Abstract:
A method of formatting data for transmission to another party including the step of incorporating in the data a flag indicative of the absence of data for authentication of the sender. An authentication tag length is also included to permit variable length tags to be used.

Description:
[0001]     This application claims priority from U.S. Provisional Patent Application No. 60/519,609 filed on Nov. 14, 2003 and Canadian Application No. 2,449,524 filed on Nov. 14, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to cryptographic methods and apparatus.  
       DESCRIPTION OF THE PRIOR ART  
       [0003]     A block cipher is a versatile cryptographic function that has found widespread adoption in cryptographic schemes. It may be used as a central component in a cryptographic system and permits message authentication techniques and entity authentication techniques to be included.  
         [0004]     CCM is a block-cipher mode of operation that can be used to protect the privacy and/or authenticity of messages. The CCM mode is a particular mode of operation that operates on block-ciphers, typically with a 128-bit block size, and involves a particular combination of the so-called Counter (CTR) mode of operation and the Cipher-Block Chaining (CBC) mode of operation, using a single key. The CCM mode of operation has been widely adopted, due to its incorporation as the mandatory block-cipher mode of operation in a number if current wireless standards that recently emerged, including the IEEE 802.11 WLAN standard and the IEEE 802.15 High-Rate and Low-Rate WPAN standards.  
         [0005]     The CCM mode allows for variable-length authentication tags (from 32-bits to 128-bits), thus allowing varying degrees of protection against unauthorized modifications. The CCM mode allows quite efficient implementations, due to the fact that one only needs to implement the encryption transformation of the underlying block-cipher (and not the decryption transformation) and due to its reliance on a single key, rather than multiple keys, to provide confidentiality and authenticity services. This being said, the CCM mode has also some disadvantages.  
       SUMMARY OF THE INVENTION  
       [0006]     While the original CCM mode provides for data authentication and, possibly, confidentiality, it does not provide for confidentiality only. This is unfortunate, since not all implementation environments call for data authenticity (e.g., if data authenticity is provided by an external mechanism).  
         [0007]     The original CCM mode is known to be vulnerable to specific attacks, if used with variable-length authentication tags rather than with fixed-length authentication tags only. Thus, the original CCM mode can only be securely used with the same key in settings with fixed-length authentication tags. This is unfortunate, since support for variable-length authentication tags is useful in constrained implementation environments, such as secured wireless sensor networks, where applications on a device might have different protection requirements, but would have to share the same key, due to resource constraints.  
         [0008]     It is therefore an object of the present invention to obviate or mitigate the above disadvantages.  
         [0009]     A method of formatting data for transmission to another party including the step of incorporating in the data a flag indicative of the absence of data for authentication of the sender. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The features of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:  
         [0011]      FIG. 1  is a schematic illustration of a data communication system.  
         [0012]      FIG. 2  is a representation of a cryptographic engine as used by one of the correspondents in  FIG. 1 .  
         [0013]      FIG. 3  is a representation similar to  FIG. 2  of a cryptographic engine as used by the other of the correspondents.  
         [0014]      FIG. 4  is a representation of the format of data presented to the cryptographic engine of  FIG. 2 .  
         [0015]      FIG. 5  is a representation of the format of data generated by the cryptographic engine of  FIG. 2 .  
         [0016]      FIG. 6  is an expanded representation of a portion of data used in the enciphering process.  
         [0017]      FIG. 7  is a table indicating the coding of data to be incorporated in the data shown in  FIG. 5 .  
         [0018]      FIG. 8  is a representation of data used in the initiation of the enciphering process.  
         [0019]      FIG. 9  is an expanded portion of data shown in  FIG. 8 .  
         [0020]      FIG. 10  represents the format of data used in the enciphering process.  
         [0021]      FIG. 11  is an enlarged view of a portion of the data shown in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Referring therefore to  FIG. 1 , a pair of correspondents  10 ,  12  communicate over a transmission link  16 . Each of the correspondents  10 ,  12  includes a cryptographic engine  18 ,  20  respectively that may receive information from the correspondent and perform cryptographic operations on it before transmission over the transmission line  16 . Similarly, messages received over the transmission line  16  may be processed by the cryptographic unit  18 ,  20  to provide information to the recipient.  
         [0023]     Although it will be appreciated that the cryptographic units  18 ,  20  are similar and that each can function with its respective correspondent as a recipient or a sender, it will be assumed for the purpose of the following description that the correspondent  10  is the sender of a message and that its cryptographic unit  18  acts to encode the information for transmission and that the correspondent  12  is the recipient of the information and the cryptographic unit  20  acts to process information that is received.  
         [0024]     Referring therefore to  FIG. 2 , cryptographic unit  18  includes an input  22  of a message to be forwarded to the recipient  12 . The format of the message may be seen in  FIG. 4  and includes a packet header  24  and a payload  26  consisting of the message m. The packet header  24  has packet control information  28  and a plurality of addressing fields including the destination address  30 , in this case, the destination of the correspondent  12 , and the source address  32 , in this case the address of the correspondent  10 . The header  24  is to be transmitted over the link  16  as plain text whereas the payload  26  is to be transmitted in a secure manner.  
         [0025]     The enciphering of the message m is performed by an encryption module  34  that may implement a suitable encryption algorithm. In the present example, a block cipher mode of operation is performed preferably implementing a block cipher mode compatible with CCM. The encryption module  34  requires as inputs a nonce  36  and a key  38 . As a further input, selected information, a, contained in the header may be forwarded to the encryption module  34  to provide a degree of authentication. The output from the encryption module  34  is encrypted data ‘c.’ 
         [0026]     In order to derive a key at input  38 , key information is supplied over line  44  permitting to a key to be derived from for example the addressing information of the sending party and the recipient or by other previously agreed upon means.  
         [0027]     In order to provide a unique value to the nonce  36 , a frame counter  46  is provided and is not permitted to be reused within the context of utilising the same key input.  
         [0028]     An input signal indicating the desired protection level is also provided at  48  and is used to indicate whether confidentiality is required and whether authenticity is required and at what level. The encoding of the input signal  48  is shown in  FIG. 7  and provides an unambiguous indication of the nature of the security level required. As seen in  FIG. 7 , the protection level SEC provides eight possible options, as represented by the Lexa-decimal codes. This enables lower most bits of the code to represent uniquely and unambiguously the different combination. Moreover, the combinations are ordered so that those with a 1xx indicate the encryption is turned on and those 0xx have the encryption turned off to further facilitate recognition of the cooling.  
         [0029]     The key information  44 , frame counter  46  and protection level  48  are provided to a buffer  50  where they are concatenated to provide a security information output.  
         [0030]     The protection level of signal  48  is also fed to a encoding module  52  that determines the authentication tag length and provides an input signal M indicating the length of the authentication tag to be appended to the message and included in the ciphertext ‘c’. Again, as may be seen from  FIG. 7 , each of the possible tag lengths, in this case 0, 4, 8 or 16 bytes, is provided with a corresponding M value that may be represented as a combination of three bits. Th values are provided to the encryption module  34  for inclusion in the data string to enciphered. A greater range of values may be used with additional bits provided in the field as appropriate.  
         [0031]     The outputs of the header, encrypted data  42  and security information from the buffer  50  are assembled at a database  54  and transmitted over the communication line  16 . The format of the resultant transmission may be seen from  FIG. 5  and comprises the packet header  24  and addressing fields  30 ,  32  corresponding of to the plain text header and the security information formed from the concatenation of the frame counter, key identifier information and the protection level indication, that is output of the buffer  50 . The output of the encryption module  34  appears as the payload  42  and includes the cipher text of the message m and the encrypted authentication tag U obtained from the authentication data, a.  
         [0032]     Upon receipt of the cipher text at the cryptographic unit  20  of correspondent  12 , the process is reversed as shown in  FIG. 3 . The header is processed to remove the associated data and provide an input to the encryption module  34   a . The nonce is reconstructed by the construction module  36   a  from the information in the plane text header and the information derived from the security information. The security information is processed through buffer  50   a  which extracts the frame counter and derives the protection level included in the security information header. From the protection level, the tag length is derived at module  52   a  and provided as an input to the encryption module  34   a . The encryption module  34   a  may then perform the decryption and extract the plain text of the message m.  
         [0033]     As noted above, the input to the encryption module  34  includes the key, the nonce  36 , and the message m to be encrypted and additional authentication data a. The binary string representing the tag length M is also provided as an input. The first step of the encryption module is to compute an authentication field T. In the case of a block cipher implementing the CCM protocol, this is done using the CBC-MAC mechanism and truncating the output to the appropriate size. To perform this operation, a series of blocks B o , B 1 , . . . B n  are defined and the CBC-MAC operation applied to these blocks.  
         [0034]     The first block B o  is formatted as shown in  FIG. 8  and has a first octet to contain a set of flags followed by nonce  36 , a security field indication of the form indicated in  FIG. 7  and an indication of the length of the message m. The flag field is itself formatted as shown in  FIG. 9  and includes a first field set of bits that indicate the number of octets in the length field of the message and thr authentication length M, i.e. number of octets in the authentication field, corresponding to the tag length derived from the module  52 . A further bit indicated as the A data is used to indicate whether or not authentication is included in the operation. Where authentication data is included, the blocks encoding the authentication A data are formed by right concatenating the octet string that encodes l (a) with a itself and splitting the result into 16 octet blocks. The last block may be right padded with zeros if necessary. These blocks are appended to the first block B o . After the additional authentication blocks, if any, have been added to the first block B, the message blocks are right concatenated. The message blocks are formed by splitting the message into 16 octet blocks, right padding the last block with zeros if necessary. If the message m is an empty string, then no blocks are added in this step. As a result, a sequence of 16 octet blocks B o , B 1  . . . B n  is prepared.  
         [0035]     The CBC-MAC is now computed as follows: 
 
 X   1   :=E ( K, B   0 ); 
 
 X   i+1   :=E ( K,X   i   ⊕B   i ) for  i= 1 , . . . , n,  
 
 where E ( ) is the block cipher function to provide a 16 octet string. An authentication tag T is obtained by truncating the 16 octet string to the left most M octets as indicated in the tag length output from the function  52 . 
 
         [0036]     To encrypt the message data, the CTR mode is used and the key stream blocks are defined by Si=E(k, a i ) for i=O, 1,2 . . . . The encryption blocks A 1  are formatted as shown in  FIG. 10  with the Sec Field being formatted as indicated in  FIG. 8 . The flag field as shown in  FIG. 11  includes a 3 bit representation of the length L of the message. The bits  3 ,  4 , and  5  are each set to O.  
         [0037]     The encrypted message is then prepared by XORing the octets of the message m in order, with the left most octets of the right concatenation of S 1 , S 2 , S 3 . The key block S o  is not used to encrypt the message but is used to encrypt the authentication field T previously obtained. The encrypted authentication value U results from XORing the octets of the authentication field T with the left most m octets of the key stream block S o  and is appended to the encrypted message as part of the enciphered payload c.  
         [0038]     Upon receipt of the encrypted message, the encryption key K, the nonce  36 , the additional authenticated data, a, and the encrypted message c is obtained and decryption starts by recomputing the key steam to recover the message m and the authentication field T. The message and additional authentication data a is then used to recompute the CBC-MAC value and check T. If the T value is not correct, the message is rejected without revealing further information.  
         [0039]     The above format of data permits the encryption module to be used without authenticating data. This is simply achieved by setting the flag bit Adata in the authentication block shown in  FIG. 9  at 0 to indicate the absence of any authentication data. A bit value of 1 is indicative of the presence of authentication data. A corresponding value of M indicative of no authentication data is also generated and included in the data to be enciphered. The provision of the protection level encoding and the inclusion of the tag length M within the message generation also permits variable length authentication tags to be utilised within the CCM block cipher mode. As indicated in  FIG. 7 , the encoding of the protection level not only indicates the nature of the protection, i.e. authentication with or without encryption but also may be used to uniquely identify the tag length associated with the authentication data. Accordingly, as part of the authentication process, the desired tag length can be verified and messages rejected if they are not compliant.  
         [0040]     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. The entire disclosures of all references recited above are incorporated herein by reference.