diff --git a/Lib/quopri.py b/Lib/quopri.py
index 5002cc8..62c0503 100755
--- a/Lib/quopri.py
+++ b/Lib/quopri.py
@@ -6,10 +6,10 @@
 
 __all__ = ["encode", "decode", "encodestring", "decodestring"]
 
-ESCAPE = '='
+ESCAPE = b'='
 MAXLINESIZE = 76
-HEX = '0123456789ABCDEF'
-EMPTYSTRING = ''
+HEX = b'0123456789ABCDEF'
+EMPTYSTRING = b''
 
 try:
     from binascii import a2b_qp, b2a_qp
@@ -19,23 +19,25 @@
 
 
 def needsquoting(c, quotetabs, header):
-    """Decide whether a particular character needs to be quoted.
+    """Decide whether a particular byte ordinal needs to be quoted.
 
     The 'quotetabs' flag indicates whether embedded tabs and spaces should be
     quoted.  Note that line-ending tabs and spaces are always encoded, as per
     RFC 1521.
     """
-    if c in ' \t':
+    assert isinstance(c, bytes)
+    if c in b' \t':
         return quotetabs
     # if header, we have to escape _ because _ is used to escape space
-    if c == '_':
+    if c == b'_':
         return header
-    return c == ESCAPE or not (' ' <= c <= '~')
+    return c == ESCAPE or not (b' ' <= c <= b'~')
 
 def quote(c):
     """Quote a single character."""
-    i = ord(c)
-    return ESCAPE + HEX[i//16] + HEX[i%16]
+    assert isinstance(c, bytes) and len(c)==1
+    c = ord(c)
+    return ESCAPE + bytes((HEX[c//16], HEX[c%16]))
 
 
 
@@ -56,12 +58,12 @@
         output.write(odata)
         return
 
-    def write(s, output=output, lineEnd='\n'):
+    def write(s, output=output, lineEnd=b'\n'):
         # RFC 1521 requires that the line ending in a space or tab must have
         # that trailing character encoded.
-        if s and s[-1:] in ' \t':
-            output.write(s[:-1] + quote(s[-1]) + lineEnd)
-        elif s == '.':
+        if s and s[-1:] in b' \t':
+            output.write(s[:-1] + quote(s[-1:]) + lineEnd)
+        elif s == b'.':
             output.write(quote(s) + lineEnd)
         else:
             output.write(s + lineEnd)
@@ -73,16 +75,17 @@
             break
         outline = []
         # Strip off any readline induced trailing newline
-        stripped = ''
-        if line[-1:] == '\n':
+        stripped = b''
+        if line[-1:] == b'\n':
             line = line[:-1]
-            stripped = '\n'
+            stripped = b'\n'
         # Calculate the un-length-limited encoded line
         for c in line:
+            c = bytes((c,))
             if needsquoting(c, quotetabs, header):
                 c = quote(c)
-            if header and c == ' ':
-                outline.append('_')
+            if header and c == b' ':
+                outline.append(b'_')
             else:
                 outline.append(c)
         # First, write out the previous line
@@ -94,7 +97,7 @@
         while len(thisline) > MAXLINESIZE:
             # Don't forget to include the soft line break `=' sign in the
             # length calculation!
-            write(thisline[:MAXLINESIZE-1], lineEnd='=\n')
+            write(thisline[:MAXLINESIZE-1], lineEnd=b'=\n')
             thisline = thisline[MAXLINESIZE-1:]
         # Write out the current line
         prevline = thisline
@@ -105,9 +108,9 @@
 def encodestring(s, quotetabs = 0, header = 0):
     if b2a_qp is not None:
         return b2a_qp(s, quotetabs = quotetabs, header = header)
-    from io import StringIO
-    infp = StringIO(s)
-    outfp = StringIO()
+    from io import BytesIO
+    infp = BytesIO(s)
+    outfp = BytesIO()
     encode(infp, outfp, quotetabs, header)
     return outfp.getvalue()
 
@@ -124,44 +127,44 @@
         output.write(odata)
         return
 
-    new = ''
+    new = b''
     while 1:
         line = input.readline()
         if not line: break
         i, n = 0, len(line)
-        if n > 0 and line[n-1] == '\n':
+        if n > 0 and line[n-1:n] == b'\n':
             partial = 0; n = n-1
             # Strip trailing whitespace
-            while n > 0 and line[n-1] in " \t\r":
+            while n > 0 and line[n-1:n] in b" \t\r":
                 n = n-1
         else:
             partial = 1
         while i < n:
-            c = line[i]
-            if c == '_' and header:
-                new = new + ' '; i = i+1
+            c = line[i:i+1]
+            if c == b'_' and header:
+                new = new + b' '; i = i+1
             elif c != ESCAPE:
                 new = new + c; i = i+1
             elif i+1 == n and not partial:
                 partial = 1; break
             elif i+1 < n and line[i+1] == ESCAPE:
                 new = new + ESCAPE; i = i+2
-            elif i+2 < n and ishex(line[i+1]) and ishex(line[i+2]):
-                new = new + chr(unhex(line[i+1:i+3])); i = i+3
+            elif i+2 < n and ishex(line[i+1:i+2]) and ishex(line[i+2:i+3]):
+                new = new + bytes((unhex(line[i+1:i+3]),)); i = i+3
             else: # Bad escape sequence -- leave it in
                 new = new + c; i = i+1
         if not partial:
-            output.write(new + '\n')
-            new = ''
+            output.write(new + b'\n')
+            new = b''
     if new:
         output.write(new)
 
 def decodestring(s, header = 0):
     if a2b_qp is not None:
         return a2b_qp(s, header = header)
-    from io import StringIO
-    infp = StringIO(s)
-    outfp = StringIO()
+    from io import BytesIO
+    infp = BytesIO(s)
+    outfp = BytesIO()
     decode(infp, outfp, header = header)
     return outfp.getvalue()
 
@@ -169,21 +172,23 @@
 
 # Other helper functions
 def ishex(c):
-    """Return true if the character 'c' is a hexadecimal digit."""
-    return '0' <= c <= '9' or 'a' <= c <= 'f' or 'A' <= c <= 'F'
+    """Return true if the byte ordinal 'c' is a hexadecimal digit in ASCII."""
+    assert isinstance(c, bytes)
+    return b'0' <= c <= b'9' or b'a' <= c <= b'f' or b'A' <= c <= b'F'
 
 def unhex(s):
     """Get the integer value of a hexadecimal number."""
     bits = 0
     for c in s:
-        if '0' <= c <= '9':
+        c = bytes((c,))
+        if b'0' <= c <= b'9':
             i = ord('0')
-        elif 'a' <= c <= 'f':
+        elif b'a' <= c <= b'f':
             i = ord('a')-10
-        elif 'A' <= c <= 'F':
-            i = ord('A')-10
+        elif b'A' <= c <= b'F':
+            i = ord(b'A')-10
         else:
-            break
+            assert False, "non-hex digit "+repr(c)
         bits = bits*16 + (ord(c) - i)
     return bits
 
@@ -214,18 +219,18 @@
     sts = 0
     for file in args:
         if file == '-':
-            fp = sys.stdin
+            fp = sys.stdin.buffer
         else:
             try:
-                fp = open(file)
+                fp = open(file, "rb")
             except IOError as msg:
                 sys.stderr.write("%s: can't open (%s)\n" % (file, msg))
                 sts = 1
                 continue
         if deco:
-            decode(fp, sys.stdout)
+            decode(fp, sys.stdout.buffer)
         else:
-            encode(fp, sys.stdout, tabs)
+            encode(fp, sys.stdout.buffer, tabs)
         if fp is not sys.stdin:
             fp.close()
     if sts: