Process and system for transmission of signed messages

The transmission of signed messages occurs between a transmitter (1) and a receiver (3) connected by a transmission line system (2). The transmitter (1) and the receiver (3) respectively comprise a coding device 7 and a decoding device 11. Means 4 situated at the transmitter supply a signature SG as a function of the message M which is to be transmitted. The message associated with its signature, as well as a common key E, are transmitted by the transmitter towards the receiver after coding by means of an intrinsic key R.sub.1. Upon being received, the message M linked with its signature SG is decoded by means of an intrinsic key R.sub.2 determined at the receiver location in such manner as to establish equality between intrinsic keys R.sub.1 and R.sub.2.

BACKGROUND OF THE INVENTION 
The present invention relates to a process and system for transmission of 
"signed" messages. The present invention is related to the invention to 
which commonly-assigned copending application Ser. No. 200,785, filed Oct. 
27, 1980, by Robert J. L. HERVE, and entitled "SYSTEM AND PROCESS FOR 
IDENTIFICATION OF PERSONS REQUESTING ACCESS TO TICULAR FACILITIES" is 
directed, the entire disclosure of which is hereby expressly incorporated 
by reference. 
More particularly, the invention relates to systems in which messages are 
transmitted in a coded binary form, as well as in other forms. These 
messages are coded by means of coding keys upon transmission, and then 
decoded for restoration to their original clear form upon reception. A 
system of this kind is disclosed in commonly-assigned U.S. patent 
application Ser. No. 235,505, filed Feb. 18, 1981 by Robert J. L. HERVE, 
and entitled "METHOD AND SYSTEM FOR TRANSMISSION OF CONFIDENTIAL DATA", 
the entire disclosure of which is also hereby expressly incorporated by 
reference. 
The system described in the above-identified application Ser. No. 235,505 
makes use of two coding keys, as follows: 
The first is a standard or common key E transmitted directly in clear from 
the transmitter to the receiver. The second is an intrinsic key R 
calculated simultaneously at the transmitter and the receiver locations. 
The intrinsic key R is a function of the common key E, of an 
identification code I.sub.n linked with the message which is transmitted, 
and of a secret code S stored in two devices situated at the transmitter 
and receiver respectively, or at the transmitter and receiver locations. 
The intrinsic key R.sub.1 calculated at the transmitter location is used 
for encoding the message transmitted, and the intrinsic key R.sub.2 
calculated at the receiver location is used to decode the coded message 
transmitted. 
If the calculating function employed at the transmitter and receiver 
locations is the same and if the secret code S stored at these locations 
is the same, the intrinsic keys R calculated are identical. Under these 
conditions, all that is needed is to utilize, at the transmitter and 
receiver locations, an operator which allows of simultaneous encoding of 
the message by means of the intrinsic key R.sub.1 calculated upon 
transmission and decoding by means of the intrinsic key R.sub.2 calculated 
upon reception. These coding and decoding operations may easily be 
performed by means of logic circuits of the combinatory type, for example 
EXCLUSIVE-OR circuits. 
The system described makes it possible to perform constant modification of 
the intrinsic keys R calculated at the transmitter and receiver locations 
by random alteration in the course of time of the value of the common key 
E. 
According to another feature of the system, the calculation of each of the 
intrinsic keys R at the transmitter and receiver locations is accomplished 
in each case by a processor situated within a portable object which also 
comprises a memory wherein is stored a program P for calculation of the 
function for determining the intrinsic key as a function of the secret 
code S also stored in the memory, of the common key E, and of the 
identification code I.sub.n linked with the message. These portable 
objects are each placed at the disposal of the persons entrusted with 
transmitting and receiving the messages. 
The system may then operate only if both persons have strictly matched 
portable objects, which amounts to saying, for example, that the 
processing units must be actuated by the same program P and that the 
memories must contain the same secret code S. It is obvious that a 
fraudulent person lacking both the program P and the secret code S will be 
unable to calculate the intrinsic key R enabling him to decode the message 
transmitted. His task becomes ever more difficult since the system alters 
the common key E in a random manner. 
According to another feature of the system, and for the purpose of 
increasing the security of the system in a practically absolute manner, 
each portable object of the system contains within its memory a table of 
the identification codes I.sub.n related to the messages which each person 
will be authorized to transmit and receive. 
The identification code related to the message contained in the memories of 
the portable objects are located or "pointed" to by address generators at 
the transmitter and receiver locations. These address generators establish 
correspondence between the identification code I.sub.n linked with the 
message and the address of the identification code I.sub.n contained in 
the memory of the portable object. The message transmission may then only 
occur correctly if the address generator and the portable objects have 
been able to establish the identity of the identification code I.sub.n 
related to the message. 
The system described in the above-identified patent application Ser. No. 
235,505 resolves the problems of transmission of coded messages, but it is 
incomplete in that it does not render it possible to establish certain 
authentication of the person who had transmitted the messages, and this 
may constitute a handicap to the application of this system in particular 
spheres. 
As a matter of fact, in the social structure--be it commercial or 
private--the agreements concluded between legal or physical entities may 
form the object of contracts. One of the contracting parties may, if it so 
wishes, enforce application in indisputable manner of the test specifying 
the contractual engagement. The contract is in effect liable to be 
implemented. The contract comprises a text and a particular number of 
signatures. The material form of the document (i.e., the written document) 
only resolves a practical problem of being made available for recognition. 
The legal ambit of the contract resides entirely in the data corresponding 
to the text and in the validity of the signatures appended. The 
transposition of a text in binary code, without loss of information, does 
not raise any difficulties. By contrast, if this transposition is not 
absolute for the signature, there is a loss of information, and more 
serious, the absence of a link between the text and the signature offers 
the possibility of combining a text and signature of different origins. 
SUMMARY OF THE INVENTION 
To eliminate these shortcomings, an object of the present invention is to 
provide a system for transmission of signed messages, which system couples 
the text of the message to its signature in indissoluble manner. 
In accordance with one aspect of the invention, the system operates in a 
manner such that the sender of the message may easily produce the 
signature accompanying the same and such that this signature may be 
verified by the addresses. 
In accordance with another aspect of the invention, the signature produced 
is strictly a function of the message transmitted; it is not accessible to 
third parties, and certainly not to the addressee who could make use of 
the same as of right to sign undertakings by mistakenly assuming the 
consent of the genuine holder of the signature. 
In preferred embodiments, the system of the invention consequently has the 
following features: 
(1) The production of the signature is automatic. 
(2) The addressee is able to verify the signature on two levels: 
(a) On the level of the system as such, since the system as described in 
the above-identified patent application Ser. No. 235,505 establishes a 
dialogue between two matched portable objects which results in mutual 
recognition of the two correspondents; and 
(b) On the level of an agency which personalizes the portable objects by 
inserting a secret code or parameter J into the portable objects at the 
time of their delivery to their holders. If this agency alone is 
authorized to be cognizant of the totality of the secret code J of the 
portable objects which is issues to the applicants and for which it is 
consequently the repository, this agency may be consulted by any addressee 
who wishes to verify the signed message he had received. 
This agency with which are deposited all the parameters J of the cards 
produced can discover the secret code J of any sender who has had to 
declare his identity to the addressee. If this identity is accurate, the 
storing agency is then able to determine the numerical value of the 
signature by means of the message entrusted to it and of the secret device 
allocated to the sender. In case of identity, the signature may be 
certified. 
(3) The signature of the message is a function of the message. A single 
letter altered in the message removes from the whole, text and signature, 
its subjacent coherence. This may be done in the same way as an optional 
alteration to a message at the time of its transmission should be detected 
by the verification test determined at the time of transmission, 
transmitted after the message and checked upon reception (by means of the 
application of Hamming codes for example), in order to determine 
compatibility between the message and the test. 
(For reasons of practical application, the message will most often be split 
into several blocks, each block being signed.) 
(4) Access to the signature is denied to third parties in guaranteed 
manner. 
(5) Although the addressee owns a portable object matched to that of the 
sender transmitting a message to him, he should not be able to validly 
sign the message he receives instead of the sender. This condition is 
fulfilled a priori without any trouble if the addressee does not know the 
secret code J, belonging to the sender, which would enable him to 
calculate the correct signature, and if he also lacks the sender's card 
which is the only means of producing this signature. The sender and 
addressee have in common only the secret code S and program P permitting 
the transposition of the common key E into an intrinsic key R for the 
coding and decoding operations. 
The process and system for transmission of signed messages in accordance 
with the invention achieves the foregoing features. 
Briefly, a process in accordance with the invention comprises the following 
steps: 
(1) Producing a signature SG at the location of the message transmitter as 
a function of the message M which is to be transmitted, of a secret code J 
and of an identification code I.sub.n of the message, and then associating 
the message M with the signature SG thus obtained; 
(2) Causing the transmitter to send a common key E and the identification 
code I.sub.n to the addressee; and 
(3) Causing to be calculated at the transmitter an intrinsic key R.sub.1 
for coding the message M which is to be transmitted associated with its 
signature SG at the transmitter location, and causing to be calculated at 
the receiver location an intrinsic key R.sub.2 for decoding the message 
associated with its signature, each of the intrinsic keys R.sub.1 and 
R.sub.2 being obtained from a secret code S, from the identification code 
I.sub.n of the message and from the common key E supplied by the 
transmitter to the receiver. 
The system in accordance with the invention employs the process summarized 
immediately above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The system illustrated in FIG. 1 comprises a transmitter 1 and a receiver 3 
interconnected by a transmission line system 2. The transmitter 1 and 
receiver 3 are adapted to be coupled temporarily to portable objects 4 and 
5 comprising, respectively, processing means in the form of 
microprocessors 4' and 5', as well as memories or storage devices 4" and 
5". Portable objects of this kind are described in commonly-assigned Ugon 
U.S. Pat. No. 4,211,919, issued July 8, 1980, and entitled "PORTABLE DATA 
CARRIER INCLUDING A MICROPROCESSOR", the entire disclosure of which is 
hereby expressly incorporated by reference. 
The storage devices 4" and 5" of the portable objects 4 and 5 are divided 
into at least five sections. The first is a secret section containing a 
secret code S which is written when the portable object 4 or 5 is 
initialized. The secret code S may thereafter be read only by the 
processor 4' or 5' of the portable object 4 or 5 itself, and cannot in any 
event be read from the outside. 
The second section contains the identification codes I.sub.n related to the 
messages which the bearer of the object is authorized to transmit or 
receive. The writing of the identification codes I.sub.n in the second 
memory section may be protected for reasons of security by a key code 
unknown to the bearer of the portable object. 
The third section contains a program P.sub.1, the function of which is to 
calculate an intrinsic key R from the parameters S, I.sub.n and from the 
common key E. This is summarized as the function R=P.sub.1 (S,I.sub.n,E). 
The fourth section contains the secret code J related to the legitimate 
holder of the portable object. 
The fifth section contains a program P.sub.2, the function of which is to 
calculate the signature SG from the parameters J, I.sub.n and from the 
message M which is to be transmitted, as summarized by the function 
SG=P.sub.2 (J,I.sub.n,M). Each message, even from the same signatory, thus 
has a different signature SG, and identical messages transmitted by 
different persons thus have different signatures. 
The fourth and fifth memory sections are not in use when the portable 
object is utilized for reception. 
Once the programs P.sub.1 and P.sub.2 have been written, they cannot be 
altered and may advantageously be contained in a Read Only Memory (ROM). 
The writing of the identification codes I.sub.n into the second section of 
the memory may for security reasons be protected by a standard key unknown 
to the bearer of the portable object. 
While in no way intended to limit the scope of the invention, the 
information stored in the memories 4" and 5" and operated on by the 
processors 4' and 5' may be similar to that which is disclosed in the 
above-identified commonly-assigned Herve application Ser. No. 200,785, 
filed Oct. 27, 1980. Below is repeated a specific example of a suitable 
program P. This particular example corresponds to the program P.sub.1 for 
calculating an intrinsic key R from S, I.sub.n and E. The program P.sub.2 
for calculating a signature SG from J, I.sub.n and M. 
First, the data words have the following lengths in this specific example: 
The identification code I.sub.n is 32 bits long; the common key E is also 
32 bits long; and the secret code S is 64 bits long. The result of the 
computation, the intrinsic key R, is 64 bits long. 
The computation effected by the instructions comprising the stored program 
P executed in the microprocessors 4' and 5' has the following three 
overall steps: 
(1) First, an intermediate result, R.sub.1, is calculated by concatenating 
E with I.sub.n. R.sub.1 =(E, I.sub.n). 
(2) Second, another intermediate result, S.sub.1, is calculated by an 
EXCLUSIVE-OR operation of R.sub.1 with S. S.sub.1 =R.sub.1 .sym.S. 
(3) Third, the result, R, is calculated by multiplying S with S.sub.1 with 
R.sub.1, modulo 2.sup.64 -1. R=S.times.S.sub.1 .times.R.sub.1, modulo 
2.sup.64 -1. 
This computation can be performed in a microprocessor carried on a card 
such as is disclosed in the above-referenced Ugon U.S. Pat. No. 4,211,919. 
By way of example, a suitable commercially-available memory device which 
may be employed as the memories 4" and 5" is a Mostek Type No. MK4022. 
Similarly, the microprocessors 4' and 5' may each comprise an Intel Type 
No. 8080 or an Intel Type No. 8085. 
The following lists one form of machine-language program which may be 
employed as the programs P. This program listing assumes that registers 
denoted A, B, C, D and T (not shown) are available. 
______________________________________ 
EXAMPLE PROGRAM LISTING 
Instructions Comments 
______________________________________ 
001 Load I.sub.n .fwdarw. A 
Load registers A, B and C with 
the parameters I.sub.n, E, S 
002 Load E .fwdarw. B 
003 Load S .fwdarw. C 
004 MOV B .fwdarw. A 
Concatenate E with I.sub.n to 
generate R.sub.1, and leave in 
register A 
005 MOV A, T Also store R.sub.1 in register T 
006 X CT S.sub.1 = R.sub.1 .sym. S, to T 
007 MUL (T C) S .times. S.sub.1 to T 
008 MUL (T A) R = S .times. S.sub.1 .times. R.sub.1 to T 
009 Load 2.sup.64 - 1 .fwdarw. D 
Load value 2.sup.64 - 1 into 
register D 
010 COMP (T, D) if R .gtoreq. 2.sup.64 - 1 
011 IF (1) return to 1 
012 END 
______________________________________ 
The particular transmission line system 2 employed is not important insofar 
as the present invention is concerned, and may be selected as desired from 
among available forms of transmission line systems such as those using an 
electric cable, or an optical, acoustic, magnetic, electromagnetic, or 
radio wave connection. 
The FIG. 1 transmitter 1 principally comprises a register 6' for 
concatenation of the message M which is to be transmitted with its 
signature SG, a coding device 7, a first address generator 8, a random 
number generator 9, and a sequencer S.sub.0 10. The function of the 
transmitter 1 is to transmit the message M accompanied by its 
identification code I.sub.n present in a message-in-clear register 6. 
The FIG. 2 receiver comprises a decoding device 11, second address 
generator 12 and, a sequencer S.sub.1 13. 
The address generators 8 and 12 are described in detail hereinbelow with 
reference to FIG. 2. The sequencers S.sub.0 10 and S.sub.1 13 are 
described in detail hereinbelow with reference to FIG. 3. 
The portable objects 4 and 5 are adapted to be coupled temporarily to the 
transmitter 1 and to the receiver 3 by respective coupling means C.sub.1 
and C.sub.2. 
The register 6' is connected to the register 6 in a manner such that the 
contents comprising the message M are transferred from the register 6 into 
the register 6'. 
The coding device 7 receives, at its input terminal 1, the message M in 
clear associated with its signature SG from the register 6', which message 
M and signature SG are to be coded. The coding device 7 receives at its 
input terminal 2 the intrinsic coding key R.sub.1 via a multiplexer 7'. An 
encoded message g(M,SG,R.sub.1) is generated by the coder 7 and 
transmitted from the coder 7 output terminal 3 to the input terminal 1 of 
the decoding device 11 via the transmission line system 2. The encoded 
message g(M,SG,R.sub.1) is then decoded by the device 11 which receives at 
its input terminal 2 the necessary matching intrinsic key R.sub.2. The 
message M accompanied by its signature SG appears in clear at the decoder 
11 output terminal 3 and is depicted at 14. (The coding device 7, the 
decoding device 11, and the form of encoded message g(M,SG,R) are 
described in detail hereinbelow with reference to FIG. 4.) 
The intrinsic coding key R.sub.1 is calculated and supplied by the 
processor 4' of the portable object 4. Similarly, the intrinsic decoding 
key R.sub.2 is calculated and supplied by the processor 5' of the portable 
object 5. 
The first address generator 8 calculates the address (Adv) of the 
identification code I.sub.n of the message situated in the memory 4" of 
the portable object 4 from the identification code I.sub.n linked to the 
message in clear 6 and transmits this addres via its output 2 to the data 
and address bus of the portable object 4 (see commmonly-assigned Ugon U.S. 
Pat. No. 4,211,919), via the coupling means C.sub.1. The first address 
generator 8 is reset to an initial state by the signal RAZ transmitted to 
its input 3 from the output 2 of the sequencer S.sub.0 10, and is 
activated by clock signals H.sub.0 transmitted to its input 4 from the 
output 1 of the sequencer S.sub.0 10. The sequencer S.sub.0 10 is 
energized by a START signal as soon as a message is transmitted to the 
input E.sub.2 (identification code I.sub.n in the register 6) of the 
transmitter 1. 
The system performs constant modification of the intrinsic keys R 
calculated in the processors 4' and 5' associated respectively with the 
transmitter 1 and receiver 3 by random alteration of the common key E as a 
function of time. Specifically, the common key E is generated by a random 
number generator 9 which may comprise a simple ring counter. The generator 
9 supplies a random number forming the common key E in bit serial form as 
clock signals H.sub.0 transmitted by the sequencer S.sub.0 10 are input to 
the random number generator 9. This random number comprising the common 
key E is transmitted via the output terminal 2 of the generator 9 to the 
corresponding input terminals of a multiplexer 9' and of the portable 
object 5 via an AND gate 9". At its second input terminal, the multiplexer 
9' receives the message M which is to be transmitted coming from the 
register 6. (In a modified embodiment of the invention, it will be 
possible to contemplate field reductions of the message M before applying 
the same to the input side of the multiplexer 9' in such manner as to 
change it to a format utilizable by the portable object 4.) 
In the same way as for the first address generator 8 in the transmitter 1, 
the second address generator 12 in the receiver 3 is supplied at its input 
1 with the identification code I.sub.n (transmitted to it from the 
register 6 via the transmission line system 2) so that it may calculate 
and deliver at its output 2 the address of the corresponding 
identification code I.sub.n in the memory 5" of the portable object 5. 
This address generator 12 is reset to an initial state by the signal RAZ 
applied to its input 3 by the sequencer S.sub.1 13, and is activated by a 
clock signal H.sub.1 fed to its input 4 by the sequencer S.sub.1 13. The 
sequencer S.sub.1 13 is energized via its input 3 as soon as a message is 
transmitted via the transmission line 2. 
With reference now to FIG. 2, there is shown one embodiment of an address 
generator suitable for use as the address generators 8 and 12 of FIG. 1. 
The FIG. 2 address generator comprises a memory 15 which may for example 
be either a Random Access Memory (RAM), a Programmable Read Only Memory 
(PROM), or an Erasable Programmable Read Only Memory (EPROM). This memory 
15 contains a table of all identification codes I.sub.0 through I.sub.n of 
the messages which the system is authorized to transmit. 
The memory 15 is addressed by means of an address counter 16. This address 
counter 16 is stepped by means of the clock signal H applied at the input 
4 of the address generator, and is reset to zero by the signal RAZ applied 
at the input 3 of the address generator. 
The address counter 16 determines the address of the identification code 
I.sub.n contained in the memory 4" or 5" of the portable object 4 or 5 and 
corresponding to the message 6 which is to be transmitted. This 
determination is performed by an identification code register 18 and a 
comparator 19. The identification code I.sub.n accompanying the message 6 
which is to be transmitted is fed into the identification register 18, 
after which the address counter 16 progresses at the rate of the clock H 
so as to address and read the identification codes I.sub.n contained in 
the memory 15. The identification codes I.sub.n are thus presented 
successively at an input of the comparator 19 which compares them to the 
identification code contained in the identification code register 18. 
If correspondence is established by the comparison, progression of the 
address counter 16 is stopped by application of the signal HIT to address 
counter 16 input 4, the signal HIT being supplied by output 3 of the 
comparator 19. At the same time, the contents of the address counter 16 
(representing the address of the location within the portable object 4 or 
5 of the identification code I.sub.n of the message which is to be 
transmitted) is transmitted via an AND gate 20 (activated at its input 1 
by the signal HIT) as a signal ADV to the output 2 of the address 
generator. 
Similarly, with reference now to FIG. 3, an embodiment of a sequencer 
suitable for use as the sequencers S.sub.0 10 and S.sub.1 13 of FIG. 1 is 
shown. The FIG. 3 sequencer comprises a clock 21, an RS flip-flop 22, an 
AND gate 23, a ring counter 24, and a decoder 25. An input of the AND gate 
23 receives the clock signals coming from the clock 21, these signals 
being retransmitted at the AND gate 23 output towards the output 1 of the 
sequencer when the other input of the gate 23 is activated via the Q 
output of the flip-flop 22. The Q output of the flip-flop 22 assumes a 
logic high state (binary "1") when it is triggered at its Set input (S) by 
the START signal applied to the input 3 of the sequencer. This START 
signal may comprise a particular bit of the identification code I.sub.n 
accompanying the message 6 which is to be transmitted. The signal H is 
transmitted to the input of the ring counter 24 of which a particular 
state is decoded by the decoder 25 to actuate the multiplexers 7' and 9', 
and the AND gate 9" of the transmitter 9. Another state of the ring 
counter 24 is decoded by the decoder 25 in such manner as to reset the 
flip-flop 22 to the zero state and deliver the zero reset signal RAZ at 
the output terminal 2 of the sequencer. 
With reference now to FIG. 4, there is shown a combinatory circuit such as 
may comprise both the FIG. 1 coding device 7 and the FIG. 1 decoding 
device 11. As shown, the FIG. 4 circuit comprises EXCLUSIVE-OR logic 
gates. In operation, upon transmission, a bit M.sub.i of the message to be 
transmitted (or SG.sub.i of the signature) and a bit R.sub.i of the 
intrinsic key are applied, respectively, to an input of an EXCLUSIVE-OR 
circuit in such manner that the resulting combination satisfies the 
Boolean logic equation S.sub.i =M.sub.i .sym.R.sub.i (or S.sub.i =SG.sub.i 
.sym.R.sub.i in the case of the signature). Upon reception, decoding is 
performed by also employing an EXCLUSIVE-OR circuit. The signals S.sub.i 
and R.sub.i are applied to two of its inputs in such manner as to restore 
the bits M.sub.i. The equation for the restored M.sub.i is M.sub.i 
=S.sub.i .sym.R.sub.i. 
The example of FIG. 4 shows an embodiment of a coding circuit for a message 
M and an intrinsic key word R each of three bits. The EXCLUSIVE-OR gates 
26, 27 and 28 deliver the signals S.sub.1 to S.sub.3 as follows: 
EQU S.sub.1 =M.sub.1 .sym.R.sub.1 
EQU S.sub.2 =M.sub.2 .sym.R.sub.2 
EQU S.sub.3 =M.sub.3 .sym.R.sub.3 
Similarly, for decoding, EXCLUSIVE-OR gates deliver the signals M.sub.1 to 
M.sub.3 of the restored message M as follows: 
EQU M.sub.1 =S.sub.1 .sym.R.sub.1 
EQU M.sub.2 =S.sub.2 .sym.R.sub.2 
EQU M.sub.3 =S.sub.3 .sym.R.sub.3 
The restored message is thus identical to the original message, regardless 
of the specific message and the specific intrinsic key R, so long as 
identical intrinsic keys R.sub.1 and R.sub.2 are used for both coding and 
decoding. 
In summary, the operation of the system for transmission of signed messages 
in accordance with the invention is as follows: 
The appearance of the message in clear at the input terminals E.sub.1 and 
E.sub.2 of the transmitter 1 causes the triggering of the respective 
address generators 8 and 12 of the transmitter 1 and the receiver 3, as 
well as of the random number or common key E generator 9. The identity of 
the message is recognized by the respective address generators 8 and 12 
which then transmit to the portable objects 4 and 5 the address (Adv) of 
the corresponding identity code I.sub.n in the memories 4" and 5" of the 
portable objects 4 and 5. 
The multiplexer 9' transmits the message M to the processor 4' of the 
portable object 4. The signature SG is then calculated by the processor of 
the portable object 4 according to the function SG=P.sub.2 (J,I.sub.n,M). 
The resultant signature SG is then transmitted via the multiplexer 7' into 
the register 6' in which the message M which is to be transmitted and the 
signature SG are concatenated. 
Upon appearance of the signal delivered by the sequencer SO 10, the random 
code number comprising the common key E is transmitted in its turn to the 
portable object 4 and to the receiver 3. The intrinsic keys R.sub.1 and 
R.sub.2 are then calculated by the processing devices of each of the 
portable objects by means of the function R.sub.x =P.sub.1 (S,I.sub.n,E), 
one example of which is given hereinabove. If the portable objects are of 
the same nature, that is to say if they each contain the same program 
P.sub.1, the same secret code S and the same identification code I.sub.n, 
the calculated intrinsic keys R.sub.1 and R.sub.2 are then the same. The 
intrinsic key R.sub.1 is applied to the coding device 7, and the intrinsic 
key R.sub.2 is applied to the decoding device 11. The message received at 
14 is then identical to the message in clear M transmitted at 6, and it 
complementarily comprises its signature. 
The example which has been given of a preferred embodiment of the invention 
is not restrictive in any way, and it will be possible to envisage 
modified embodiments of the invention without thereby exceeding its scope. 
As a matter of fact, in accordance with the same principle, it will be 
optional to embody a system permitting the transmission of cosigned 
messages. This will be the case where the message will imply a mutual 
engagement, the sender and the addressee wishing to be in possession of 
the text bearing the signature of the other party. To this end, the sender 
will ask the addressee to retransmit under his own signature, the message 
blocks he receives, as and when received. 
Similarly, it will always be possible to issue to the sender a message 
certifying receipt of signed information, the addressee having to collate 
the signature or signatures appended to the data received in a reply 
message, and to append his own signature to the same. This message will be 
an acknowledgement of receipt which is conclusive, since only the sender 
can produce the signatures associated with the message transmitted and 
only the addressee can sign the acknowledgement of receipt.