diff --git a/dkimpy_milter/__init__.py b/dkimpy_milter/__init__.py
index c4c407b..bcdf2a7 100644
--- a/dkimpy_milter/__init__.py
+++ b/dkimpy_milter/__init__.py
@@ -28,8 +28,9 @@ import dkim
 import authres
 import os
 import tempfile
-import StringIO
+import io
 import re
+import codecs
 from Milter.utils import parse_addr, parseaddr
 import dkimpy_milter.config as config
 from dkimpy_milter.util import drop_privileges
@@ -110,7 +111,7 @@ class dkimMilter(Milter.Base):
     def envfrom(self, f, *str):
         if milterconfig.get('Syslog') and milterconfig.get('debugLevel') >= 2:
             syslog.syslog("mail from: {0} {1}".format(f, str))
-        self.fp = StringIO.StringIO()
+        self.fp = io.BytesIO()
         self.mailfrom = f
         t = parse_addr(f)
         if len(t) == 2:
@@ -142,13 +143,13 @@ class dkimMilter(Milter.Base):
         elif lname == 'authentication-results':
             self.arheaders.append(val)
         if self.fp:
-            self.fp.write("%s: %s\n" % (name, val))
+            self.fp.write(b"%s: %s\n" % (codecs.encode(name, 'ascii'), codecs.encode(val, 'ascii')))
         return Milter.CONTINUE
 
     @Milter.noreply
     def eoh(self):
         if self.fp:
-            self.fp.write("\n")   # terminate headers
+            self.fp.write(b"\n")   # terminate headers
         self.bodysize = 0
         return Milter.CONTINUE
 
@@ -195,20 +196,20 @@ class dkimMilter(Milter.Base):
             h = authres.AuthenticationResultsHeader(authserv_id=
                                                     self.AuthservID,
                                                     results=self.arresults)
-            h = fold(str(h))
+            h = fold(codecs.encode(str(h), 'ascii'))
             if (milterconfig.get('Syslog') and
                     milterconfig.get('debugLevel') >= 2):
-                syslog.syslog(str(h))
-            name, val = str(h).split(': ', 1)
+                syslog.syslog(codecs.decode(h, 'ascii'))
+            name, val = codecs.decode(h, 'ascii').split(': ', 1)
             self.addheader(name, val, 0)
         return Milter.CONTINUE
 
     def sign_dkim(self, txt):
-        canon = milterconfig.get('Canonicalization')
+        canon = codecs.encode(milterconfig.get('Canonicalization'), 'ascii')
         canonicalize = []
-        if len(canon.split('/')) == 2:
-            canonicalize.append(canon.split('/')[0])
-            canonicalize.append(canon.split('/')[1])
+        if len(canon.split(b'/')) == 2:
+            canonicalize.append(canon.split(b'/')[0])
+            canonicalize.append(canon.split(b'/')[1])
         else:
             canonicalize.append(canon)
             canonicalize.append(canon)
@@ -218,11 +219,12 @@ class dkimMilter(Milter.Base):
         try:
             if privateRSA:
                 d = dkim.DKIM(txt)
-                h = d.sign(milterconfig.get('Selector'), self.fdomain,
-                           privateRSA, canonicalize=(canonicalize[0],
-                                                     canonicalize[1]))
-                name, val = h.split(': ', 1)
-                self.addheader(name, val.strip().replace('\r\n', '\n'), 0)
+                h = d.sign(codecs.encode(milterconfig.get('Selector'), 'ascii'), codecs.encode(self.fdomain, 'ascii'),
+                           codecs.encode(privateRSA, 'ascii'),
+                           canonicalize=(canonicalize[0],
+                                         canonicalize[1]))
+                name, val = h.split(b': ', 1)
+                self.addheader(codecs.decode(name, 'ascii'), codecs.decode(val, 'ascii').strip().replace('\r\n', '\n'), 0)
                 if (milterconfig.get('Syslog') and
                     (milterconfig.get('SyslogSuccess')
                      or milterconfig.get('debugLevel') >= 1)):
@@ -233,12 +235,12 @@ class dkimMilter(Milter.Base):
                                                   d.domain.lower()))
             if privateEd25519:
                 d = dkim.DKIM(txt)
-                h = d.sign(milterconfig.get('SelectorEd25519'), self.fdomain,
+                h = d.sign(codecs.encode(milterconfig.get('SelectorEd25519'), 'ascii'), codecs.encode(self.fdomain, 'ascii'),
                            privateEd25519, canonicalize=(canonicalize[0],
                                                          canonicalize[1]),
-                           signature_algorithm='ed25519-sha256')
-                name, val = h.split(': ', 1)
-                self.addheader(name, val.strip().replace('\r\n', '\n'), 0)
+                           signature_algorithm=b'ed25519-sha256')
+                name, val = h.split(b': ', 1)
+                self.addheader(codecs.decode(name, 'ascii'), codecs.decode(val, 'ascii').strip().replace('\r\n', '\n'), 0)
                 if (milterconfig.get('Syslog') and
                     (milterconfig.get('SyslogSuccess')
                      or milterconfig.get('debugLevel') >= 1)):
@@ -266,20 +268,17 @@ class dkimMilter(Milter.Base):
                     res = d.verify(idx=y, dnsfunc=lambda _x: dnsoverride)
                 else:
                     res = d.verify(idx=y)
+                algo = codecs.decode(d.signature_fields.get(b'a'), 'ascii')
                 if res:
-                    if d.signature_fields.get(b'a') == 'ed25519-sha256':
+                    if algo == 'ed25519-sha256':
                         self.dkim_comment = ('Good {0} signature'
-                                             .format(d.signature_fields
-                                                     .get(b'a')))
+                                             .format(algo))
                     else:
                         self.dkim_comment = ('Good {0} bit {1} signature'
-                                             .format(d.keysize,
-                                                     d.signature_fields
-                                                     .get(b'a')))
+                                             .format(d.keysize, algo))
                 else:
                     self.dkim_comment = ('Bad {0} bit {1} signature.'
-                                         .format(d.keysize,
-                                                 d.signature_fields.get(b'a')))
+                                         .format(d.keysize, algo))
             except dkim.DKIMException as x:
                 self.dkim_comment = str(x)
                 if milterconfig.get('Syslog'):
@@ -288,9 +287,9 @@ class dkimMilter(Milter.Base):
                 self.dkim_comment = str(x)
                 if milterconfig.get('Syslog'):
                     syslog.syslog("check_dkim: {0}".format(x))
-            self.header_i = d.signature_fields.get(b'i')
-            self.header_d = d.signature_fields.get(b'd')
-            self.header_a = d.signature_fields.get(b'a')
+            self.header_i = codecs.decode(d.signature_fields.get(b'i'), 'ascii')
+            self.header_d = codecs.decode(d.signature_fields.get(b'd'), 'ascii')
+            self.header_a = codecs.decode(d.signature_fields.get(b'a'), 'ascii')
             if res:
                 if (milterconfig.get('Syslog') and
                         (milterconfig.get('SyslogSuccess') or