Patent Application: US-201514752437-A

Abstract:
a technique to manage members of a group of decoders having access to broadcast data , each group member sharing a common broadcast encryption scheme comprising the steps of , in a stage for a decoder to become a group member , receiving keys pertaining to the position in the group according to the bes , receiving a current group access data comprising a current group access key , and in a stage of accessing broadcast data , using the current group access data to access the broadcast data , and in a stage of renewing the current group access key , sending a first group message comprising at lease a next group access key encrypted so that only non - revoked decoders can access it , said group message being further encrypted by the current group access key , updating the current group access key with the next group access key .

Description:
the present application comprises two parts , the group key chaining and key distribution allowing an efficient revocation mechanism . when a group access key is to be renewed , the message containing the new group access key is sent to the decoders of that group . the message is broadcasted so all decoders , even not belonging to that group can receive this message and the encryption will determine which decoders can really obtain the new group access key . let us take the example with a group of 256 decoders and two decoders should be revoked . each decoder contains at least a master group key and a personal key . the new group access key is encrypted by the current group access key and by the keys only available in the decoders that are not revoked . a simple example using a trivial broadcast encryption scheme can be to create firstly a cryptogram containing the new group access key and encrypted by the current group access key . this cryptogram ct is then encrypted with a decoder personal key . the message will then comprises 254 cryptograms , each being encrypted by a personal key of the non - revoked decoders . of course , the inverse method is also applicable , the new group access key is firstly encrypted by the personal key of a non - revoked decoder and then encrypted by the current group access key . for the next renewal of the group access key , so - called further next group access key , even if the revoked decoders still contain the master group key and their personal key , the next message will contain the further next group access key encrypted by the master key only and by the next group access key . since the revoked decoders have not been able to access to the next group access key , this further next group access key is also not accessible for these decoders even if they have the master group key . according to another example , the further next group access key is simply encrypted by the next group access key . the second part of the invention is to propose a scheme that reduces greatly the size of the message when a revocation is to be carried out . one can imagine a group of 5000 decoders and only one is to be revoked . in this case , with the example above , the next group access key should be duplicated 4999 times , each time associated with the personal key of the non - revoked decoders . the fig4 illustrates the process of revocation . the top part shows the audio / video product ( could be one channel or a group of channels ) encrypted by the key successively k 1 , k 2 and k 3 . it is to be noted that this key ( k 1 , k 2 or k 3 ) could be used to decrypt directly the audio / video product or serving as decryption key to decrypt the messages ( ecm ) containing the keys to decrypt the audio / video product . in the example of the fig4 , during the first time period , the decoders t 1 , t 2 , t 3 and t 4 are part of the group . the group access key c 1 is the current one when the message k 1 c 2 is arrived , containing the next group access key c 2 and the key k 1 to access the audio / video product . in fact , the product key k 1 will arrive before this key is used to decrypt the product . the decoders will store the current product key k 1 and when the next is received , the product key k 2 , ready to be applied at the time the product swap from k 1 to k 2 . during the second time period , the group access key c 3 is sent to the non - revoked decoders . these decoders are t 1 , t 2 and t 4 . the message k 2 c 3 is encrypted by the current group access key c 2 and the keys pertaining to the non - revoked decoders t 1 , t 2 and t 4 . the decoder t 3 , having the current group access key c 2 , cannot decrypt this message and have access to the group access key c 3 . during the third time period , the message carrying the next group access key c 4 can be simply encrypted by the current group access key c 3 . the position into the group of formerly t 3 can be reallocated ( to a decoder t 30 ) by transmitting the current group access key c 3 and the key or keys previously distributed to the decoder t 3 . this reallocation can be executed only after the group access key c 3 is active i . e . after the transmission of the message k 2 c 3 . the group is organized by the management system and each position into the group is associated with a position status . this status can comprises three states , namely “ free ”, “ allocated ” and “ transitional ”. at the creation of a group , all positions are marked “ free ”. when a position is allocated to a member , this position is marked “ allocated ”. as soon as a member is withdrawn of the group , the position is marked “ transitional ”. this state indicates that the position was used before and special care is to be taken while reallocating this position . this position can be reallocated as soon as the group access key has been renewed into the members of this group at the exception of this specific member . the time between the revocation of the member until the group access key is changed for all other members is the so - called “ quarantine ” period . after this quarantine period , the position is virtually “ free ” and can be reused . the management of the database of the management center regularly checks the status of the “ transitional ” positions and checks whether the group access key is no longer present into the revoked decoder attached to that position . in this case , the position can be modified from “ transitional ” to “ free ”. in the case that no regular scan of the database is carried out , the status of a specific position is determined when a new member is to be inserted into that group . this is why in the case that the position has the state “ transitional ”, a further check is carried out to determine if the quarantine period is over . the renewal message of the group access key is formed by the group access data ( cgd ) which includes at least the group access key ( cgk ). this key can be used to decrypt the entitlement messages ( ecm ) related to the services for which the group of decoders has access . as a consequence , the group access key serves for the chaining mechanism and to access the services . according to another embodiment , the group access data comprises a session key sk . this session key sk will serves to access the services and decrypt the entitlement messages ( ecm ) related to these services . according to another embodiment , when the group access data comprising the new group access key is received and stored in the non - revoked decoders , another message is sent to the decoders containing the session key sk . this message is then encrypted by the group access key , thus only the non - revoked decoders can decrypt and obtain this session key sk . although the group access key can be distributed according to any broadcast encryption scheme as described above , in order to efficiently generate a revocation message , the present invention will now describe an efficient way to organize the key distribution . the main property of an ideal broadcast encryption system can be summarized for the purpose of this invention : assuming each terminal in the system has been provisioned with a unique set of secrets , a server , knowing the secrets of each terminal , may encrypt a single message in a way that is both efficient ( the message is small ) and that can be decrypted by authorized terminals but not by excluded ( revoked ) terminals even if all revoked terminals collude together . a particular scheme is considered here to illustrate the working principle of the invention . it is described in [ 3 ], however , it is to be noted that due to its severe lack in collusion resistance its use is not recommended in practice and it is only used here for its simplicity and for illustrative purposes . n is the total population of terminals in the broadcast encryption scheme r is the number of terminals revoked in an encrypted message log is the logarithm base 2 k is the size in bytes of keys in the system ( value assumed here is 128 bits = 16 bytes ) each terminal must store ( log ( n )+ 1 )* k bytes of key material the size of the encrypted message is at most : n / 8 + k + payload size bytes the terminal must perform at most r *( log ( n )− 1 ) crypto operations to retrieve the message encryption key the mechanism operates on a population of n = 2 m terminals . a binary tree of keys is built as illustrated in the fig1 for this population using a one way function to derive the key of each branch from the key of the node above . the f ( k , n ) function is a public one - way function ( e . g . hash primitive ) that derives a key from its two parameters . each terminal is assigned a leaf key , as depicted above , however , this key is not given to the terminal , instead , each terminal is given the key of all the other terminals in the group , or the means to compute them . for instance , as illustrated in the fig2 , the keys provided to terminal t 2 are k 10 , k 3 and k 2 . using k 3 , t 2 can compute k 7 and k 8 , and using k 2 , it can compute k 11 to k 14 , through k 5 and k 6 . when joining the group , each terminal then effectively receives log 2 ( n ) keys , plus an additional group key k g used for addressing a message to all members of the group . once this is in place , any message that must be sent to the group or subset of the group is encrypted in the following way : if the message is targeted to all terminals in the group , it is encrypted with the group key , k g which is known to all terminals if the message is targeted to a subset of the terminals in the group , a key is built by hashing together the keys assigned to each excluded terminals , and the message is encrypted with this key : k = hash ( k a , kb , . . . , k z ). for example , if terminals t 0 and t 6 are excluded , keys k 7 and k 13 are hashed together to compute a key and the message is encrypted with it . since t 0 and t 6 do not know their respective keys , they can not compute the final key , while all the other terminals in the group can compute these keys and thus access the content of the message . the resulting encrypted message is essentially the same size as the original , only padding and the use of a session key slightly increase its size . in addition to the message itself , some signaling must be added so that receiving terminals know whether they are excluded or not and how to compute the keys . this is done using a bitmap where each bit corresponds to a terminal and indicates whether that terminal is included in the recipient or not . the bitmap may be compressed under certain conditions . some mechanism must be introduced to reach an addressable population of tens of millions while keeping the number of revoked terminals to a minimum ( and thus the bandwidth to an acceptable level ). the first goal is easily met by splitting the total population into a number of subsets of the adequate size and managing each subset as an independent population . the second goal is more difficult to meet without a dedicated mechanism for revoked population control . the dynamic group management mechanism described below proposes to solve this problem . the content is put up for sale in packages , typically by grouping a number of services in independent products . the unit of sale , and thus the unit of control , is the product . for each product , the population of terminals subscribed to this product is split in a number of groups , for which an independent broadcast encryption system is generated ( for instance using methods well - known in the art ). the number of groups is proportional with the actual population of subscribers for this product ( population divided by the group size ), not with the total population of terminals . upon subscribing to a product , a slot is allocated to the terminal in one of the groups associated to this product ( a new group is created if needed ). the unique set of keys corresponding to this slot is sent to the terminal using a message addressed to this particular terminal . an additional key is also provided , the group access key , which use is described below on a regular basis ( e . g . every day ), a positive addressing message is generated for each group of terminals of each product . this pa message contains all the keys required to access the content of the product over the next period of control ( e . g . the next week or month ). this pa message is encrypted using the broadcast encryption primitive for this group of terminals , and is further over - encrypted with the group access key . upon cancellation of a subscription by the user , the terminal is put in the list of revoked terminals for its group ( for this particular product ). in the next pa message , those terminals that are revoked may decrypt the first layer of encryption using the group access key , however , they are not capable of decrypting the underlying message , by virtue of the broadcast encryption scheme . as a consequence , these terminals cannot retrieve the content keys for the next period of control and are thus unable to access the content . furthermore , they cannot retrieve the next group access key which is covered by the broadcast encryption and are thus effectively definitively excluded from this group . as soon as the last group access key given to a revoked terminal is replaced by a new one , the slot of the revoked terminal may be assigned to a new subscribing terminal . t n indicates a terminal , the solid arrows indicate the ability of the targeted terminal to access the message in the middle layer of the diagram . this message is the pa message addressing a subset of the terminal population with the broadcast encryption scheme , containing the service keys k n and over encrypted with the group access key c n . the first benefit is that the number of the pa emm generated for any product is directly proportional to the number of subscribers to that product , not to the total population of subscribers . thus , if a product is purchased by a minority , the pa bandwidth required to maintain it is small . the second benefit is that the population of receivers targeted by any pa emm is extremely homogeneous : indeed , all receivers have purchased that product and only a small percentage of them have cancelled it . this means that the addressing bit field , which indicates which receivers in the pa group are revoked is essentially composed of bits set to ‘ 1 ’ and thus can be compressed . a simple and efficient compression algorithm will provide a compression ratio of 1 / 14 for a 0 % revocation rate , 1 / 6 for a 2 % revocation rate and still 1 / 3 for a 5 % revocation rate . the third benefit is that slots in the group are recycled : when a terminal is excluded from the group , its slot is reassigned to a new terminal , constantly keeping the number of revoked slots in the group to a minimum ( no more than 2 %- 3 % in the ideal case ). fourth benefit is that any broadcast encryption method can be used , such as previously known in the art , as well as new ones , hence improving even more the efficiency ( bandwidth , terminal key storage and / or encryption / decryption complexity ) of the entire system . all these put together allow for a very efficient use of the broadcast bandwidth . dan boneh , craig gentry , brent waters : collusion resistant broadcast encryption with short ciphertexts and private keys . crypto 2005 dalit naor , moni naor , jeffery lotspiech : revocation and tracing schemes for stateless receivers . crypto 2001 oma drm v2 . 0 extensions for broadcast support , oma - 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