Abstract:
A cryptosystem prevents replay attacks within existing authentication protocols, susceptible to such attacks but containing a random component, without requiring modification to said protocols. The entity charged with authentication maintains a list of previously used bit patterns, extracted from a portion of the authentication message connected to the random component. If the bit pattern has been seen before, the message is rejected; if the bit pattern has not been seen before, the bit pattern is added to the stored list and the message is accepted.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International PCT Application No. PCT/CA2005/000180 filed on Feb. 14, 2005 which claims priority from U.S. Provisional Application No. 60/543,914 filed on Feb. 13, 2004 the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to cryptographic systems and protocols used in such systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    In order to ensure confidentiality of or authentication of messages transferred between a pair of correspondents in a data communication system, it is usual to employ one or more cryptographic protocols to secure the message. Such protocols must be capable of withstanding a variety of attacks that may be made by an interloper in an attempt to break the protocol. One such attack is a replay attack. “Replay attacks” attempt to replicate an action performed by an earlier transmission to obtain information about the correspondents private information or keys used in the encryption by recording messages and reproducing the protocol  at a later date. 
       DESCRIPTION OF THE PRIOR ART 
       [0004]    Such attacks can be thwarted at the cost of added complexity to the protocol such as the use of nonces. However, in many situations, especially legacy operations, the addition of nonces to the protocol requires a significant change to the protocol and/or the data structure of the message, something often unacceptable with legacy systems. Such changes are especially problematic in legacy systems with a large deployed base of authentication tokens where such a  change would require amendment of and redeployment of the tokens. A method is needed to prevent replay attacks which does not require changes to the established base. 
         [0005]    It is therefore an object of the present invention to obviate or mitigate the above disadvantages. 
         [0006]    In general terms, the invention utilises the presence of an identifiable random component generated during signing of a message for use in verification of the signature to verify the originality of a message and inhibit replay attacks in a protocol. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the invention applies to signature schemes wherein the signature contains a random component, that is, a component that is derived from a randomly generated bit stream every time a signature is computed. To comply with the protocol, the signature must contain the random component. A portion of the component provides a bit pattern that may be used to inhibit a replay attack. 
         [0008]    The entity charged with authentication maintains a list of bit patterns previously used by the sending correspondent and extracted from a portion of the signed message connected to the random component. If the bit pattern has been seen before, the message is not considered original and is rejected, i.e. it has previously been received, if the bit pattern has not been seen before and the signature verifies, the bit pattern is added to the stored list and the message is accepted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: 
           [0010]      FIG. 1  is a schematic representation of a data communication system. 
           [0011]      FIG. 2  is a schematic representation of a data stream representing a signed message. 
           [0012]      FIG. 3  is a schematic representation of the flow of information in the system shown in  FIG. 1 . 
           [0013]      FIG. 4  is a detailed representation of the implementation with and ECDSA signature protocol. 
           [0014]      FIG. 5  is a representation similar to  FIG. 4  applied to an RSA signature scheme. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring therefore to  FIG. 1 , a data communication system generally indicated at  10  includes a pair of correspondents  12 ,  14  interconnected by a data communication link  16 . Each of the correspondents  12 ,  14  includes a computing device  18  to implement a set of programmed instructions and an encryption module  20  to interface between the computing device  18  and communication link  16 . 
         [0016]    It will be appreciated that the correspondents  12 ,  14  may be general purpose computers or dedicated equipment in a client server relationship, such as a point of sale device, PDA or cell phone interfacing through the link  16  with a financial institution. 
         [0017]    In operation, the computing device  18  prepares a message which is processed by the encryption unit  20  and transmitted as a data stream  26  through the communication link  16 . The encryption unit  20  at the correspondent  14  process the data stream to recover and authenticate the message received before passing it to the computing device  18 . 
         [0018]    The correspondent  14  includes a database  22  that contains lists  24  of bit patterns of selected portions of signatures received by the processor  20 . The database  22  is accessible by the computing device  18  and the lists  24  are conveniently organised to conduct a comparison for a particular initiating correspondent  12  between the bit patterns in a message received and those that are contained in the database. 
         [0019]    The encryption device  20  may implement a number of different protocols, such as a key generation, encryption/decryption or signature and verification. It will be assumed for the purpose of illustrating a preferred embodiment that the correspondent  12  prepares an information package in the computing device  18  which is signed by the encryption device  20 . Upon receipt at the correspondent  14 , the cryptographic processor  20  verifies the signature and passes the information to the computing device  18 . 
         [0020]    In operation, the correspondent  12  generates the information I in the computing device  18  and forwards it to the cryptographic processor  20 . The processor  20  signs the information I, utilising a protocol that generates a random component r. The bits representing the information I and signature components including the random component are assembled in to a data stream  26  to represent a signed message  28 . 
         [0021]    The signed message  28  is transmitted over the link  16  as a data stream and is received by the cryptographic unit  20  at the correspondent  14 . The signature is verified according to the signature scheme in the normal manner. If the verification is authenticated, the portion of the signed message corresponding to the random component r is located. The bit stream representing the portion is then compared with the bit streams contained in the database  22  to ensure that the same random component has not been utilised in previous signed messages. If the bit stream has not been previously utilised, that is if no match is found in the database  22 , then the signature is considered to be an original message, in that it has not been received before, and is accepted. If a match is found then the signed message is not accepted. 
         [0022]    An example of an established signature protocol that may be utilised to implement the above technique is described below with respect to  FIG. 4  utilising the ECDSA signature protocol. 
         [0023]    Information I is to be signed by a long term private key d of the correspondent  12  in an elliptic curve cryptosystem (ECC) with know parameters including a generating point P of order n. 
         [0024]    The correspondent  12  randomly generates a ephemeral private key k and computes a  corresponding ephemeral public kP which represents a point with coordinates (x,y). 
         [0025]    To compute a first component r of the signature, the first co-ordinate of the ephemeral public key kP is converted into an integer. The first component is itself random as it is determined from the random private key k. 
         [0026]    A second component s, of the signature is generated by solving the signing equation ks=H(I)+dr (mod n) for the second component s of the signature, where H is an appropriate cryptographic hash function such as SHA1. 
         [0027]    The information and signature is assembled as a data stream  26  containing: (I,r,s) in defined locations and is then transmitted as the signed message  28  through the link  16 . 
         [0028]    Upon reception of the signed message  28 , at the correspondent  14 , the cryptographic processor  20  proceeds to authenticate the signature. The authentication normally proceeds as follows. 
         [0029]    Initially the ephemeral public key kP is computed by calculating s −1 (H(I)P+rA), where A is the long term public key of the correspondent  12 . 
         [0030]    After recovery of kP, the first co-ordinate of kP is converted into an integer following the same procedure as used by the correspondent  12 . The integer obtained should correspond to the number r contained in the transmission and if so the signature is accepted. If it does not, the signature is not verified and so is rejected. 
         [0031]    To inhibit a replay attack, a subject f(r) of the number r is extracted or derived from the signed message  28 . The subset f(r) is compared with a previously stored list  24  of subsets in the database  22  for the correspondent  12 . The database  22  is conveniently organised by the correspondent for comparison. Well-known masking and shifting techniques may be used to extract and compare the bit streams efficiently. If only a replay attack is of concern, then it may be sufficient to compare the subsets received from the same correspondent but for greater security all previous subsets may be compared. 
         [0032]    The authentication is rejected if the subset f(r) is in the list, indicating it had previously been used. If the subset is not on the list  24 , the process continues and the subset f(r) is added to the database  22  using well-known storage-and-retrieval techniques to store the data in such a manner as to allow subsequent efficient retrieval. 
         [0033]    It will be appreciated that the signature verification may be performed after the comparison of the subsets if preferred. It will also be noted that the subset used to detect potential replay is part of the signature component r used for verification of the signature and as such already exists in the signed message. Accordingly, neither the bandwidth nor protocol are affected by the additional authentication and redundancy is avoided. 
         [0034]    The number of bits chosen from the random component depends on the security level required for the application and the storage available. The number of bits chosen from the random component should also be large enough to give assurance against the Birthday Surprise, where the expected number of events that will occur before a match is calculated to be √{square root over (2 m )}π asymptotically, where m+1 bits are stored. For example, in storing 40 bits, one would not expect a match short of 1.3 million signatures; in storing 60 bits, one would not expect a match short of 1.3 billion signatures. 
         [0035]    In a second preferred embodiment shown in  FIG. 5 , the signature scheme is the well-known integer-factorisation scheme of RSA with appendix, RSA-PSS, as specified in PKCS #1, Ver. 2.1. 
         [0036]    The information I is encoded as follows.
       i) The information I is hashed, the hash is bracketed by prepending padding bytes and appending random bytes r, resulting in a bracketed hash E.   ii) The bracketed hash E is further hashed, resulting in the bit string H.   iii) The bit string H is used in a mask generation function, and the output of the function employed to mask the random bytes appended to the hash of the information I.   iv) The encoded message is assembled comprising the concatenation of the masked output from Step (iii), the further hash from Step (ii) i.e. the bit string H, and a padding byte.       
 
         [0041]    The encoded message is then converted into a number. The RSA operation is performed on the number with the private exponent of the correspondent  12 , and the result converted to a bit string s which is used as a signature, s for the information I. 
         [0042]    The message with signature (I,s) is then transmitted over the link  16  as a data stream  28  to the correspondent  14 . 
         [0043]    Upon reception of the data stream (I,s), by the correspondent  14 , the verification and authentication proceeds as follows. 
         [0044]    At the cryptographic processor of correspondent  14 , the signature s is converted into a number. 
         [0045]    The RSA operation is then performed on the number with the public exponent of correspondent  12 , resulting in another number which is converted into the alleged bracketed hash E′. 
         [0046]    The alleged bracketed hash E′ is hashed and split into the alleged masked output and the alleged hash of the original message. 
         [0047]    Using the alleged masked output and the alleged hash, the alleged random bytes are extracted. 
         [0048]    The concatenation of the appropriate padding, the hash of the alleged bracketed hash and the alleged random bytes is hashed and compared with the alleged hash of the original message. If the two agree, the signature is considered verified and accepted. 
         [0049]    To inhibit a replay attack, either before or after verification, a subject f(s) of the number s, is extracted, where f is a predetermined function. The subset f(s), is selected from the portion of the signature s that corresponds to the appended random bytes and compared with a previously stored list  24  of subsets for the correspondent  12  in the database  22 . 
         [0050]    The authentication is rejected if the subset is in the list. If it is not in the list, the signature is accepted and the subset to the list is added. Again therefore the reply attack is inhibited by use of the portion of the signature components that are random and used by the protocol in the signature verification. 
         [0051]    The above examples have been described in the context of a signature verification but may also be used in other protocols where a random bit pattern is generated. For example, the MQV protocols may be used a key agreement protocol as well as signature protocols. 
         [0052]    In the key agreement protocols, the ephemeral public key of each correspondent is exchanged and forms part of the message. The ephemeral public key is random and is used to authenticate the respective party. Accordingly, a subset of the data representing the key may be extracted and compared with the existing database to verify the originality of the exchanged message. 
         [0053]    It will be appreciated that although in the above description the data base  22  is shown associated with the correspondent  14 , a similar database may be associated with each correspondent in the system where protection from such attacks is required.