ZipExtFile to read from the start of the file"""
+        self._running_crc = crc32(b'')
+        self._compress_left = self._zinfo.compress_size
+        self._left = self._zinfo.file_size
+        self._eof = False
+        self._readbuffer = b''
+        self._offset = 0
+
+        if self._pwd:
+            self._init_decrypter()
+        self._decompressor = _get_decompressor(self._compress_type)
 
     def _init_decrypter(self):
         self._decrypter = _ZipDecrypter(self._pwd)
@@ -845,7 +842,17 @@ def _init_decrypter(self):
         #  and is used to check the correctness of the password.
         header = self._fileobj.read(12)
         self._compress_left -= 12
-        return self._decrypter(header)[11]
+        computed_check_byte = self._decrypter(header)[11]
+
+        if self._zinfo.flag_bits & 0x8:
+            # compare against the file type from extended local headers
+            check_byte = (self._zinfo._raw_time >> 8) & 0xff
+        else:
+            # compare against the CRC otherwise
+            check_byte = (self._zinfo.CRC >> 24) & 0xff
+
+        if computed_check_byte != check_byte:
+            raise RuntimeError("Bad password for file %r" % self._zinfo.orig_filename)
 
     def __repr__(self):
         result = ['<%s.%s' % (self.__class__.__module__,
@@ -1072,17 +1079,8 @@ def seek(self, offset, whence=0):
             read_offset = 0
         elif read_offset < 0:
             # Position is before the current position. Reset the ZipExtFile
-            self._fileobj.seek(self._orig_compress_start)
-            self._running_crc = self._orig_start_crc
-            self._compress_left = self._orig_compress_size
-            self._left = self._orig_file_size
-            self._readbuffer = b''
-            self._offset = 0
-            self._decompressor = _get_decompressor(self._compress_type)
-            self._eof = False
-            read_offset = new_pos
-            if self._decrypter is not None:
-                self._init_decrypter()
+            self._fileobj.seek(self._compress_start)
+            self.read_init()
 
         while read_offset > 0:
             read_len = min(self.MAX_SEEK_READ, read_offset)
diff --git a/Misc/NEWS.d/next/Library/2021-05-14-04-37-51.bpo-44128.cxicgo.rst b/Misc/NEWS.d/next/Library/2021-05-14-04-37-51.bpo-44128.cxicgo.rst
new file mode 100644
index 00000000000000..b6da75e9c0a198
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-05-14-04-37-51.bpo-44128.cxicgo.rst
@@ -0,0 +1 @@
+Minor refactor of zipfile.ZipExtFile
\ No newline at end of file