diff --git a/onetimepad.py b/onetimepad.py
index 62dfc0e..50cbe9b 100644
--- a/onetimepad.py
+++ b/onetimepad.py
@@ -7,6 +7,7 @@ TEXTLINE_OFFSET = -1
 DEFAULT_PAD_WIDTH = 72
 CIPHER_FLOOR = 1
 CIPHER_CEIL = 25
+NULLCHAR = '.'
 
 
 def encode(msg, pad):
@@ -52,7 +53,7 @@ def _as_line_chunks(chars, width=DEFAULT_PAD_WIDTH):
             yield ''.join(chunk)
             chunk = []
     if len(chunk) < width:
-        chunk += (['.'] * (width - len(chunk)))
+        chunk += ([NULLCHAR] * (width - len(chunk)))
     yield ''.join(chunk)
 
 
@@ -66,24 +67,38 @@ def _padfill(text, padlines):
 
 
 def _cipherfill(padfilled_lines):
-    padline = textline = None
     for i, line in enumerate(padfilled_lines):
-        if _is_padline(i):
-            padline = line
-        elif _is_textline(i):
-            textline = line
-        else:
-            cipherline = \
-                _get_cipherline_from_padline_and_textline(padline, textline)
-            padfilled_lines[i] = cipherline
-
-    return padfilled_lines
+        lineno = i + 1
+        if _is_cipherline(lineno):
+            padline = padfilled_lines[i - abs(PADLINE_OFFSET)]
+            textline = padfilled_lines[i - abs(TEXTLINE_OFFSET)]
+            yield padline
+            yield textline
+            yield _cipherline_from_padline_and_textline(padline, textline)
 
 
-def _get_cipherline_from_padline_and_textline(padline, textline):
+def _cipherline_from_padline_and_textline(padline, textline):
     ret = []
     for padchar, textchar in izip(padline, textline):
-        idx = (_AS_NUMS[padchar] + _AS_NUMS[textchar]) % CIPHER_CEIL
+        if textchar == NULLCHAR:
+            ret.append(NULLCHAR)
+            continue
+        charnum = _AS_NUMS[padchar] + _AS_NUMS[textchar]
+        idx = charnum if charnum <= CIPHER_CEIL else charnum % CIPHER_CEIL
+        ret.append(_AS_ALPHA[idx])
+    return ''.join(ret)
+
+
+def _textline_from_cipherline_and_padline(cipherline, padline):
+    ret = []
+    for ciphercar, padchar in izip(cipherline, padline):
+        if ciphercar == NULLCHAR:
+            ret.append(NULLCHAR)
+            continue
+        charnum = _AS_NUMS[ciphercar] - _AS_NUMS[padchar]
+        idx = charnum if charnum <= CIPHER_CEIL else charnum % CIPHER_CEIL
+        if idx < 0:
+            idx = CIPHER_CEIL + idx
         ret.append(_AS_ALPHA[idx])
     return ''.join(ret)
 
diff --git a/test_onetimepad.py b/test_onetimepad.py
index b5b6faf..b6c78ca 100644
--- a/test_onetimepad.py
+++ b/test_onetimepad.py
@@ -56,11 +56,24 @@ class TestOneTimePad(unittest.TestCase):
             self.assertFalse(OT._is_cipherline(lineno),
                              'line {0} is not cipherline'.format(lineno))
 
+    def test_make_cipherline_from_padline_and_textline(self):
+        actual = OT._cipherline_from_padline_and_textline(TEST_PADLINE,
+                                                          TEST_TEXTLINE)
+        self.assertEqual(TEST_CIPHERLINE, actual)
+
+    def test_make_textline_from_cipherline_and_padline(self):
+        actual = OT._textline_from_cipherline_and_padline(TEST_CIPHERLINE,
+                                                          TEST_PADLINE)
+        self.assertEqual(TEST_TEXTLINE, actual)
+
 
 PADLINES = (1, 4, 7, 10, 13, 16, 19, 22, 25)
 TEXTLINES = (2, 5, 8, 11, 14, 17, 20, 23, 26)
 CIPHERLINES = (3, 6, 9, 12, 15, 18, 21, 24, 27)
 
+TEST_PADLINE = 'HUCHUGGHOBUXADDHOHPGQBKXQKAOLRL'
+TEST_TEXTLINE = 'THISCAKEISUNSATISFACTORYTOMEYES'
+TEST_CIPHERLINE = 'BCMAXHRNXUPLTEXRGOQKKQBWKYNTKWD'
 
 if __name__ == '__main__':
     unittest.main()