A method for securely transmitting a secret item between a first telecommunications device and at least one second telecommunications device.

Abstract

The invention relates to a secure transmission of a secret element between a key server (SERV) and a telecommunications device (DISPO1) comprising a secure cryptographic module (SE), with reciprocal authentication including in particular: by the server: generation of a first random value; encryption of said first random value with the public key of the device; determination of a first signature, by use of the server's private key, of said first encrypted random value; transmission to the device of a message containing said first encrypted random value and said first signature; by the device, in secure mode: verification of the authenticity of the server based on said first encrypted random value, the first signature and the public key of the device;then decryption, by the secure cryptographic module (SE) and using the server's private key, of said first encrypted random value and storage of the first decrypted random value in secure memory; encryption of a second random value with the server's public key; determination of a second signature by the secure cryptographic module (SE), with the device's private key stored in said module, of said second encrypted random value. Figure for the abbreviation: Fig. 1;

Description

Title of the invention: Method for the secure transmission of a secret element between a first telecommunications device and at least one second telecommunications device. Technical field

[0001] The invention relates to the establishment of a secure communication channel between two pieces of equipment, allowing the exchange of secret data, for example a temporary key, between the two pieces of equipment, without the intervention of a third party, as opposed to the distribution of a key by a trusted authority.

[0002] The invention is particularly applicable to critical embedded systems (CES). Previous technique

[0003] Industrial control systems (ICS), such as SCADA, are distributed systems by nature, connecting sensors and actuators to a supervisor via dedicated buses (fieldbuses). Historically, all this equipment formed an isolated network and was confined to a secure area within a company building, where physical and organizational measures ensured its physical and IT security. From the 1990s onward, Industrial Ethernet enabled greater interoperability of these systems and their integration with IT networks. Around 2010, the maturation of lightweight and decentralized communication protocols such as OPC UA PubSub and MQTT made it possible to envision an Industrial Internet of Things (IIoT), where industrial equipment is located outside the control of its operators and communicates with each other via public, unsecured networks.

[0004] Establishing a secure channel for exchanging a secret element, such as Diffie-Hellman keys, is applicable in the computer systems of most industries, such as automotive, rail, and automation. It is of particular interest in the case of so-called "IIoT" (Industrial Internet of Things) infrastructures, where equipment connectivity cannot be established before the deployment phase and may evolve during the lifecycle. The security of this communication channel is especially critical when it is established for the purpose of enrolling a connected device (one of the devices participating in the channel then acts as a key server, distributing session keys that subsequently allow the other device to establish communication sessions with the other members of the group).

[0005] However, embedded systems are now deployed in an environment that is less and less controlled by their legitimate operators, and increasingly accessible to attackers. While a range of techniques at both the network level (firewalls, DMZs, etc.) and the equipment level (execution flow control) have significantly increased the security of these systems, the risk of equipment compromise remains high, given the inherent criticality of SECs. Compromising (reading or modifying) a session key by an attacker allows them to control all communication between two devices. Existing key exchange techniques (Diffie-Hellman RSA or ECC) are generally effective in protecting communication security against an attacker present on the network but do not offer protection against an attacker capable of accessing the device's memory (for example, by compromising a process on it). This is because the participants in the communication calculate a temporary encryption key, which is stored in the device's memory.Once an attacker has read the temporary encryption key, they will be able to read or falsify information transmitted through that channel—for example, session keys, during communication with a key server. The attacker will then be able to provide or falsify any data transmitted over the network to or from the compromised machine. If the session keys are group keys, this power will extend to all machines in the group.

[0006] There is therefore a need to increase the protection of telecommunications in telecommunications systems, particularly of the SEC type. Summary of the invention

[0007] To this end, according to a first aspect, the present invention describes a method for the secure transmission of a secret element between a first telecommunications device and at least one second telecommunications device, a respective private key-public key pair being associated with each of said first and second devices

[0008] the second equipment comprising a secure cryptographic module, adapted to execute predefined cryptographic functions, to store cryptographic information including the private key of the second equipment and to be unable to deliver at least some of said stored cryptographic information;

[0009] the second piece of equipment comprising a processor, at least two memory areas including secure memory and common memory, and software functions running on the processor, said second piece of equipment being adapted to operate alternately in a first mode called non-secure mode and a second mode called secure mode, such that the software functions running in the non-secure mode of the processor can only access the common memory, while at Conversely, software functions running in the processor's secure mode can access both secure memory and common memory;

[0010] said process comprising the following steps:

[0011] i / mutual authentication between the first device and the second device, said authentication comprising:

[0012] i-1 / by the first piece of equipment: generation of a first random value and storage of said first random value; encryption of said first random value with the public key of the second equipment previously obtained; determination of a first signature by using the private key of the first equipment of said first encrypted random value; transmission to the second equipment of a message containing said first encrypted random value and said first signature;

[0013] i-2 / by the second piece of equipment:

[0014] in non-secure mode:

[0015] receiving the message and copying said message into the common memory;

[0016] then in safe mode:

[0017] verification of the authenticity of the first piece of equipment based on said first encrypted random value, the first signature and the public key of the first piece of equipment previously obtained; then

[0018] if authenticity has been verified, decryption, by the secure cryptographic module and by means of the private key of the second equipment stored in said module, of said first encrypted random value and storage of the first decrypted random value in secure memory;

[0019] obtaining a second random value and storing said second random value in secure memory; encrypting said second random value with the public key of the first equipment previously obtained; determining a second signature by the secure cryptographic module, with the private key of the second equipment stored in said module, of said second encrypted random value; copying said second encrypted random value and said second signature into common memory;

[0020] in non-secure mode:

[0021] preparation, for transmission to the first equipment, of a message containing said second encrypted random value and said second signature;

[0022] transmission to the first piece of equipment, of the message;

[0023] i-3 / by the first equipment:

[0024] receipt of the message containing said second encrypted random value and said second signature;

[0025] verification of the authenticity of the second piece of equipment according to said second a random encrypted value, of said second signature and of the public key of the second device previously obtained; then

[0026] if authenticity has been verified, decryption of said second encrypted random value according to the private key of the first equipment;

[0027] ii / determination by the first equipment of a symmetric encryption key, called initialization key, as a function of the first random value stored at step i-1 and the second random value decrypted at step i-3, by implementing an initialization key calculation function having the said first and second random values ​​as variables;

[0028] iii / determination by the second equipment in secure mode of said initialization key as a function of the first random value stored at step i-2 and the second random value stored at step i-2, by implementing said initialization key calculation function having as variables said first and second random values ​​and whose result will be in a strictly deterministic way the initialization key also calculated by the first equipment; and storage of said initialization key in the secure memory;

[0029] iv / encryption of the secret element by the first equipment with said initialization key determined in step ii; and transmission of said encrypted secret element to the second equipment;

[0030] v / reception by the second equipment of said encrypted secret element; decryption, in secure mode, of said secret element using the initialization key stored in step iii and storage of said secret element in secure memory or in the secure cryptographic module.

[0031] The term "secret information" refers to sensitive data, typically cryptographic keys, used by cryptographic functions, for example to encrypt, decrypt, sign and / or authenticate a signature, and whose acquisition by a malicious third party would compromise the security of communications implemented in particular by the second piece of equipment.

[0032] Such a process makes it possible to increase the protection of telecommunications in telecommunications systems, in particular second SEC equipment.

[0033] The principle of the invention is to propose a security architecture based on the capabilities of hardware elements that ensures the mitigation of software and / or hardware compromise of a second device, including in the event of an undetected takeover. In particular, the invention provides a telecommunications system comprising networked second devices implementing the invention with the following properties: - a second piece of equipment, whether compromised or not, cannot disclose the secret element (typically a temporary key); - in the context of use for enrollment of second devices: a second device, compromised or not, cannot disclose the encryption key (typically the encrypted session key of the secret element);

[0034] A piece of equipment, whether compromised or not, can only receive a key to communicate with other secondary equipment from a valid key server.

[0035] In embodiments, such a method will further comprise at least one of the following features:

[0036] The first piece of equipment is a key server, the secret element comprises at least one session key, and said method further comprises the following steps:

[0037] steps i / to v / are implemented by the key server with a plurality of second devices to transmit said second devices said same session key;

[0038] a telecommunication session is then established between at least two of said second equipment according to said session key stored in each of said second equipment following their transmission by the key server;

[0039] - the first piece of equipment is a key server, the secret element comprises at least a session key and said process further comprising the following steps:

[0040] steps i / to v / are implemented by the server with a plurality of software applications from the same second equipment to transmit the same session key to said applications;

[0041] a telecommunication session is then established between at least two of said applications based on at least said copies of said session key stored following their transmission by the key server;

[0042] - the secure cryptographic module of the second device is a discrete module and / or the secure cryptographic module of the second device is adapted to only be able to exchange data within the second device and only in secure mode;

[0043] - the secure transmission method includes, before step i-1, a step i-0 including:

[0044] the transmission by the second piece of equipment to the first piece of equipment of the certificate of the second piece of equipment issued by a certification authority and including at least the public key of the second piece of equipment and a signature of said public key by the private key of the certification authority;

[0045] by the first piece of equipment: verification of the validity of the certificate of the second piece of equipment transmitted and storage of the public key of the second piece of equipment;

[0046] and the message transmitted by the first device further includes the certificate of the first device comprising the public key of the first device; and

[0047] in step i-2, after switching to safe mode: before the implementation of the verification of the authenticity of the first device, the certificate of the first device received in the message is verified using the secure cryptographic module, based on the public key of the certification authority previously stored in secure mode in the secure cryptographic module;

[0048] - the first equipment comprising a secure cryptographic module, adapted to execute predefined cryptographic functions, to store cryptographic information including the private key of the first equipment and to be unable to deliver at least some of said stored cryptographic information; the first equipment comprising a processor, at least two memory areas including secure memory and common memory and software functions running on the processor, said first equipment being adapted to operate alternately in a first mode called insecure mode and a second mode called secure mode, such that the software functions running in the insecure mode of the processor can only access the common memory, while conversely, the software functions running in the secure mode of the processor can access both the secure memory and the common memory;

[0049] and the steps described with respect to the first piece of equipment, respectively to the second piece of equipment, are further implemented symmetrically, i.e. this time by the second piece of equipment, respectively by the first piece of equipment.

[0050] According to another aspect, the invention describes a telecommunications system comprising a first telecommunications device and at least one second telecommunications device, a respective private key-public key pair being associated with each of said first and second devices

[0051] the second equipment comprising a secure cryptographic module, adapted to execute predefined cryptographic functions, to store cryptographic information including the private key of the second equipment and to be unable to deliver at least some of said stored cryptographic information;

[0052] the second equipment comprising a processor, at least two memory areas including secure memory and common memory and software functions running on the processor, said second equipment being adapted to operate alternately in a first mode called non-secure mode and a second mode called secure mode, such that the software functions running in the non-secure mode of the processor can only access the common memory, while conversely, the software functions running in the secure mode of the processor can access both the secure memory and the common memory;

[0053] said first telecommunications equipment and at least second telecommunications equipment being adapted to carry out between them an operation authentication during which

[0054] the first equipment is adapted to generate a first random value and to store said first random value, to encrypt said first random value with the public key of the second equipment previously obtained, to determine a first signature, by use of the private key of the first equipment, of said first encrypted random value, to transmit to the second equipment a message containing said first encrypted random value and said first signature;

[0055] - the second equipment is adapted, in the non-secure mode, to receive said message and copy said message into the shared memory,

[0056] - then the second piece of equipment is adapted, in safe mode, to check the authenticity of the first equipment based on said first encrypted random value, the first signature and the public key of the first equipment previously obtained, then if the authenticity has been verified, so that the secure cryptographic module decrypts, using the private key of the second equipment stored in said module, said first encrypted random value, to store the first decrypted random value in the secure memory;the second piece of equipment being adapted to, in secure mode, obtain a second random value and store said second random value in secure memory, to encrypt said second random value with the public key of the first piece of equipment previously obtained, so that the secure cryptographic module determines a second signature, with the private key of the second piece of equipment stored in said module, of said second encrypted random value, and to copy said second encrypted random value and said second signature into the common memory;

[0057] the second equipment is adapted to, in the non-secure mode, prepare, for transmission to the first equipment, a message containing said second encrypted random value and said second signature, transmit said message to the first equipment;

[0058] the first piece of equipment being adapted to receive said message containing said second encrypted random value and said second signature, to verify the authenticity of the second piece of equipment based on said second encrypted random value, said second signature and the previously obtained public key of the second piece of equipment, and then, if authenticity has been verified, to decrypt said second encrypted random value based on the private key of the first piece of equipment

[0059] the first piece of equipment being adapted to determine a symmetric encryption key, called the initialization key, based on said first stored random value and said second decrypted random value, by implementing a initialization key calculation function having the aforementioned first and second random values ​​as variables;

[0060] the second equipment is adapted to, in secure mode, determine said initialization key as a function of said first stored random value and second stored random value, by implementing said initialization key calculation function having as variables said first and second random values ​​and whose result will be in a strictly deterministic way the initialization key also calculated by the first equipment, and store said initialization key in secure memory;

[0061] the first piece of equipment being adapted to encrypt the secret element with said initialization key which it has determined, and to transmit said encrypted secret element to the second piece of equipment;

[0062] the second piece of equipment being adapted to receive said encrypted secret element, to decrypt, in secure mode, said secret element using the stored initialization key and to store said secret element in secure memory or in the secure cryptographic module.

[0063] In some embodiments, the telecommunications system will further include at least one of the following features:

[0064] - the first piece of equipment is a key server, the secret element comprises at least a session key and the authentication operation is implemented by the key server with a plurality of second devices to transmit said second devices the same session key;

[0065] at least two of said second equipment being adapted to establish a telecommunication session between them according to said session key stored in each of said second equipment following their transmission by the key server;

[0066] - the first piece of equipment is a key server, the secret element comprises at least a session key and the authentication operation is implemented by the server with a plurality of software applications on the same second device to transmit said same session key to said applications;

[0067] at least two of said applications being adapted to establish a telecommunication session between them based on at least said copies of said session key stored following their transmission by the key server;

[0068] - the secure cryptographic module of the second device is a discrete module and / or the secure cryptographic module of the second device is adapted to allow communication only within the second device and only in secure mode. Brief description of the drawings

[0069] The invention will be better understood and other features, details and advantages will become clearer from the following description, given by way of non-limiting reason, and from the accompanying figures, given by way of example.

[0070] [Fig-1] Fig. 1 is an illustration of a telecommunications system in a method of implementing the invention;

[0071] [Fig.2] Fig.2 represents the steps of a secure transmission process of secret element in an embodiment of the invention;

[0072] [Fig.3] Fig.3 illustrates an application to the railway field of a method of implementation of the invention.

[0073] Identical reference numerals may be used in different figures when they refer to identical or comparable elements. Description of embodiments

[0074] Figure 1 shows a distributed telecommunications system 1 in an embodiment of the invention. This system 1 comprises a plurality of telecommunications devices 10, a server (named SERV) 20 and a telecommunications link network 40.

[0075] As is known, each server 20 and device 10 is associated with a private / public key pair enabling an asymmetric encryption mechanism: the public key can be distributed publicly, while the private key must remain strictly secret and be used only by the device 10 (or its user) to which it was assigned. The property of asymmetric algorithms is that a message encrypted with a private key will be readable by anyone who possesses the corresponding public key. Conversely, a message encrypted with a public key is readable only by the owner of the corresponding private key. Typically, the private key also allows messages to be signed.

[0076] In the embodiment considered, the server 20 and each device 10 have a respective digital certificate, signed and issued by a third-party CA, which allows their authenticity to be verified. A certificate contains information uniquely identifying the certificate owner, such as their name, their public key, the distinguished name of the CA, a digital signature, by the CA, using its own private key, or by an affiliated trusted third party of the preceding information (indeed, verification of this signature, using the CA's public key, thus makes it possible to subsequently validate that the certificate was issued by a recognized certification authority).

[0077] The network 40 is adapted to provide wired and / or wireless telecommunication links to the telecommunication devices 10 and the server 20.

[0078] Telecommunications devices 10, for example named DISPO1, DISPO2..., These are electronic devices adapted to establish telecommunication sessions with each other via the telecommunication network 40, for example to exchange useful data (such as data provided by sensors in devices 10 or intended for actuators in devices 10, or data stored in a remote database within a device 10). One or more business applications 15 (APP1 in DISPO1, APP2 in DISPO2) running in the electronic device 10 exchange(s) with one or more business applications in other devices 10 or with the server 20.

[0079] Each telecommunications device 10 is further adapted to exchange with the server 20 via the telecommunications network 40.

[0080] The server 20 includes a random number generator, a memory, a processor, a telecommunications block including, in particular, in the embodiment considered, a modem, an RF transmit-receive antenna, (not shown).

[0081] Each device 10 comprises a TEE 11, a Secure Element (SE) 12, a common memory 13 and a secure memory 14, a business application 15, an operating system 16 and a Modem 17 (,) and not shown: a processor enabling the execution of the business application, the TEE, the OS, etc., an RF stage and a random number generator. ...

[0082] The TEE 11 is a Trusted Execution Environment adapted to allow the execution of critical functions in the secure memory 14 (by means, for example, of a control bit in the address field, for example set to 0) and which is separated from the rest of the system (corresponding to the control bit set to 1).

[0083] The device 10, and in particular its processor, is thus equipped with a mechanism allowing it to switch between two alternative operating modes: an unsecured mode and a secure TEE mode, such that in the unsecured mode, the processor and the executed software functions are prohibited from accessing the secure memory area 14 and access the common memory 13, while in the secure mode, only a very small, predefined subset of available functionalities is allowed to run (making it easier to analyze the behavior of critical functions) on the processor, and the secure memory 14 and the common memory 13 are accessible.For example, in the present case, this subset exclusively comprises the cryptographic and logical operations necessary to receive a key from a key server, including all the checks that are the subject of the present invention, as well as the use of this key to establish and secure communication with other secondary equipment.

[0084] The SE 12 (“Secure Element”) is an electronic module, for example discrete (on an electronic chip of its own, for example, separate from the rest of the components of the device), which performs basic cryptographic operations (encryption, decryption, signing, signature verification) using secrets and allows the storage and use of secrets (including the private key of device 10) in a tamper-proof memory of SE 12, without them being accessible to any user (for example, because SE 12 is designed not to release these secrets: no interfaces, no functionalities enabling this). The provisioning of secrets in SE 12 is, for example, performed at the factory where the secrets are generated within SE 12; it is also possible to add a secret dynamically. No direct connection from the business application 15 to SE 12 is permitted. In the considered optional embodiment, communication with SE 12 within device 10 is only possible in TEE mode.The business application 15 communicates via the common memory 13 with a "TEE Trusted Application" running in the TEE 11, which is itself connected to the SE 12 responsible for performing cryptographic operations.

[0085] Typically, a TPM security element can be used to implement the SE. Several types of usable SEs exist: ST33® from STM, the A700x® chip from NXP, etc.

[0086] In the considered embodiment of the invention, secure telecommunication channels are to be established between the devices 10 via the network 40, and encrypted messages will be exchanged over these channels. A secure communication channel will protect the security of communications against an attacker present on the network or against an attacker with physical access to the devices.

[0087] Establishing the secure communication channel between the devices 10 involves two phases, with reference to [Fig.2]: - retrieval of session keys by devices 10 (phase 1), which involves three steps that each device 10 must perform independently: reciprocal authentication between device 10 and the server (step lOOi);

[0088] construction of a temporary secure communication channel between the server and the device 10 (substeps lOOii and lOOiii);

[0089] transmission of session keys (substeps lOOiv and lOOv); - operation of the secure communication channel by devices 10 using session keys (phase 2).

[0090] Reciprocal authenticity (step lOOi) can be the subject of many more or less simplifying assumptions. Here we place ourselves in the most general - and most secure - framework where device 10 knows server 20, but does not trust it (i.e., it does not yet possess its certificate), and where the server does not know device 10, but knows (and trusts) its certificate authority CA.

[0091] Below is set out a reciprocal authentication mechanism reflecting a particular implementation (namely the OPC UA security model), but the main steps are the same regardless of the approach, even if the order of execution may vary: - Identification of device 10 (typically in the form of a certificate sending) and verification of the validity of device 10 by server 20 (with the CA certification authority); - Proof of authenticity of server 20 (by sending a signed message); - Verification of the validity and proof of authenticity of server 20 by device 10 (signature verification); - Proof of authenticity of device 10 (by issuing a signed message); - Verification of the proof of authenticity of device 10 by server 20 (Signature verification).

[0092] Thus, with reference to [Fig. 2], the steps of a method according to the invention are now detailed. The device 10 DISPO1 includes its private key CS21 as well as the public key of the CA, both stored in its SE 12.

[0093] During the process, if at any point an authentication or validity verification, signature etc. fails, the process terminates and the subsequent steps are not carried out.

[0094] A step 100i-0 of Device Identification 10 (typically in the form of a certificate submission) and verification of the validity of Device 10 by server 20 (with a certification authority) includes: - the transmission by device 10 DISPO1 to server 20 of the certificate of device 10 established by the certification authority CA and including at least the public key CP21 of device 10 and a signature of in particular said public key by the private key of the certification authority CA; - verification by server 20 of the validity of the certificate of device 10 transmitted (by authenticating it with the CA or with the public key of the CA stored in server 20) and storage of the public key of device 10 by server 20. In one embodiment, the certificate of device 10 is stored in the SE 12, which extracts it in TEE mode before sending it to the server 20, thus ensuring the integrity of the certificate in device 10. In another embodiment, it is stored in secure memory 14 or common memory 13 (in this case, the hash of the certificate must be stored in secure memory or in the SE).

[0095] A step lOOi-1 of proof of authenticity of server 20 (by issuing a signed message) implemented by server 20 comprises: - generation of a first random value, and storage of said value first random value; - encryption of said first random value with the CP21 public key of device 10 DISPO1 previously obtained in the received certificate; - determination of a first signature by encryption with the CS1 private key of the aggregate server of said first encrypted random value; - transmission to device 10 of a message containing said first encrypted random value, said first signature and the certificate of server 20.

[0096] A step 100i-2-1 for verifying the validity and proof of authenticity of server 20 by device 10 (signature verification) implemented by device 10 DISPO1 includes receiving the message, and after switching to TEE mode: - the certificate of server 20 received in the message is provided to SE 12 for authentication of said certificate by SE 12 according to the public key of the certification authority CA previously stored in SE 12 (in an alternative embodiment, the CA is stored in secure memory, and the SE is not involved in the authentication of the server but is involved in the authentication of the client to the server); - verification of the authenticity of server 20 based on the aforementioned first encrypted random value, the first signature, and the public key of server 20 contained in the server 20 certificate: device 10 in turn calculates an aggregate of the random value, then compares it with the signature decrypted by the CPI server's public key contained in the certificate: this allows confirmation, in case of equality of the compared terms, that the server had the private key necessary to perform the signature and prove its authenticity; then - only if authenticity has been successfully verified, decryption by SE 12, according to the private key, CS21, of device 10 stored in SE 12, of said first encrypted random value and storage of the first decrypted random value, random, in secure memory 14.

[0097] Thanks to the use of the SE proposed in the invention, the integrity of the CA's public key and the CS21 private key of the device 10 is ensured, as well as the confidentiality of the latter. Furthermore, thanks to the use of the TEE, the random secret transmitted by the server 20 remains confidential (applications not associated with the TEE in the device 10 do not have access to it) and the sequence of operations is executed atomically (in that the sequence of operations of the process implemented in TEE mode cannot be compromised).

[0098] In a step 100i-2-2 of proving the authenticity of the device 10 (by issuing a (signed message), the latter implements the following steps, in TEE mode: - generation of a second random value, alea2, (optionally the SE can be used to generate at least partially this value and optionally, this second random value is for single use only) and storage of said second random value in secure memory 14; - encryption of said second random value alea2 with the CPI public key of server 20 contained in the received certificate; - determination of a second cryptographic signature by the SE 12 with the private key of the device 10, stored in the SE 12, of said second encrypted random value alea2.

[0099] A message containing said second encrypted random value and said second signature is provided, for example via common memory 13, to the business application 15.

[0100] Then the business application 15, in non-secure mode, transmits to server 20 a message containing said second encrypted random value and said second signature.

[0101] Thanks to the use of the SE 12 proposed in the invention, the integrity and confidentiality of the private key CS21 of the device 10 are ensured. Thanks to the use of the TEE 11, the confidentiality of the generated random secret alea2 remains ensured; again, no application in the device 10 can know the content of the secret because the latter is manipulated in the TEE 11 and the SE 12. Finally, the atomic execution of all operations is ensured by the TEE according to the invention.

[0102] In a step 100i-3 of verifying the proof of authenticity of the device 10 by the server 20 (signature verification), the server 20 implements the following steps: - verification of the authenticity of device 10 based on the second encrypted random value alea2 and the second signature present in the received message and the previously obtained public key CP21 of device 10 (to verify that device 10 did indeed use the corresponding private key CS21 to sign); then - only if authenticity has been successfully verified, decryption of said second random value alea2 encrypted according to the private key CS1 of server 20 then storage.

[0103] In a step of constructing a temporary secure communication channel between the server 20 and the device 10, substeps lOOii and lOOiii are implemented, either in parallel or not. This "construction" corresponds to that of an initialization key, for example a symmetric key (typically AES256). The term "construction of a secure channel" refers to all the operations that allow Several participants (two or more) share a key without it being exposed online. These exchanges rely on the possession of a common secret, calculated (and not exchanged) by each participant.

[0104] Thus in substep lOOii, server 20 determines an encryption key, here symmetric, called initialization key, named Kinit, as a function of the first random value aleal stored in step lOOi-1 and the second random value alea2 decrypted in step lOOi-3, by implementing an initialization key calculation function having as variables the first and second random values ​​aleal and alea2.

[0105] And in substep 100iii, the device 10 also determines, in TEE mode, the Kinit initialization key based on the first random value alea2 stored in step 100i-2-1 and the second random value alea2 stored in step 100i-2-2, by implementing said initialization key calculation function which is common to the server and the devices 10; and storing the Kinit initialization key in secure memory 14 or in the secure cryptographic module (SE 12). Thanks to the calculation in TEE mode and the thus secure storage, the confidentiality of the Kinit key is superior to purely software implementations.

[0106] In a step of transmitting session keys {Ks} in the temporary secure channel comprising substeps lOOiv and lOOv, the session keys encrypted with the initialization key are transmitted. A man-in-the-middle attacker will only obtain encrypted information, and therefore unusable information if the encryption level is sufficient and the exchanged data is unknown.

[0107] In substep lOOiv, server 20 encrypts the session keys {Ks} with the Kinit initialization key determined in substep lOOii and transmits the thus encrypted session keys to device 1.

[0108] In substep lOOv, device 10 receives the encrypted session keys, then decrypts them, in TEE mode, using the Kinit initialization key stored in substep lOOiii, then stores the decrypted session keys {Ks} in secure memory 14 or in SE 12.

[0109] The information exchanged in this context must therefore have sufficient entropy (unpredictability in order to resist cryptanalysis). Typically, this initial exchange is limited to a set of encryption and / or signature key(s), which have been collectively designated as "session keys." These keys are, for example, symmetrical for performance reasons, but not necessarily identical (particularly for two distinct devices) in the case of signature keys.

[0110] Failure at this stage (typically the consequence of a failure at the earlier key calculation stage) interrupts any possibility of communication with the third party, unless the authentication process is restarted from the beginning.

[0111] In an embodiment of the invention applicable to the class of low-speed communicating objects, the session keys are provisioned in the SE 12 and all traces in memory (including in the TEE 11) are erased. In an embodiment of the invention applicable to the class of high-speed communicating objects, the key is stored in memory space (the secure memory 14 for persistence).

[0112] The difference between these classes is recalled below.

[0113] Some SECs communicate via low-bandwidth or very low-bandwidth networks for reasons of range, cost, autonomy, or robustness to electromagnetic interference (including against electronic warfare threats). Furthermore, the hardware platforms may have only limited computing resources—again, for the same reasons. Consequently, data encryption may not be necessary or even possible given the system's capabilities and requirements. Data integrity, however, is all the more critical because the criticality of these systems is typically higher than that of the more generic SECs described below.

[0114] Examples of applications falling within this class of needs are equipment involved in the connected battlefield, sensors and actuators in environments with high electromagnetic activity (nuclear, space) or invasive medical equipment (pacemaker type).

[0115] The class of high-speed connected SECs, more widespread than the previous one, concerns systems communicating confidential data and offering sufficient resources (computing capacity, high-speed communication) to implement encryption.

[0116] The steps of phase 1 described above having been implemented between the server 20 and several devices 10, the latter have thus obtained the session keys allowing them, in a phase 2, to set up a communication session between them.

[0117] For example, a secure communication channel 30 is implemented between devices DISPO1 and DISPO2, such that the data exchanged between them on network 30 between business applications APP1 and APP2 is encrypted, in TEE mode, using one of the session keys, and / or is signed using another session key. For example, for the class of high-speed communicating objects, decryption / encryption is performed in the TEE using the session keys, and in the case of the class of low-speed communicating objects, decryption / encryption is not performed in the TEE, but in the SE 12, which alone stores the session key. Access to the Secure Element, however, is indeed performed in TEE mode.

[0118] The TEE 11 and the SE 12 in the device 10 are used by the invention to ensure strong confidentiality and integrity of the various secrets involved in the steps of the process, including session keys.

[0119] A method for the secure transmission of session keys between a first piece of equipment, such as a key server, and a second piece of equipment, such as a communication device in a network of communication terminals, has been described above, thereby enabling increased protection of subsequent communications between the network devices. The invention can, of course, be implemented for the transmission of secret elements other than session keys between two telecommunications devices: for example, information concerning the configuration of a device and / or the updating of its software.

[0120] As previously mentioned, to protect the confidentiality of communications and the integrity of equipment, a number of secrets are necessary. These secrets are, depending on the circumstances, shared with third parties or exclusive to device 10.

[0121] A classification of secrets based on their persistence and use is proposed here:

[0122] [Tables 1] Secret Type Source Secret / Target Secret Initialization Key Session Key Private Key Lifetime A few seconds A few seconds A few hours to a few months Permanent Use Arbitrary operation (temporary key generation) Symmetric encryption Symmetric encryption and signature Asymmetric decryption and signature Placement TEE TEE SE or TEE SE

[0123] Several elements emerge from this classification:

[0124] The source secret / target secret pair (called above aleal / aléa2) and the initialization key are not stored in an OS in an embodiment of the invention because the loading times of these would be too significant today in view of their lifetime (at least for TPM2 - this point could change with the evolution of OS);

[0125] Arbitrary operations applied to the source secret / target secret pair are hardly compatible with the functioning of an SE (except for personalization, which would harm the trust that could be placed in the SE);

[0126] The lifespan of private keys makes the use of an OS entirely appropriate;

[0127] Session keys, which have a very variable lifespan, could be protected by an OS or in the TEE depending on the operational need.

[0128] Generally speaking, the more ephemeral a secret is, the less protected it is in current architectures. However, ephemeral secrets allow the inference of more permanent secrets: the target secret / source secret pair makes it possible to calculate with certainty the temporary key, which allows the session key to be decrypted. The invention makes it possible to protect all secrets (protecting the equipment's private key in a Secure Element is not sufficient: this protects the key, but does not prevent access to session keys): regardless of their lifespan, using means compatible with their operational use and without significantly restricting the real-time and / or embedded capabilities of the final system.

[0129] Moreover, the invention makes it possible to protect the integrity of certificates, which in the prior art are rarely protected because they are not confidential (neglecting that an attacker who modifies these certificates could nevertheless initiate a connection with malicious equipment: a classic and particularly critical example is allowing a malicious update server, which makes it possible to transform the compromise of a process into the installation of malware - and thus to make the compromise persistent).

[0130] The invention provides connected equipment that implements it with the benefit of a secure channel protected by the SE and TEE pair.

[0131] In the general case where the secure channel is used to exchange a secret, the invention offers the following benefits: - execution of cryptographic operations in the TEE and the SE: ability to implement checks on the nature of the data, the source and / or the destination in a trusted environment, protection against the risk of private key theft; - protection of certificate integrity: protection against certificate injection.

[0132] Application to key distribution: all the general advantages of the invention described above are also provided, with the following additional advantages: • in the main case (High Confidentiality High Bandwidth Connected SEC): key protection throughout the lifecycle: mitigation of the risk of equipment compromise, better protection of received secrets, better protection of remote equipment against data forgery, better protection against the theft of private equipment keys;

[0133] protection of certificate integrity: mitigation of the risk of equipment compromise, protection against certificate injection;

[0134] coupled with a level 4 firewall solution, the invention also provides better protection against information exfiltration from the equipment;

[0135] Trusted Datalake: by protecting the integrity of data on equipment and in transit, the invention makes it possible to have confidence in the data present in the datalake, and therefore to use it in the context of critical applications. • In the variant case (SEC Connected high integrity low bandwidth), the properties described in the main case are ensured, apart from the improvement of the key protection property throughout the lifecycle, because the session key is stored in the SE 12 rather than in the TEE 14. In both cases, these benefits apply even if the attacker has physical access to the equipment's memory.

[0136] The invention also allows adaptation to the lifecycle of secrets. It allows logical isolation of wireless communication flows, and allows not only encryption of one or more communications between two remote devices, but also between two applications located on the same device 10, or between an application on a device 10 and a database local to the device 10 or remote.

[0137] In one embodiment, the invention is implemented to secure communications in a producer-consumer (also called publisher / subscriber) architecture, where information is organized into groups (or topics) that can admit one or more producers and one or more consumers. In this type of architecture, encryption keys are shared among the different devices subscribed to the group, which makes the compromise of one device particularly attractive to an attacker (because it allows them to then compromise the entire group). The use of our invention, through the judicious use of secure hardware elements (TEE and SE) in the computer, ideally of all communicating entities, makes it possible to secure the exchange and confidentiality at execution, including against an attacker with physical access to one of the devices protected by the invention.

[0138] The OPC UA PubSub protocol (OPC UA Specification, Part 14, Version 1.04, 2018 / 02 https: / / reference.opcfoundation.org / vl04 / Core / docs / Partl4 / ) is used to implement communications. It describes the mechanism for distributing encryption keys by means of a dedicated server, the Security Key Service. This authenticates clients requesting a data subscription, establishes a secure communication channel, and distributes encryption keys via this channel. Communications are then encrypted by producers using these keys, sent to a 204 broker (potentially implemented via multicast transmissions), forwarded by the broker to consumers, and decrypted by the consumers. The broker only handles encrypted messages and therefore plays no role in the maintaining data integrity.

[0139] In the case of high-confidential, high-speed connected SEC, a hardware platform in each communicating device (producer and / or consumer) includes an NXP i.MX8M card (including an ARM TrustZone TEE) and the ST33 TPM2 secure element.

[0140] This platform is operated with the following software stack (in insecure mode)

[0141] Operating system: Embedded Linux

[0142] Cryptographic library: mbedTLS

[0143] Communications: UDP / IP + OPC UA PubSub + MQTT

[0144] In secure mode, several options are possible, for example OP-TEE. Furthermore, access to the TPM must be performed within the TrustZone. To this end, it is proposed to implement this access using an open-source TSS2 or WolfTPM stack.

[0145] In this context, the different keys are protected and used in the following manner, following the main embodiment:

[0146] [Tables2] Secret Type Source Secret / Target Secret Temporary Key Session Key Private Key Protection TrustZone TrustZone TrustZone TPM2 Usage TrustZone TrustZone TrustZone TPM2

[0147] The cryptographic parameters are as follows:

[0148] The TPM2 of the equipment protected by the invention will use its private key to sign the connection initiating the secure channel with an asymmetric RSA2048 encryption.

[0149] The TPM2 of the equipment protected by the invention will use its private key to decrypt random 1 sent by the remote equipment with an asymmetric RSA2048 encryption.

[0150] The TrustZone of the equipment protected by the invention will generate the random secret 2, and use the public key of the remote equipment to encrypt this secret.

[0151] The TrustZone of the equipment protected by the invention will calculate the temporary key using the secrets aleal and alea2.

[0152] The TrustZone of the equipment protected by the invention will use its temporary key to encrypt the secure channel with AES256 symmetric encryption.

[0153] The TrustZone of the equipment protected by the invention will use its session (or group) key to encrypt messages with AES256 symmetric encryption.

[0154] The TrustZone of the equipment protected by the invention will use its session key (or group) to sign messages with the OPC UA PubSub Basic256Sha256 policy.

[0155] In the case of a high-integrity, low-throughput SEC with long-life keys (two parameters that are generally closely related), a different function placement is proposed. The session key, instead of being stored in the TrustZone, only passes through this memory and is then loaded, stored, and used in the TPM2.

[0156] This variant can be made with the same hardware and software architecture as the previous one.

[0157] In this context, the use of the session key (for encryption, decryption, verification and signing) is carried out by the TPM2.

[0158] [Tables3] Secret Type Source Secret / Target Secret Temporary Key Session Key Private Key Protection TrustZone TrustZone TrustZone (ms) and TPM2 (hours / days / weeks) TPM2 Usage TrustZone TrustZone TPM2 TPM2

[0159] Compared to the general case, the session key is less exposed to hidden channels (it is present in the TrustZone for only a few milliseconds before being loaded into the TPM2). However, since the TPM2 is typically less efficient than a cryptographic accelerator or a CPU, it can only process a limited data rate frequency—which is why we restrict this architecture to low-bandwidth SECs.

[0160] The cryptographic parameters are as follows:

[0161] The TPM2 of the equipment protected by our invention will use its private key to sign the connection initiating the secure channel with an asymmetric RSA2048 encryption.

[0162] The TPM2 of the equipment protected by the invention will use its private key to decrypt random 1 sent by the remote equipment with an asymmetric RSA2048 encryption.

[0163] The TrustZone of the equipment protected by the invention will generate the random secret 2, and use the public key of the remote equipment to encrypt this secret.

[0164] The TrustZone of the equipment protected by the invention will calculate the temporary key using the secrets aleal and alea2.

[0165] The TrustZone of the equipment protected by our invention will use its temporary key to encrypt the secure channel with AES256 symmetric encryption.

[0166] The TPM2 of the equipment protected by our invention will use its session (or group) key to encrypt messages with AES256 symmetric encryption.

[0167] The TPM2 of the equipment protected by the invention will use its session (or group) key to sign messages with the OPC UA PubSub Basic256Sha256 security policy.

[0168] In the architecture chosen for the equipment 10, a Trusted Gateway Application 205 executes the application code in insecure mode within a general-purpose operating system. The Trusted Gateway Application (TEE) executes the code critical for establishing communication, while asymmetric encryption operations are performed solely by the operating system upon request from the TEE. In insecure mode, an application firewall filters all data transmissions and accepts only messages signed with a valid encryption key. Key verification is delegated to the TEE (TrustZone). This protection is not absolute, as the firewall can be compromised, but it limits the risk of information exfiltration and thus adds a layer of defense in depth.

[0169] Let us consider the implementation of this application of the invention in the railway sector, for a predictive maintenance application, with reference to [Fig. 3]. A big data application 202 uses data collected from sensors 206 to provide a maintenance expert with a flexible, real-time view of the condition of railway tracks (rails, overhead lines, etc.). The railway operator expressed the need to secure communications on the public network, including between the secure gateway 205 and the APN 203 (Access Public Network, provided by a telecom operator), without sacrificing the flexibility of the architecture (ease of deployment and equipment enrollment). To this end, the secure gateway must connect to a key server, the Security Key Server 201 (SKS), using the OPC UA PubSub protocol. This exchange allows the new client to obtain the group keys (or session keys).In this context, session keys are particularly sensitive, since they are shared between all members of a group, and the implementation of the invention allows one to benefit from the advantages mentioned above.

[0170] At the operational level, by protecting data confidentiality, the invention ensures compliance with the customer's confidentiality requirements, as well as protection against the theft of a company asset, which can then be valued (including sold). By protecting data integrity on the equipment and in transit, the invention allows for confidence in the data present in the data lake, and therefore its use for the maintenance and optimization of the system 200 thus presented with reference to [Fig. 3].

Claims

1. Demands A method for the secure transmission of a secret element between a first telecommunications device (SERV) and at least one second telecommunications device (DISPO1), a respective private key - public key pair being associated with each of said first and second devices, the second device comprising a secure cryptographic module (SE), adapted to execute predefined cryptographic functions, to store cryptographic information including the private key of the second device and to be unable to deliver at least some of said stored cryptographic information;the second piece of equipment comprising a processor, at least two memory areas including secure memory (14) and common memory (13) and software functions running on the processor, said second piece of equipment being adapted to operate alternately in a first mode called non-secure mode and a second mode called secure mode, such that the software functions running in the non-secure mode of the processor can only access the common memory, while conversely, the software functions running in the secure mode of the processor can access both the secure memory and the common memory; said method comprising the following steps:; i / mutual authentication between the first equipment (SERV) and the second equipment (DISPO1), said authentication comprising: i-1 / by the first equipment (SERV): generation of a first random value and storage of said first random value; encryption of said first random value with the public key of the second equipment previously obtained; determination of a first signature by using the private key of the first equipment (SERV) of said first encrypted random value; transmission to the second equipment (DISPO1) of a message containing said first encrypted random value and said first signature; i-2 / by the second device (DISPO1): in non-secure mode: - receiving the message and copying said message into the common memory; then in safe mode: - verification of the authenticity of the first device based on said first encrypted random value, the first signature, and the previously obtained public key of the first device; then - if authenticity has been verified, decryption, by the secure cryptographic module (SE) and by means of the private key of the second equipment stored in said module, of said first encrypted random value and storage of the first decrypted random value in secure memory; - obtaining a second random value and storing said second random value in secure memory; encrypting said second random value with the public key of the first equipment previously obtained; determining a second signature by the secure cryptographic module (SE), with the private key of the second equipment stored in said module, of said encrypted second random value; copying said encrypted second random value and said second signature into the common memory; in insecure mode: preparation, for transmission to the first equipment (SERV), of a message containing said second encrypted random value and said second signature; - transmission of the message to the first equipment (SERV); i-3 / by the first piece of equipment (SERV): - receipt of the message containing said second encrypted random value and said second signature; - verification of the authenticity of the second device based on said second encrypted random value, said second signature, and the previously obtained public key of the second device; then

2. - if authenticity has been verified, decryption of said second random value encrypted according to the private key of the first equipment; ii / determination by the first equipment of a symmetric encryption key, called the initialization key, based on the first random value stored at step i-1 and the second random value decrypted at step i-3, by implementing an initialization key calculation function having the said first and second random values ​​as variables; iii / determination by the second equipment in secure mode of said initialization key as a function of the first random value stored at step i-2 and the second random value stored at step i-2, by implementing said initialization key calculation function having as variables said first and second random values ​​and whose result will be in a strictly deterministic way the initialization key also calculated by the first equipment; and storage of said initialization key in the secure memory; iv / encryption of the secret element by the first equipment (SERV) with said initialization key determined in step ii; and transmission of said encrypted secret element to the second equipment (DISPO1); v / reception by the second equipment (DISPO1) of said encrypted secret element; decryption, in secure mode, of said secret element using the initialization key stored in step iii and storage of said secret element in secure memory or in the secure cryptographic module (SE). A secure transmission method according to claim 1, wherein the first piece of equipment (SERV) is a key server, the secret element comprises at least one session key, and said method further comprising the following steps: - steps i / to v / are implemented by the key server with a plurality of second devices (DISPO1, DISPO2, ...) to transmit the same session key to said second devices; - a telecommunications session is then established between at least two of said second devices (DISPO1, DISPO2, ...) according to said session key stored in

3.

4.

5. each of the said second pieces of equipment following their transmission by the key server. Secure transmission method according to claim 1 or 2, according of which the first piece of equipment (SERV) is a key server, the secret element includes at least one session key, and said process further includes the following steps: steps i / to n! are implemented by the server with a plurality of software applications from the same second equipment to transmit the same session key to said applications; a telecommunications session is then established between at least two of said applications based on at least said copies of said session key stored following their transmission by the key server. Secure transmission method according to any of the following previous indications, according to which the secure cryptographic module (SE) of the second device (DISPO1) is a discrete module and / or the secure cryptographic module (SE) of the second device (DISPO1) is adapted to be able to exchange only within the second device and only in secure mode. Secure transmission method according to any of the following preceding instructions, including, before step i-1, a step i-0 comprising: the transmission by the second piece of equipment (DISPO1) to the first piece of equipment (SERV) of the certificate of the second piece of equipment issued by a certification authority and including at least the public key of the second piece of equipment and a signature of said public key by the private key of the certification authority; by the first device: verification of the validity of the certificate of the second device transmitted and storage of the public key of the second device; and according to which the message transmitted by the first piece of equipment (SERV) further includes the certificate of the first device, including the public key of the first device; and in step i-2, after switching to secure mode: - before the implementation of the verification of the authenticity of the first device, the certificate of the first device received in the message is verified using the secure cryptographic module (SE), based on the public key of the certification authority previously stored in secure mode in the secure cryptographic module (SE).

6. A secure transmission method according to any one of the preceding claims, the first piece of equipment comprising a secure cryptographic module (SE), adapted to perform predefined cryptographic functions, to store cryptographic information including the private key of the first piece of equipment and to be unable to deliver at least some of said stored cryptographic information;the first equipment comprising a processor, at least two memory areas including secure memory (14) and common memory (13) and software functions running on the processor, said first equipment being adapted to operate alternately in a first mode called non-secure mode and a second mode called secure mode, such that the software functions running in the non-secure mode of the processor can only access the common memory, while conversely, the software functions running in the secure mode of the processor can access both the secure memory and the common memory; and the steps described with respect to the first piece of equipment, respectively to the second piece of equipment, are further implemented symmetrically, i.e. this time by the second piece of equipment, respectively by the first piece of equipment.

7. Telecommunications system (1) comprising a first telecommunications device (SERV) and at least one second telecommunications device (DISPO1), a respective private key-public key pair being associated with each of said first and second devices, the second device comprising a secure cryptographic module (SE) adapted to perform cryptographic functions defined, to store cryptographic information including the private key of the second equipment and to prevent the release of at least some of said stored cryptographic information; the second equipment comprising a processor, at least two memory areas including a secure memory (14) and a common memory (13) and software functions running on the processor, said second equipment being adapted to operate alternately in a first mode called insecure mode and a second mode called secure mode, such that the software functions running in the insecure mode of the processor can only access the common memory, while conversely, the software functions running in the secure mode of the processor can access both the secure memory and the common memory; said first telecommunications equipment (SERV) and at least second telecommunications equipment (DISPO1) being adapted to implement between themselves an authentication operation during which - the first equipment (SERV) is adapted to generate a first random value and to store said first random value, to encrypt said first random value with the public key of the second equipment previously obtained, to determine a first signature, by use of the private key of the first equipment (SERV), of said first encrypted random value, to transmit to the second equipment (DISPO1) a message containing said first encrypted random value and said first signature; - the second device (DISPO1) is adapted, in non-secure mode, to receive said message and copy said message into the common memory, - then the second device (DISPO1) is adapted, in secure mode, to verify the authenticity of the first device based on the first encrypted random value, the first signature, and the previously obtained public key of the first device; then, if authenticity has been verified, the secure cryptographic module (SE) decrypts using the private key of the second device. equipment stored in said module, said first encrypted random value, to store the first decrypted random value in secure memory; the second equipment (DISPO1) being adapted to, in secure mode, obtain a second random value and store said second random value in secure memory, to encrypt said second random value with the public key of the first equipment previously obtained, so that the secure cryptographic module (SE) determines a second signature, with the private key of the second equipment stored in said module, of said second encrypted random value, and to copy said second encrypted random value and said second signature into common memory; the second equipment (DISPO1) is adapted to, in the non-secure mode, prepare, for transmission to the first equipment (SERV), a message containing said second encrypted random value and said second signature, transmit said message to the first equipment (SERV); the first equipment (SERV) being adapted to receive said message containing said second encrypted random value and said second signature, to verify the authenticity of the second equipment based on said second encrypted random value, said second signature and the public key of the second equipment previously obtained, and then if authenticity has been verified, to decrypt said second encrypted random value based on the private key of the first equipment; the first equipment (SERV) being adapted to determine a symmetric encryption key, called the initialization key, based on said first stored random value and said second decrypted random value, by implementing an initialization key calculation function having said first and second random values ​​as variables; The second piece of equipment is adapted to determine, in secure mode, the initialization key based on the first and second stored random values, by implementing the initialization key calculation function which takes the first and second random values ​​as variables and whose result will be, in a strictly deterministic manner, the initialization key also calculated by the first piece of equipment, and to store said key initialization in secure memory; the first equipment (SERV) being adapted to encrypt the secret element with said initialization key which it has determined, and to transmit said encrypted secret element to the second equipment (DISPO1); the second equipment (DISPO1) being adapted to receive said encrypted secret element, to decrypt, in secure mode, said secret element using the stored initialization key and to store said secret element in secure memory or in the secure cryptographic module (SE).

8. Telecommunications system (1) according to claim 7, wherein the first equipment (SERV) is a key server, the secret element comprises at least one session key and the authentication operation is implemented by the key server with a plurality of second equipment (DISPO1, DISPO2, ...) to transmit said same session key to said second equipment; at least two of said second equipment (DISPO1, DISPO2, ...) being adapted to establish a telecommunications session between them based on said session key stored in each of said second equipment following their transmission by the key server.

9. Telecommunications system (1) according to claim 7 or 8, wherein the first equipment (SERV) is a key server, the secret element comprises at least one session key and the authentication operation is implemented by the server with a plurality of software applications of the same second equipment to transmit said same session key to said applications; at least two of said applications being adapted to establish a telecommunications session between them based on at least said copies of said session key stored following their transmission by the key server.

10. Telecommunications system (1) according to any one of claims 7 to 9, wherein the secure cryptographic module (SE) of the second equipment (DISPO1) is a discrete module and / or the secure cryptographic module (SE) of the second equipment (DISPO1) is adapted to be able to exchange only within the second equipment and only in secure mode.

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