The present invention relates to a method to encrypt informa- tion that is transferred between two communication units. The communication units may be mobile telephones, personal computers, electronic tablets, palm-top computers, etc. The present invention will be described below, however, for the case in which the communication units are mobile telephones. Therefore, the term "mobile telephone" is used below and in the claims to denote a communication unit of the type specified above. The term "information" is used to denote a voice conversation and the transfer of data between two communication units. The transfer of data comprises the transfer of images. With respect to the transfer of information between two mobile telephones, this is also referred to using the term "telephone call".

There is a need to encrypt information that is transferred between two mobile telephones such that it is not possible for a third party to eavesdrop on conversations and data.

One way of achieving the encryption of telephone calls is to realise a security application on an SD card, which is in- serted into a card-holder location in the mobile telephone. The information is in this way passed across an interface between the CPU of the mobile telephone, with its associated memory, and the said SD card. One problem with sending streams of data between an application on an SD card and the mobile telephone is that the limiting resources of the mobile telephone are used, in that the said interface has a limited capacity for the transfer of data. In many cases, the transfer requires so much resources that the function cannot be carried out. A second problem is that such a transfer of data leads to the consumption of power becoming so high that the battery duration is severely reduced. A third problem is that an encryption key, which is present in the memory of the mobile telephone, is exposed in an insecure environment.

The present invention solves these three problems.

The present invention thus relates to a method to encrypt information that is transferred between two communication units such as mobile telephones, personal computers, palm-top computers, etc., where a security module is present in each mobile telephone and where a security application contains encryption and decryption algorithms, and is characterised in that the security application is caused to be stored on an SD (Secure Digital) card that is caused to be introduced into a card-holder location in the relevant mobile telephone, in that the security application is caused to be transferred from the SD card to the security module of the mobile tele- phone in addition to a session key stored on the SD card, in that a key stream is caused to be formed in the SD card on the basis of the session key, in that the key stream is caused during encryption to be transferred from the SD card to the security module of the mobile telephone, in that the encryption is caused to take place in the security module of the mobile telephone, after which such an encrypted stream of data is transmitted by the mobile telephone, in that the stream of data is received by a second mobile telephone, in that decryption is caused to take place with- the aid of a corresponding key stream in the security module of the second mobile telephone and in that the said corresponding key stream is caused to be formed in the SD card of the second mobile telephone on the basis of the said session key. The invention will be described below in more detail, partly in association with embodiments of the invention shown in the attached drawings, where:

- Figure 1 illustrates schematically the present invention together with a mobile telephone,

Figure 2 constitutes a more detailed illustration of that which is shown in figure 1,

Figure 3 illustrates an embodiment, and

- Figure 4 illustrates an SD-card.

Figure 1 illustrates the present method for encrypting information that is transferred between two communication units such as mobile telephones, personal computers, palm-top com- puters, etc., where a security module 4 is present in each mobile telephone 1 and where a security application contains encryption and decryption algorithms. Only a first mobile telephone 1 is shown in Figure 1. A second mobile telephone, not shown, is arranged in a corresponding manner such that the mobile telephones can communicate with each other across a communication network. The invention can be applied for various communication networks such as GSM, 3G, 4G or over IP, so called "VoIP telephony". The invention will be described below, however, for the case of VoIP telephony.

The mobile telephone 1 is provided with a card-holder location for an SD card 2. Further, the mobile telephone comprises a memory, in a known manner. An interface 3 is present between the memory of the mobile telephone and an SD (Secure Digital) card that has been introduced into the card-holder location . According to the invention, the said security application is caused to be stored on an SD card 2, which is caused to be introduced into a card-holder location in the relevant mobile telephone. The security application is caused to be trans- ferred from the SD card to the said security module 4 of the mobile telephone, in addition to a session key stored on the SD card 2. A key stream is caused to be formed in the SD card on the basis of a session key, for encryption and for decryption .

The said SD card 2 is illustrated in Figure 4. The SD card 2 comprises a processor 32, a memory 33 and a smart-card circuit 34 of conventional type. The said security application is stored in the memory 33, which security application is to be transferred to the memory of the mobile telephone, as is also the security application, which is used by the SD card during encryption and decryption.

During encryption and decryption, the key stream is caused to be transferred to the security module 4 of the mobile telephone across the interface 3, and the encryption or decryption is caused in this way to take place in the security module 4 of the mobile telephone. An encrypted stream of data is sent out by the mobile telephone 1 following encryption of information, such as a voice. This stream of data is received by the second mobile telephone, not shown. Decryption is caused to take place with the aid of a corresponding key stream in the security module of the second mobile telephone, where the said corresponding key stream is caused to be formed in the SD card of the second mobile telephone on the basis of the session key. The reference number 5 in Figure 1 denotes algorithms for the generation of session keys and key streams for encryption and decryption, which algorithms are stored on the SD card. The reference number 6 denotes algorithms for encryption that contain key streams, which have been retrieved from the SD card 2. The reference number 7 denotes algorithms for decryption that contain key streams, which have been retrieved from the SD card 2. The reference number 8 denotes an application for IP telephony. The reference number 9 denotes a signal from the microphone of the mobile telephone, and the reference number 10 denotes a signal to the loudspeaker of the mobile telephone. The arrow 11 illustrates encrypted information sent out by the mobile telephone and the arrow 12 illustrates encrypted information received by the mobile tele- phone.

Through the session key and the generation of key streams being protected on the SD card, the method according to the invention is very secure.

According to one preferred design, the security application in the security module 4 of the mobile telephone is caused to be deleted after the transfer of information between the mobile telephones has been completed. One way of achieving the deletion is to ensure that the security module in the mobile telephone no longer has contact with the SD card. This can be achieved through the telephone application breaking the connection with the SD card when a call is disconnected, or through the SD card being removed from the mobile tele- phone, whereby the security module in the mobile telephone no longer has contact with the SD card. It is obvious that the actual encryption or decryption takes place in the security module 4 of the mobile telephone, and for this reason the flow of information across the interface is limited to an order of magnitude of one half of that re- quired if the encryption or decryption were to take place in the SD card 2. Also the power consumption will in this way be highly reduced when the present invention is applied.

The said session key is a secret that is known to both par- ties, while no external actors may know it. It is preferred that this session key is established through Diffie-Hellman key generation, using, for example, the ikey protocol. In this way can both parties generate a secret session key without the key being exposed in the public network. The present invention, however, is not limited to any particular encryption and decryption algorithms or methods, nor to any methods of generating keys since all suitable methods can be used.

According to one preferred embodiment, the used session key is caused to be changed between two occasions of connection, one after the other, between two mobile telephones.

It is furthermore preferred that information about the session key that is to be used during a particular call is caused to be transferred from the calling mobile telephone to the receiving mobile telephone during the establishment of the connection between the telephones.

A key stream is derived from this session key, which key stream is used as a mask to permute ( change/move ) the bit stream in the data, such that only the one who can generate an identical bit stream can recreate the bit pattern in the stream, i.e. the inverse function of the permutation. An example of such a masking function is the logical XOR function (Exclusive OR) .

An example of an encryption operation is given below:

RTP (Real Time Protocol) 01101101 bit stream of data

Key stream 10101001 derived from the session key

Gives an ouput S (Safe) -RTP 11000100 encrypted bit stream of data

The encrypted bit stream of data is received and decrypted:

Incoming S-RTP 11000100 bit stream of data

Key stream 10101001 derived from the session key

RTP 01101101 Decrypted bit stream of data.

XOR is used in this example to mask the stream of data with

the key stream that has been generated by for example AES CTR

(American Encryption Standard Counter Mode) . It is Counter

Mode that defines how a key stream is to be derived from the relevant session key. A VoIP application 8 (IP telephony application) is present in the telephone, which application generates RTP frames, which represent microphone and loudspeaker in digital form. These

RTP frames must be encrypted and decrypted in order for the

communication to be secure, since the RTP frame is transmit- ted across an IP connection. A security application is available for the VoIP application, which security application is partly installed in the security module 4 of the telephone

and partly installed in the safety module of the SD card 2.

The said session key for communication is generated and pro- tected in the security module on the SD card, using, for

example, Diffie-Hellman or other key exchange. Furthermore,

the security module on the SD card generates the key stream, which constitutes the bit stream that is to be permuted with the stream of data in the RTP frames in order to obtain the encrypted RTP frames (S-RTP). This is sent to the receiving telephone, and vice versa for incoming packets. The permutation is carried out in the part of the application that has been placed into the security module 4 of the mobile telephone .

Through the bit stream of data and the key stream being per- muted in the security module 4 in the mobile telephone, the exchange of information across the interface between the telephone and the card will be approximately halved, while at the same time the session key never leaves the safe environment of the SD card. This constitutes a safer implementation that makes it possible to protect the cryptography function in the environment of the SD card without risk-filled exposure of the session keys and without data encryption in the unsafe environment of the mobile telephone. The streams of data and the information streams for encryption and decryption are illustrated in more detail in Figure 2. When, for example, a person speaks into the microphone of the mobile telephone 1 a signal 20 passes from the microphone to a circuit 21, denoted "CODEC" in Figure 2. CODEC is an acronym for "Coder-Decoder", which is an analogue-digital converter and the signal is transferred onwards to a circuit RTP with reference number 22, where the speech is packaged into an RTP frame. The RTP frames are transferred onwards to the security module 4 of the mobile telephone 1, where the RTP frames are encrypted by means of a key stream 23 for encryption, which has previously been obtained from a key stream generator 24 on the SD card 2. The encrypted RTP frames are transferred through a circuit 25 HMAC, which inte- graty protects the information, to an IP application 26, IP, whereby the encrypted message 13 is transmitted from the mobile telephone 1. HMAC is an acronym for Hash-Based Message Authentication Code.

An incoming encrypted signal 12 that the mobile telephone receives is transferred through the IP application 26 to the HMAC circuit 25 and onwards to the security module of the mobile telephone, where it is decrypted by means of a key stream 27 for decryption, which has previously been obtained from a key stream generator 28 on the SD card 2. The decrypted RTP frames are transferred through the CODEC circuit 21 and a conductor 29 to the loudspeaker of the mobile telephone, where the message can be heard in plaintext.

The reference number 30 in Figure 2 denotes a memory area of session keys, which area cannot be accessed from outside. The reference number 31 refers to an application for the authentication of a user for the transmission of an encrypted mes- sage.

According to a further preferred embodiment, a transfer that is to be encrypted is started by the user being caused to select a telephone number that has been provided with a pre- determined prefix.

An embodiment is that when the user has input information, for example retrieved information from the telephone book or input a string through the keyboard of the mobile telephone, this information is transferred to a so called "event- handler" in the mobile telephone. This event-handler currently checks only whether the user has specified an emergency telephone number, 112, 911, and - if this is the case - connects an emergency call independently of the normal telephone connection procedures. According to one embodiment of the invention, the event-handler is arranged to carry out a further check of the prefix in the string that the user specified. According to the invention, this prefix constitutes a suitable known symbol, * or #, for example, or a direct-dialling number or another alphanumerical code. On the condition that the prefix precedes a telephone number, the event-handler is arranged to call the applications that have been described above and that are required in order to carry out an encrypted exchange of information between two mobile telephones .

If a prefix is not given, the complete string can be matched to known strings, such as 343, which in this case corresponds to the application EID, Electronic Identification, which is called in this case with an empty argument. Thus the task of requesting the input data that is required, such as a security code, is handed over to the application.

This embodiment is illustrated in Figure 3. The user has previously input names and telephone numbers in the conventional manner, and has thereby input the telephone numbers in a traditional manner, as is shown with the number for Alice, denoted by the reference number 14. The user has input also Alice's number with the prefix *, as described above, and this is denoted by the reference number 13. If the user selects the marked telephone number, with reference number 13, an encrypted transfer will be initiated, as has been de- scribed above.

It is obvious that the present invention solves the problems described in the introduction. A number of embodiments have been described above. The inven ^¬ tion can, however, be varied. Other suitable encryption and decryption algorithms, for example, may be used. Furthermore, parts of the key streams may be transferred through the in ^¬ terface to the security module of the mobile telephone after remaining parts of the key streams have been transferred to the security module of the mobile telephone.

The present invention, therefore, is not to be considered to be limited to the embodiments specified above since it can be varied within the scope specified by the attached patent claims .