|Publication number||US3808365 A|
|Publication date||Apr 30, 1974|
|Filing date||Dec 6, 1971|
|Priority date||Dec 8, 1970|
|Also published as||CA956891A, CA956891A1, DE2160132A1|
|Publication number||US 3808365 A, US 3808365A, US-A-3808365, US3808365 A, US3808365A|
|Original Assignee||Gretag Ag|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (14), Classifications (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent [191 [111 3,808,365
Ehrat Apr. 30, 1974  METHOD AND APPARATUS FOR ENCODING AND DECODING MESSAGES  ABSTRACT  Inventor: Kurt Ehrat, Zurich, Switzerland A method and app r us or encoding and decoding messages are provided, the method comprising contin-  Assignee. Gretag jktlegge sellslchtgt, uously changing the state of coding data in the storage Regens wltzer an cells of a coding pulse generator by a term derived  Filed: Dec. 6, 1971 fron tfirle date and cilocktimz. Thei rigding data is derive om a secret asic co e an e output coding  Appl' 205098 pulses from the pulse generator are used to encode clear message pulses, the encoded message then being  Foreign Application Priority Data transmitted to a receiving station. The encoded mes- Dec. 8 1970 Switzerland 18134/70 e is deemed by mixing with mdinfilqlieil' duced in the same way as the coding pulse were pro- 52 US. Cl. 178/22 duced by the generamrl Means are Provided  Int. Cl. H04l 9/02 Synchronismg the generation of the coding Pulses at 58 Field of Search 178/22 the transmiting and receiving smimls- The rules forming the coding pulse sequences are selected so  References Cited that these sequences depend only for a limited period of time on the state of their terms at the beginning of UNITED STATES PATENTS that period of time. This results in the initial coding 2 23 358 :32; g pulses produced by the code pulse generator at an instant in time being dependant only on the coding data 3,170,033 2/1965 Vasseur 178/22 which occurs during a predetermined period of time i prior to that instant and is independant of all coding igfizgl g x g i g data applied earlier to the coding pulse generator. Attorney, Agent, or Firm-Pierce, Scheffler & Parker 30 Claims, 21 awing Figures TIME CORRELATOR u BER k 82 I I Z8 suconme PULSE l 1 1 440 GENERATOR 54 mm 6 k 154 I 7 f I SECRET cone STORE 6 44 5' 3 SYNQrIZIgNIZING DIFFERENCE 81 sronz 132 f 9 8 I h -GLEAR LANGUAGE V TEXT-CLEAR MlXER I PuL g ssE flTER 102 4? r u m PATENTEDAPR 30 I974 sum 01 or 18 PATENTEDAPR 30 m4 sum 02 0F 18 llll-ll llll NQW w E2: 2104 nmkm b2 ER Nt DNv N55: 9
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PATENTEDAPR 30 I974 sum 10 UF18 PATENTEDAPR 30 1974 sum 12 or 18 FIG. l
PATENTEDAPR 30 I974 sum-1s or 18 $66 I 855KB ms;
PATENTEDAPR 30 I974 sum '17 [1F 18 llo PATENTEDAPR 30 1974 3808.365
sum '18 or 18 METHOD AND APPARATUS FOR ENCODING AND DECODING MESSAGES This invention relates to a method for the encoded transmission of messages in which the clear language message pulses are mixed on the transmitting side with coding .pulses and the clear message pulses are restored on the receiving side by mixing the received coded pulse train with identical coding pulses, the coding pulses being generated at the transmitting and receiving stations by identical pulse generators in accordance with identical rules based on a secret basic code and a term derived from the date and time.
BACKGROUND OF THE INVENTION In a previously proposed method of this kind the rules for forming the coding pulse sequences are selected so that they depend on the initial state of coding pulse generators, the initial state itself being defined by basic and supplementary codes. Since the coding pulse generators are set to a new initial state for each new message transmission it is impossible even for authorised third parties to gain access to a connection which is already established and over which a coded message is being transmitted. This is a disadvantage, in particular in multiple transmission networks.
The invention seeks to avoid this disadvantage by forming the coding pulse sequences in accordance with rules selected so that the last mentioned sequence depends only for a limited period of time on the condition of its terms at the beginning of the said period of time and that the instantaneous state of the data contained in all the storage cells of the coding pulse generator is constantly influenced by the term derived from the data and time, at least during message transmission.
SUMMARY OF THE INVENTION In a method according to the invention the initial state of the coding pulse generators is no longer preset, instead the instantaneous state of the data in the storage cells of the coding pulse generator are constantly influenced by a term derived from the date and time, that is to say the initial state of the coding pulse generator is constantly reset. Moreover, the rules for forming the coding pulse sequences are selected so that these sequences depend only for a limited period of time on the state of their terms at the beginning of the said period of time. This means that the initial data produced by the coding pulse generator at an instant in time depends only on the input data which occurs during a predetermined period of time prior to that instant and is independent of all input data applied earlier. An authorised third party may without difficulty gain access to an existing encoded message transmission since he need only wait for those moments in time at which the coding pulse sequence which occurs at the output of the coding pulse generator at the transmitting station is independent of the secret and supplementary coding states occuring prior to the aforementioned moments in time and in which the instantaneous state of the coding pulse generator is influenced by the term derived from the date and time.
In practice this means that the third party switches on his receiver at any desired moment in time, the receiver then automatically switching itself into the existing connection at the next possible moment in time.
This invention furthermore provides apparatus for performing the aforementioned method, the apparatus having a clock, preferably electronic, and a coding pulse generator which may be driven by a control pulse sequence formed from a secret basic code and from a term derived from the date and time. The coding pulse generator may be programmed so that each coding pulse at the output of the coding pulse generator remains unaffected by that part of the control pulse sequence which occurs prior to the affected coding pulse by an amount of time equal to the so-called passage time and that the passage time switches from a larger to a smaller value for a predetermined period of time and at intervals which depend on the secret code.
In a preferred embodiment of the invention described in detail hereinafter the clock is connected as a timing generator for the coding pulse generator.
BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described in detail with reference to the accompanying drawings in which:
FIG. 1 is a block diagram of a coding apparatus.
FIGS. 2a and 2b are details of FIG. 1.
FIGS. 3 to 6 are graphs illustrating the pulses used in synchronising two sets of coding apparatus.
FIGS. 7, 8 illustrate detail modifications of FIG. 1.
FIGS. 9, 10 are diagrams showing the function of synchronising sets of coding apparatus.
FIGS. 11, 12 are detail variations of FIG. 1.
FIGS. l3, 14 are diagrams for explaining the synchronising stage illustrated in FIG. 12, and
FIGS. 15 to 20 show detail variations of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT As shownin FIG. 1, each coding station is provided with a clear language text-clear language pulse converter 9, the converter being adapted to convert the clear language message text into clear language message pulses at the transmitter or vice versa at the receiver. This converter may be, for example, a teleprinter. The output of the transmitter station converter feeds the clear language message pulses through a conductor 147 into a mixer 8 the output of which is connected to the transmission line 148, which may be a cable, wire or radio link. When using the above described coding apparatus as a receiver, the transmission line 148' is connected to a mixer 8 the output of which feeds a converter 9 through a conductor 102. To facilitate encoding, the mixer 8 is provided with encoding pulse sequences from coding apparatus which are mixed in the transmitter station mixer with the clear language message pulse sequences generated by the clear language text-clear language pulse converter 9. The pulse sequences thus encoded and received on conductor 148 are mixed with decoding pulse sequences identical to the encoding pulse sequences and fed to the clear language text-clear language pulse converter at the receiving station which restores the received encoded pulse sequences to the clear language message text.
The coding apparatus mainly comprises an encoding pulse generator 5 and a synchronising stage 6. The purpose of the coding pulse generator 5 is to produce a coding pulse sequence with the longest possible cycle and which is built up in such a way that variation between successive elements or element groups is as random as possible. The coding pulse generator comprises
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|U.S. Classification||380/47, 380/260|