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1 StdProcedure Class
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2 ==================
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3
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4 The ``StdProcedure`` class encapsulates the encrypting and decrypting
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5 procedures outlined in :ref:`references-label`. In particular, see
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6 references [5] and [7]. This class takes care of the high level details of
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7 inserting various message indicators into an encrypted message, and stripping
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8 them off during decrypt. These message indicators tell the recipient what key
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9 list and initial key wheel settings to use when configuring their M-209 for
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10 decrypt.
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11
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12 .. class:: m209.procedure.StdProcedure([m_209=None[, key_list=None]])
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13
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14 :param m_209: an instance of a :class:`~m209.converter.M209` can optionally be
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15 provided to the procedure object. If ``None`` the procedure object
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16 will create one for internal use.
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17 :param key_list: an instance of a :class:`~m209.keylist.KeyList` can be
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18 provided if known ahead of time. Before an encrypt or decrypt operation
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19 can be performed, a key list must be provided. This can be done after
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20 object creation via :meth:`set_key_list`. Note that the ``letter_check``
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21 attribute of the :class:`~m209.keylist.KeyList` is not accessed by the
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22 procedure object, and can be ``None`` if not known.
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23
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24 Before an :meth:`encrypt` operation can be performed, a valid key list must be
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25 installed, either during procedure object construction, or by the
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26 :meth:`set_key_list` method.
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27
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28 Decrypt operations are performed in a 3-step process.
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29
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30 #. First, a call to the :meth:`set_decrypt_message` method passes the message
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31 to be decrypted to the procedure and establishes the parameters to be used
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32 for the actual :meth:`decrypt` operation. These decrypt parameters are
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33 returned to the caller.
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34
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35 #. The caller can examine the decrypt parameters to determine which key list
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36 must be installed before a successful :meth:`decrypt` operation can be
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37 carried out. The caller may call :meth:`get_key_list` to examine the
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38 currently installed key list. It is up to the caller to obtain the required
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39 key list and install it with :meth:`set_key_list`, if necessary. This is
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40 done by ensuring the installed key list indicator matches the
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41 ``key_list_ind`` field of the decrypt parameters.
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42
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43 #. Finally :meth:`decrypt` can be called. If the procedure does not have the
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44 key list necessary to decrypt the message, a ``ProcedureError`` is
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45 raised.
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46
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47 ``StdProcedure`` objects have the following methods:
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48
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49 .. method:: get_key_list()
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50
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51 :returns: the currently installed :class:`~m209.keylist.KeyList` object
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52 or ``None`` if one has not been set
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53
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54 .. method:: set_key_list(key_list)
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55
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56 Establishes the :class:`~m209.keylist.KeyList` to be used for future
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57 :meth:`encrypt` and :meth:`decrypt` operations
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58
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59 :param key_list: the new :class:`~m209.keylist.KeyList` to use
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60
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61 .. method:: encrypt(plaintext[, spaces=True[, ext_msg_ind=None[, sys_ind=None]]])
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62 :noindex:
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63
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64 Encrypts a plaintext message using the installed
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65 :class:`~m209.keylist.KeyList` and by following the standard procedure.
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66 The encrypted text with the required message indicators are returned as
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67 a string.
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68
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69 :param plaintext: the input string to be encrypted
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70 :param spaces: if ``True``, space characters in the input plaintext are
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71 allowed and will be replaced with ``Z`` characters before encrypting
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72 :param ext_msg_ind: this is the external message indicator, which, if
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73 supplied, must be a valid 6-letter string of key wheel settings. If not
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74 supplied, one will be generated randomly.
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75 :param sys_ind: this is the system indicator, which must be a string of length
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76 1 in the range ``A`` - ``Z``, inclusive. If ``None``, one is chosen at random.
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77 :returns: the encrypted text with the required message indicators
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78 :raises ProcedureError: if the procedure does not have
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79 a :class:`~m209.keylist.KeyList` or the input indicators are invalid
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80
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81 .. method:: set_decrypt_message(msg)
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82
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83 Prepare to decrypt the supplied message.
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84
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85 :param msg: the messsage to decrypt. The message can be grouped into
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86 5-letter groups separated by spaces or accepted without spaces.
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87 :returns: a ``DecryptParams`` named tuple to the caller (see below)
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88
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89 The ``DecryptParams`` named tuple has the following attributes:
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90
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91 * ``sys_ind`` - the system indicator
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92 * ``ext_msg_ind`` - the external message indicator
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93 * ``key_list_ind`` - the key list indicator
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94 * ``ciphertext`` - the cipher text with all indicators removed
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95
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96 The caller should ensure the procedure instance has the required
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97 :class:`~m209.keylist.KeyList` before calling :meth:`decrypt`. The
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98 ``key_list_ind`` attribute of the returned ``DecryptParams`` named tuple
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99 identifies the key list that should be installed with
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100 :meth:`set_key_list`.
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101
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102 .. method:: decrypt()
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103 :noindex:
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104
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105 Decrypt the message set in a previous :meth:`set_decrypt_message` call. The
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106 resulting plaintext is returned as a string.
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107
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108 :returns: the decrypted plaintext as a string
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109 :raises ProcedureError: if the procedure instance has not been
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110 previously configured with the required :class:`~m209.keylist.KeyList`
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111 via :meth:`set_key_list`
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112
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113 Here is a simple interactive example of performing an encrypt operation. Here
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114 we choose a random key list from our key list file, and use random indicators:
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115
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116 >>> from m209.keylist.config import read_key_list
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117 >>> from m209.procedure import StdProcedure
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118 >>>
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119 >>> key_list = read_key_list('m209keys.cfg')
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120 >>> proc = StdProcedure(key_list=key_list)
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121 >>> ct = proc.encrypt('ORDER THE PIZZA AT TWELVE HUNDRED HOURS')
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122 >>> ct
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123 'YYGBM ENNHT VBMTJ PEEFV JWLUU PAFTS VOHEA QEPEQ OKVUA XDAUX YYGBM ENNHT'
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124 >>>
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125
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126 The first and last two groups of this message contain the indicators. Here we
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127 can see the system indicator was ``Y``, the external message indicator is
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128 ``GBMENN``, and the key list indicator is ``HT``.
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129
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130 An example session for decrypting the above message might look like:
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131
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132 >>> proc = StdProcedure()
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133 >>> ct = 'YYGBM ENNHT VBMTJ PEEFV JWLUU PAFTS VOHEA QEPEQ OKVUA XDAUX YYGBM ENNHT'
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134 >>> params = proc.set_decrypt_message(ct)
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135 >>> params
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136 DecryptParams(sys_ind='Y', ext_msg_ind='GBMENN', key_list_ind='HT', ciphertext='VBMTJ PEEFV JWLUU PAFTS VOHEA QEPEQ OKVUA XDAUX')
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137 >>> key_list = read_key_list('m209keys.cfg', params.key_list_ind)
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138 >>> proc.set_key_list(key_list)
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139 >>> pt = proc.decrypt()
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140 >>> pt
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141 'ORDER THE PI A AT TWELVE HUNDRED HOURS '
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142 >>>
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143
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