GB2432496A

GB2432496A - Communication indicating a security level

Info

Publication number: GB2432496A
Application number: GB0623685A
Authority: GB
Inventors: Marinus Struik
Original assignee: Certicom Corp
Current assignee: Certicom Corp
Priority date: 2003-08-19
Filing date: 2004-08-19
Publication date: 2007-05-23
Anticipated expiration: 2024-08-19
Also published as: GB0623685D0; GB2432496B

Abstract

In a secure communication system, an adaptable security level is synchronised between devices by inserting security bits (35-37, fig. 3) into the control bits of a message frame or header (32) which indicate the selected security level (eg.0, 32, 64 or 128 bits) and whether encryption is turned on or off, thus allowing modification on a frame-by-frame basis. For multiple recipients, a minimum integrity level common to all of them can be selected. The security bits may also indicate parameters in an algorithm. Communication is organised by a network stack (Fig. 6) which passes payloads [m] and security levels (SEC) between various layers (application, network, MAC, physical etc.) and encryption keys K. Security level information may also be passed via acknowledgement (ACK) messages over a feedback channel.

Description

METHOD AND APPARATUS FOR SYNCHRONIZING AN ADAPTABLE

2 SECURITY LEVEL IN AN ELECTRONIC COMMUNICATION

4 BACKGROUND OF THE INVENTION

6 FIELD OF THE INVENTION

7 (0001] The present invention relates to a method and apparatus for providing synchronizing 8 an adaptable security level in an electronic communication.

DESCRIPTION OF THE PRIOR ART

11 100021 In electronic communications, it is often necessary to prevent an eavesdropper from 12 inteicepting a message. It is also desirable to have an indication of the authenticity of a message, 13 that is a verifiable identification of the sender. These goals are usually achieved through the use 14 of cryptography. Private key cryptography requires sharing a secret key prior to initating communications. Public key cryptography is generally preferred as it does not require such a 16 shared secret key. Instead, each correspondent has a key pair including a private key and a public 17 key. The public key may be provided by any convenient means, and does not need to be kept 18 secret.

19 (0003) There are many variations in cryptographic algorithms, and various parameters that determine the precise implementation. In standards for wireless communications, it has been 21 customary to set these parameters in advance for each frame type. However, this approach limits 22 the flexibility of the parameters.

23 (0004] When one device is communicating with several other devices, it will often need to 24 establish separate parameters for each communication.

(0005] It is an object of the present invention to obviate or mitigate the above disadvantages.

27 SUMMARY OF THE INVENTION

28 [0006) In accordance with one aspect of the present invention, there is provided a method of 29 communicating in a secure communication system, comprising the steps of assembling as I message at a sender, then determining a security level, and including an indication of the security 2 level in a header of the message. The message is then sent to a recipient.

3 (0007] in accordance with another aspect of the present invention, there is provided a method 4 of providing a security level to a sender by including information in an acknowledgement message.

7 BRIEF DESCRIPTION OF THE DRAWtNGS

8 100081 These and other features of the preferred embodiments of the invention will become 9 more apparent in the following detailed description in which reference is made to the appended drawings wherein: 11 (0009] Figure 1 is a schematic representation of a communication system; 12 100101 Figure 2 is a schematic representation of an information frame exchanged in the 13 communication system of Figure 1; 14 (00111 Figure 3 is a schematic representation of a frame control portion of the frame of Figure 2; 16 (0012) Figure 4 is a schematic representation of a method performed by a sender in Figure 1; 17 (0013] Figure 5 is a schematic representation of a method performed by a recipient in Figure 18 1; 19 (0014) Figure 6 is a schematic representation of a network protocol used in one embodiment of the communication system; 21 10015) Figure 7 is a schematic representation of an embodiment of the communication 22 system; 23 (00161 Figure 8 is a schematic representation of another embodiment of the communication 24 system.

26 DESCRIPTION OF THE PREFERRED EMBODIMENTS

27 10017] Referring to Figure 1, a communication system 10 includes a pair of correspondents 28 12, 14 connected by a communication link 16. Each correspondent 12, 14 includes a respective 29 cryptographic unit 18, 20.

1 100181 Each correspondent 12, 14 can include a processor 22, 24. Each processor may be 2 coupled to a display and to user input devices, such as a keyboard, mouse, or other suitable 3 devices. If the display is touch sensitive, then (he display itself can be employed as the user 4 input device. A computer readable storage medium is coupled to each processor 22, 24 for providing instructions to the processor 22, 24 to instruct and/or configure processor 22, 24 to 6 perform steps or algorithms related to the operation of each correspondent 12, 14, as further 7 explained below. The computer readable medium can include hardware and/or software such as, 8 by way of example only, magnetic disks, magnetic tape, optically readable medium such as CD 9 ROM's, and semi-conductor memory such as PCMCIA cards. In each case, the medium may take the form of a portable item such as a small disk, floppy diskette, cassette, or it may lake the 1] form of a relatively large or immobile item such as hard disk drive, solid state memory card, or 12 RAM provided in a support system. It should be noted that the above listed example mediums 13 can be used either alone or in combination.

14 100191 Referring to Figure 2, a frame used in communications between the correspondents 12, 14 is shown generally by the numeral 30. The frame 30 includes a header 32 and data 34. The 16 header 32 includes information about the source and destination of the frame 30 and is used for 17 processing frames. The header 32 may contain other control information as will be understood 1 8 by those skilled in the art.

19 [0020) Referring to Figure 3, the header 32 also contains frame control bits 33. The frame control bits 33 include security bits 35, 36, and 37. Security bit 35 indicates whether encryption 21 is on or off. Security bits 36 and 37 together indicate the integrity level, such as 0, 32, 64, or 128 22 bits. It will be recognized that providing security bits in each frame allows the security level to 23 be modified on a frame-by-frame basis rather than on the basis of a pair of correspondents, 24 therefore providing greater flexibility in organizing communications.

100211 In order to provide security, certain minimum security]evels may be used. These 26 levels should be decided upon among all of the correspondents through an agreed-upon rule.

27 This rule may be either static or dynamic.

28 [0022) In operation, the correspondent 12 performs the steps shown in Figure 4 by the 29 numeral 100 to send infonnation to the correspondent 14. First, the correspondent 12 prepares I data and a header at step 102. Then it selects the security level at step 104. The security level is 2 determined by considering the minimum security level required by the recipient, the nature of the 3 recipient, and the kind of data being transmitted. If the security level includes encryption, then 4 the correspondent 12 encrypts the data at step 106. If the security level includes authentication, then the correspondent 12 signs the data at step 108. Then the correspondent 12 includes bits 6 indicating the security level in the frame control at step 1 10. The correspondent 12 then sends the 7 frame to the correspondent 14.

8 100231 Upon receiving the frame, the correspondent 14 performs the steps shown in Figure 5 9 by the numeral 120. The correspondent 14 first receives the frame at step 122. It then extracts the security bits at step 124. If the security bits indicate encryption, then the correspondent 14 11 decrypts the data at step 126. If the security bits indicate authentication, then the correspondent 12 14 verifies the signature at step 126. Finally, the correspondent 14 checks the security level to 13 ensure it meets predetermined minimum requirements. If either the encryption or authentication 14 fails, or if the security level does not meet the minimum requirements, then the correspondent 14 1 5 rejects the message.

16 100241 It wiJl be recognized that providing security bits and an adjustable security level 17 provides flexibility in protecting each frame of the communication. Ills therefore possible for the 18 sender to decided which frames should be encrypted but not authenticated. Since authentication 19 typically increases the length of a message, this provides a savings in constrained environments when bandwidth is at a premium.

21 10025] In a further embodiment, the correspondent 12 wishes to send the same message to 22 multiple recipients 14 with varying minimum security requirements. th this case, the 23 correspondent 12 chooses a security level high enough to meet all of the requirements. The 24 correspondent 12 then proceeds as in Figure 4 to assemble and send a message with the security level. The message will be accepted by each recipient since it meets each of their minimum 26 requirements. It will be recognized that this embodiment provides greater efficiency than 27 separately dealing with each recipient's requirements.

28 10026) In another embodiment, a different number of security bits are used. The actual 29 number of bits is not limited to any one value, but rather may be predetermined for any given I application. The security bits should indicate the algorithm parameters. They may be used to 2 detennine the length of a key as 40 bits or 128 bits, the version of a key to be used, or any other 3 parameters of the encryption system.

4 0027] It will be recognized that in the above embodiments, a network stack may be used to organize communications between the correspondents. Referring therefore to Figure 6, the a 6 network stack of correspondent A is showii by the numeral 130. A network stack of 7 correspondent B is shown by the numeral 140. The network stacks are organized into layers and 8 have similar structures. The network stack 130 includes an application layer (APL) 132, a 9 network layer (NWK) 134, a message authentication layer (MAC) 136, and a physical layer (PHY) 138. The network stack 140 includes similar components with similar numbering.

11 (0028) The sender determines how he wants to protect payload (and where to protect it, i.e., 12 which layer). For the APL layer, security should be transparent; its role is limited to indicating at 13 which level it wants to protect data (i.e., security services: none, confidentiality, data 14 authenticity, or both). The actual cryptographic processing then is delegated to lower layers.

10029) The recipient determines whether or not to accept protected payload, based on the 16 received frame and locally maintained status information. The outcome of the cryptographic 17 processing (done at the same layer as that of the sender), including info on the apparently offered 18 protection level, is passed to the application layer, who determines whether the offered 19 protection level was adequate. The recipient may acknowledge proper receipt of the frame to the original sender, based on this adequacy test'.

21 10030) The acknowledgement (ACK), if present, is then passed back to the sender and 22 passed up to the appropriate level (if protected message sent at A.PL layer, then ACK should also 23 arrive back at that level; similar for lower layers of course).

24 10031) The sender A determines that it wants to protect payload m using the protection level indicated by SEC (taking into account its own security needs and, possibly, those of its intended 26 recipient(s). The payload m and desiTed protection level SEC is then passed to a lower layer 27 (e.g., the MAC layer, as in the diagram) which takes care of the actual cryptographic processing.

28 (This message passing could include additional status information that aids in the processing of 29 the frame, such as the intended recipient(s), fragmentation info, etc. Note that the delegation of I the cryptographic processing to a lower layer is only a conceptual step if cryptographic 2 processing takes place at the same layer at which the payload m originates.) Cryptographic 3 processing involves protecting the payload m and, possibly, associated information such as frame 4 headers, using the cryptographic process indicated by the desired protection level SEC. The key used to protect this information is derived from shared keying material maintained between the 6 sender and the intended recipient(s). After cryptographic processing, the protected frame, 7 indicated by [m}K, SEC in Figure 6, is communicated to the intended recipient(s) B. 8 10032J The intended recipient (s) retrieves the payload m' from the received protected frame, 9 using the cryptographic process indicated by the observed protection level SEC', using a key that is derived from shared keying material maintained between the sender and the recipient(s) in 11 question. The retrieved payload m' and the observed protection level SEC' is passed to the same 12 level at which the payload was originated by the sender, where the adequacy of the observed 13 protection level is determined. The observed protection level SEC' is deemed sufficient, if it 14 meets or exceeds the expected protection level SEC0, where the parameter SEC0 might be a fixed pre-negotiated protection level that does or does not depend on the retrieved payload rn' in 16 question. (Defining SEC0 in a message-dependent way would allow fine-grained access control 1 7 policies, but generally involves increased storage and processing requirements.) 18 OO33J The above approach works in contexts where expected and observed protection levels 19 can be compared, i.e., where the set of protection levels isa partial ordering. An example is the context where protection involves a combination of encryption and/or authentication, with as 21 ordering the Cartesian product of the natural ordering for encryption (encryption OFF < 22 Encryption ON) and the natural ordering of authentication (ordered according to increasing 23 length of data authenticity field). Moreover, if the set of protection levels has a maximum 24 element, then the sender can use this maximum protection level to ensure that (unaltered) messages always pass the adequacy test.

26 [0034) In the above embodiments, each sender has to pre-negotiate the minimum expected 27 protection level SEC0 with each intended recipient. Thus, the approach might not be as adaptive 28 as desirable for some applications and may involve additional protocol overhead at every change 29 of the SEC0 parameter. These disadvantages can be overcome by using the acknowledgement I (ACK) mechanism from recipient(s) to sender as a feedback channel for passing the SEC0 info.

2 This is performed by incorporating in each acknowledgement message an indication as to the 3 expected protection level. This information can then be collated by the original sender to update 4 the minimum protection level expected by its recipient(s), whether or not this is message-dependent or not.

6 J0035) In a further embodiment, a method of synchronizing security levels is shown.

7 Referring to Figure 7, another embodiment of the communication system is shown generally by 8 the numeral 160. The system includes a sender A 162 and recipients 168 in a group labelled G. 9 The sender A includes parameters SECA 164 and SEC0 166.

10036] Sender A wants to securely communicate a message m to a group G of devices. The 11 sender A has access to the two parameters, i.e., 12 (0037] (1) The minimum level SECA at which it would like to protect this message (in 113 general, SECA might depend on the group it sends information to and the message itself, so 14 proper notation would be SECA (m,G)); 10038] (2) The minimum protection level SEC0 that the group G of recipients expects 16 (again, the proper notation would be SEC0(m,A) if this level would depend on the sender and the 17 message itself as well). Here, the minimum expectation level of a group is the maximum over all 18 group members of the minimum expectation level for each group member.

19 0039) Initialization 100401 Sender A assumes that each parameter SEC0 is set to the maximum protection level 21 (for each group G it securely communicates with).

22 [0041] Operational usage 23 10042] -Sender A determines the minimum protection level SECA at which it Wants to 24 protect the message m. The actual protection level SEC applied to the message m meets both its own adequacy test (i.e., SEC =SECA) and the minimum expected level by the group G (i.e., SEC 26 =SECG).

27 [0043] -Each recipient B that is in the group G of recipients (i.e., B G) indicates in its 28 secure acknowledgement message the minimum expected protection level (for sender A and 29 message m) at that particular moment of time.

1 10044) A updates the parameter SEC0 such that it is consistent with all the minimum 2 protection levels indicated in each of the acknowledgement messages it received back (i.e., 3 SECG =SECB for all responding devices B).

4 (0045] Note that the procedure described above sends messages with a protection level that satisfies both the needs of the sender and expectations of recipient(s) and is adaptable to changes 6 herein over time. Alternatively, the sender might only take its own protection needs into account, 7 at the cost of potentially sending messages that will be rejected by one or more recipients due to 8 insufficient -since less than expected -protection level.

9 (0046] The procedure descnbed above can be generalized towards a general self-synchronization procedure for status information among devices in any network topology, where 11 the feedback info on status information may be partially processed along the feedback path from 12 recipient(s) towards sender already, rather than at the sender itself only (in the example above, 13 this graph is a tree with root A and leaves the recipient(s) and the synchronization involves a 14 specific security parameter).

(0047] As seen in Figure 8, A sends a payload secured at protection level SEC to a group of 16 devices consisting ofBl-B4. The recipients B1-B4 provide feedback to the sender A on the 17 expected protection level (indicated in the diagram as the integers 1, 3, 2, 5, where these integers 18 are numbered in order of increasing protection level). The feedback is communicated back to A 19 via intermediate nodes Cl and C2, who collect the respective feedbacks of devices in their respective groups GI and G2 and process this, before returning a condensed acknowledge 21 message representing both groups to sender A. The condensed feedbacks provided by these 22 intermediate devices provides A with the same information on the minimum protection level that 23 satisfies the expectations of all recipients as would have been the case if this information would 24 have been forwarded to A without intermediate processing. (Here, we assume that the intermediate devices do not cheat in their calculations.) 26 10048) Although the invention has been described with reference to certain specific 27 embodiments, various modifications thereof will be apparent to those skilled in the art without 28 departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims

I

Claims 1. A method of communicating between a sender and at least one recipient in a secure communication system, said method comprising the steps of: (a) said sender determining that an initial message is to be protected by a security level based on known security requirements, said determination made at a first layer of said sender; (b) said sender sending said initial message and an indication of said security level to a second layer of said sender; (c) said initial message undergoing cryptographic processing to generate a protected message at said second layer based on said indication of said security level using a shared key maintained between said sender and said at least one recipient; and (d) said sender sending said protected message to said at least one recipient.

2. A method according to claim 13 further comprising the steps of: (e) said at least one recipient receiving said protected message at a first level, said first level similar to said second level of said sender; (f) said at least one recipient obtaining said initial message from said protected message using cryptographic processing based on said indication of said security level using said shared key; and (g) said at least one recipient passing said initial message and said indication of said security level to a second level, checking to ensure said security level meets predetermined minimum requirements, and rejecting said initial message is said security level is below said minimum requirements, said second level similar to said first level of said sender.

3. A method according to claim 13 wherein said known security requirements include minimum security requirements for each of said at least one recipient and said sender.

4. A method according to claim 15 wherein said security level is chosen to be high enough to meet all the said minimum security requirements.

5. A method according to claim 13 wherein said indication of said security level comprises a number of security bits which change in length according to the application of said method.

6. A method according to claim 13 wherein said sender receives a feedback response from said at least one recipient subsequent to sending said protected message, said feedback response including an update of said known security requirements.

7. A method according to claim 14 wherein said at least one recipient sends a feedback response subsequent to checking said security level, said feedback response including an update of said known security requirements.

8. A method of communicating between a sender and a group of at least one recipient in a secure communication system, said method comprising the steps of: (a) said sender determining a minimum security level at which to send a message, said minimum security level based on a value SECA indicative of security requirements of said sender and based on a value SEC0 indicative of security requirements of said group; (b) said sender sending said message with an indication of said security level to said at least one recipient of said group; (c) said at least one recipient receiving said message and said indication of said security level, determining if said message meets its own minimum security requirements, and sending a feedback response to said sender with an indication of said minimum security requirements; and (d) said sender receiving said feedback response and updating said value SECG based on said feedback response.

9. A method according to claim 20 wherein said minimum security level is based on said value SECA only.

10. A method according to claim 20 wherein said sender and said group are of the same entity, said method thus providing a self synchronization of said security requirements of said sender.

11. A method according to claim 20 wherein said group comprises multiple groups and a central node intermediate of said sender and said multiple groups, said central node receiving multiple feedback responses from said multiple groups and communicating a single feedback response to said sender for updating said value SEC0.

12. A method of communicating between a sender and at least one recipient in a secure communication system, said method comprising the steps of: (a) said at least one recipient receiving a message from said sender, said message having a header, said header including an indication of a security level determined by said sender based on known security requirements; (b) said at least one recipient extracting said indication of said security level from said header; and (c) said at least one recipient checking said security level to ensure it meets predetermined minimum requirements and rejecting said message if said security level is below said minimum requirements.

13 A method of communicating between a sender and at least one recipient in a secure communication system, said method comprising the steps of: (a) said at least one recipient receiving a protected message from said sender at a first level, said protected message generated from an initial message, said initial message determined by said sender at a first layer of said sender to be protected by a determined security level based on known security requirements, said initial message sent with an indication of said security level to a second layer of said sender whereby said initial message undergoes cryptographic processing using a shared key maintained between said sender and said at least one recipient to generate said protected message, said first level of said at least one recipient similar to said second layer of said sender; (b) said at least one recipient obtaining said initial message from said protected message using cryptographic processing based on said indication of said security level using said shared key; and (c) said at least one recipient passing said initial message and said indication of said security level to a second level, checking to ensure said security level meets predetermined minimum requirements, and rejecting said message if said security level is below said minimum requirements, said second level of said at least one recipient similar to said first level of said sender.

14. A method of communicating between a sender and a group of at least one recipient in a secure communication system, said method comprising the steps of: (a) said sender determining a minimum security level at which to send a message, said minimum security level based on a value SECA indicative of security requirements of said sender and based on a value SECG indicative of security requirements of said group; (b) said sender sending said message with an indication of said security level to said at least one recipient of said group to determine if said message meets minimum security requirement of said at least one recipient of said group; and (c) said sender receiving a feedback response with an indication of said minimum security requirements of said at least one recipient and updating said value SEC based on said feedback response.

15. A method of communicating between a sender and a group of at least one recipient in a secure communication system, said method comprising the steps of: (a) said at least one recipient of said group receiving from said sender a message and an indication of a security level determined by said sender, said security level based on a value SECA indicative of security requirements of said sender and based on a value SECG indicative of security requirements of said group; (b) said at least one recipient determining if said message meets its own minimum security requirements; and (c) said at least one recipient sending a feedback response to said sender with an indication of said minimum security requirements for updating said value SEC0. VL.

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY

OR PRiVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: lh A method of communicating in a secure communication system, comprising the steps of a) assembling as message at a sender; b) determining a security level; c) including an indication of the security level in a header of the message; d) sending the message to a recipient.

17 A method according to claim 1, wherein the security level is chosen to be at least a minimum security level known to the sender and the recipient.

/ A method of communicating in a secure communication system, wherein each communication is divided into a plurality of frames, and each fiame includes an indication of a security level, thereby allowing each frame to have a different security level to adapt to the nature of each frame.

/ 1 A method of providing a sccunty level to a sender by including information in an acknowledgement message.