A Privacy-Preserving Trusted Aggregation Method for Electric Vehicle Charging Load Data

Abstract

A privacy-preserving, trusted aggregation method for electric vehicle charging load data establishes a multi-level aggregation architecture for electric vehicle charging data. This architecture aggregates charging load data within the distribution network through multi-level, multi-blockchain collaboration. The method involves: uploading only encrypted charging plan data at the charging pile layer using local charging plan sharing; performing encrypted aggregation of charging data based on a practical Byzantine fault-tolerant consensus at the encrypted aggregation layer; and decrypting the aggregated charging load data at the decryption and submission layer based on an improved practical Byzantine fault-tolerant consensus. This reliably decrypts the charging load data for each region within the distribution network, ensuring both privacy protection and trustworthiness in the aggregation process. This invention utilizes a two-layer blockchain technology, effectively preventing malicious tampering and privacy leaks by representative nodes during the calculation of regional net charging load, while simultaneously ensuring both privacy protection and the trustworthiness of the calculation results.

Description

Technical Field

[0001] This invention relates to a technology in the field of information security, specifically a method for the trusted aggregation of electric vehicle charging load data with privacy protection. Background Technology

[0002] In the future scenario of power distribution networks with massive electric vehicle (EV) connections, aggregating EV charging load data from various regions of the distribution network and calculating the net charging load of each region is a crucial foundation for formulating EV demand response strategies and guiding orderly charging. Traditional data aggregation methods will face two problems in this future scenario: for power grid companies, it will increase their data storage and computational burden, while also leading to insufficient data reliability; for vehicle owners, submitting charging load data may result in the leakage of their personal privacy information to power grid companies or other third parties. Furthermore, existing privacy-protecting load data aggregation solutions can only prevent theft and tampering by malicious third parties, but vehicle owners still have to disclose their personal privacy information to power grid companies, which will continue to cause concerns about privacy leaks. Summary of the Invention

[0003] This invention addresses the limitations of existing technologies, which are restricted by encryption methods and can only prevent data theft by external third parties but cannot prevent malicious blockchain nodes from actively leaking privacy. It proposes a privacy-preserving trusted aggregation method for electric vehicle charging load data, using a two-layer blockchain as a technical tool. This method can effectively prevent tampering and privacy leakage by malicious representative nodes during the calculation of regional net charging load, while taking into account both the privacy protection of the regional net charging load calculation process and the reliability of the calculation results.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a privacy-preserving, trusted aggregation method for electric vehicle charging load data. It establishes a multi-level aggregation architecture for electric vehicle charging data, aggregating charging load data within the distribution network through multi-level, multi-blockchain collaboration. This is achieved by: uploading only encrypted charging plan data at the charging pile layer using a local charging plan sharing method; performing encrypted aggregation of charging data based on Practical Byzantine Fault Tolerance (PBFT) consensus at the encrypted aggregation layer; and decrypting the aggregated charging load data based on an improved PBFT consensus at the decryption and submission layer. This trusted aggregation of electric vehicle charging data ensures both privacy protection and reliability during the aggregation process.

[0006] The aforementioned multi-level structure refers to: the charging pile layer, the encryption aggregation layer, and the decryption submission layer. Specifically, the charging pile layer includes all charging pile nodes in each area of ​​the distribution network (such as residential communities, industrial and commercial parks, etc.); the encryption aggregation layer contains several encryption aggregation groups, each group consisting of an encryption master representative and several encryption slave representatives; and the decryption submission layer contains a decryption master representative and several decryption slave representatives.

[0007] The encrypted master representative, encrypted slave representative, decrypted master representative, and decrypted slave representative are all charging pile nodes in the charging pile layer.

[0008] In the aforementioned encrypted aggregation layer, each encrypted aggregation group maintains its own encrypted aggregation blockchain, collects encrypted charging plans for charging stations, and submits the aggregated results of the encrypted charging plans to the decryption submission layer.

[0009] In the aforementioned decryption submission layer, the primary decryption representative and the secondary decryption representative jointly maintain the decryption submission blockchain, collect the aggregated results of the encrypted charging plan submitted by the encryption aggregation layer, decrypt and calculate the total charging load of each region, and submit it to the power grid company.

[0010] Participants in the charging data aggregation scenario are limited to charging pile nodes and power grid companies within the distribution network area. Under this multi-level aggregation architecture, each charging pile node submits only an encrypted local charging plan (i.e., a predicted charging load curve), ensuring its privacy is not compromised. The charging data aggregation result is jointly decrypted, calculated, and verified by representative charging pile nodes. Even if some representative nodes tamper with the result, other representative nodes can still calculate the correct aggregation result, ensuring the reliability of the aggregation result.

[0011] When the distribution network is divided into K regions, each encrypted aggregation group contains N encrypted representatives (including encrypted master representatives and encrypted slave representatives). Furthermore, there exists no more than f F Each representative node is a malicious representative, and there are f nodes in the encrypted aggregation group k. k Malicious representatives may collude to alter the accuracy of data aggregation or infringe upon the rights of charging piles by stealing privacy information.

[0012] The aforementioned local charging plan sharing refers to: charging stations formulating local charging plans and generating K encrypted charging plans based on the Shamir secret sharing mechanism, which are then submitted to each encrypted master representative, specifically including:

[0013] 1) A set of polynomials is randomly constructed for each charging pile: Where: p it (x) is the polynomial constructed for charging pile i, L it Let T be the charging power of charging pile i during time period t. min and T max For the start and end periods during which a charging plan must be submitted, aitl Let q be the coefficient of the polynomial of degree l, which is randomly generated for charging pile i during time period t, and q be the highest power of the polynomial.

[0014] 2) The charging station calculates the encrypted charging plan, adds a digital signature, and submits it to each encryption master representative: Where: L i Let be the charging power vector of charging pile i during the start and end time periods.

[0015] {L i} k For charging pile i, the encrypted master representative of encrypted aggregation group k Submitted encrypted charging power vector.

[0016] The aforementioned PBFT consensus-based encrypted aggregation of charging data refers to: when the encrypted aggregation group k's encrypted master representative Received encrypted charging plans from all charging stations {L i} k At that time, the cryptographic master representative will reach an agreement with the cryptographic slave representatives of the group on the aggregation results of the encrypted charging plans for each region based on the PBFT consensus, ensuring the trustworthiness of the aggregation results, specifically including:

[0017] a) Encryption Master Representative The received encrypted charging plans for all charging stations will be forwarded to the encrypted representatives of the encrypted aggregation group.

[0018] b) Encryption representatives verify the legitimacy of the encrypted charging plans. The encrypted charging plans are forwarded to other encrypted representatives only if each encrypted charging plan contains the correct digital signature of the charging station.

[0019] c) When the encrypted representative receives the complete encrypted charging plan for the charging pile from the encrypted master representative, and from no less than 2f k When one encrypted representative sends a commitment message to another encrypted representative when the same encrypted charging plan is being implemented, the encrypted representative will send the commitment message to the other encrypted representatives.

[0020] d) When the encrypted representative receives not less than 2f k When a cryptographic representative sends a commitment message, it is assumed that the cryptographic representative has reached an agreement on the encrypted charging plan for each charging station, based on... Calculate the aggregated results of the encrypted charging plan for each region and submit them to the decryption master representative, where: {L k R} n For the cryptographic representative D in cryptographic aggregation group k kn The calculated aggregated results of the encrypted charging plan for region k, Ω k R Let k be the set of charging stations in region k.

[0021] The aforementioned decryption of aggregated charging load data based on improved practical Byzantine fault-tolerant consensus refers to: all representatives jointly decrypting the aggregated charging load of each region based on the ciphertext information submitted by the encryption aggregation layer, specifically including:

[0022] i) The decryption master representative collects the aggregation results of the regional ciphertext charging plans submitted by the encryption aggregation layer. For encryption aggregation group k, the decryption master representative collects the aggregation results of the regional ciphertext charging plans submitted by the encryption aggregation layer. k +1 encryption represents the aggregated result of the same ciphertext sent, that is At that time, the ciphertext aggregation result is considered to have reached a consensus within the encryption aggregation group and is therefore a trustworthy aggregation result, where: {L k R} k R The consensus ciphertext aggregation result for the charging load in region k, Ω k D Let k be the set of encrypted representatives of the encrypted aggregation group;

[0023] ii) If the decryption master representative receives no less than (2K+q+1) / 3 consensus ciphertext aggregation results, then forward all consensus ciphertext aggregation results to all decryption slave representatives; otherwise, the encryption master representatives of the encryption aggregation group that has not reached a consensus are regarded as malicious representatives, and a list of malicious encryption master representatives is sent to all charging pile nodes, proceeding to step vii).

[0024] iii) Decryption is performed by representatives to verify the legitimacy of the ciphertext aggregation result. The consensus ciphertext aggregation result is forwarded to other decryption representatives only if and only if at least (2K+q+1) / 3 copies of the ciphertext aggregation result have been confirmed to have reached a consensus;

[0025] iv) When a decryption representative receives (2K+q+1) / 3 consensus ciphertext aggregation results from the primary decryption representative and the same ciphertext aggregation results from no less than 2f secondary decryption representatives, it sends a commitment message to the other decryption representatives.

[0026] v) When a decryption representative receives commitment messages from no fewer than 2f decryption representatives, it is considered that the decryption representatives have reached a consensus on the aggregation results of no fewer than (2K+q+1) / 3 consensus ciphertexts. Based on the aggregated charging load of each region, the decryption is then sent to the power grid company, specifically as follows:

[0027] Where: Ω l G For any q+1 encrypted aggregation groups, Ω G For the set of cryptographic aggregation groups that have submitted consensus ciphertext aggregation results, s l,n G For set Ω l GThe nth cryptographic aggregation group in the group, For the set Ω l G The obtained decryption result of aggregated charging load data, L k R The charging load is aggregated for the finally confirmed region k, and m is an auxiliary parameter; the decryption representative will enumerate the decryption results calculated from the aggregation results of any q+1 consensus ciphertexts, and... The same decryption result is regarded as the aggregated charging load of region k.

[0028] vi) If the power grid company receives no less than f F If the same aggregated charging load result is obtained from +1 decryption representatives, the result is considered reliable. Otherwise, a message to change the decryption master representative is sent to all charging pile nodes, and the next representative in sequence becomes the decryption master representative, and proceeds to step viii);

[0029] vii) Upon receiving the list of malicious encryption master representatives, the encryption aggregation group on the list will sequentially designate the next encryption representative as the new encryption master representative; all charging pile nodes will resend the encrypted charging power vector to the new encryption master representative.

[0030] Restart the process of encrypting and aggregating charging data;

[0031] viii) After receiving the message to change the decryption master representative, each encryption representative resends the regional ciphertext charging plan aggregation results to the new decryption master representative and returns to step i). Attached Figure Description

[0032] Figure 1 A schematic diagram of a multi-level aggregation architecture for electric vehicle charging data;

[0033] Figure 2 A schematic diagram of a charging data encryption and aggregation algorithm based on PBFT consensus;

[0034] Figure 3 This is a schematic diagram of an aggregated charging load data decryption algorithm based on improved PBFT consensus;

[0035] Figure 4 Flowchart of a privacy-preserving method for trusted aggregation of electric vehicle charging load data;

[0036] Figure 5 This is a schematic diagram of the aggregated charging load enumeration in region 4 of the embodiment.

[0037] Figure 6 This is a schematic diagram of the actual charging plan and encrypted charging plan of a certain charging pile in Area 3 of the embodiment. Detailed Implementation

[0038] like Figure 1 As shown in the figure, this embodiment relates to a privacy-preserving trusted aggregation method for electric vehicle charging load data. A multi-level aggregation architecture for electric vehicle charging data is established. After aggregating the charging load data within the distribution network in a multi-level, multi-blockchain collaborative manner, encrypted charging plan data is uploaded only at the charging pile layer using a local charging plan sharing method. At the encrypted aggregation layer, charging data encryption aggregation based on a practical Byzantine fault-tolerant consensus is performed. At the decryption and submission layer, the aggregated charging load data based on an improved practical Byzantine fault-tolerant consensus is decrypted in a trusted manner to decrypt the charging load data of each area within the distribution network. This achieves trusted aggregation of electric vehicle charging data, ensuring privacy protection and trustworthiness during the aggregation process.

[0039] To verify the effectiveness of this invention, this embodiment tests the above method in a computational system with 10 regions and 78 charging piles in each region, as described in the literature PING J, YAN Z, CHEN S, et al., "Coordinating EV charging via blockchain" ([J]. Journal of Modern Power Systems and Clean Energy, 2020, 8(3):573–581.). The hardware environment is a personal computer equipped with an Intel i7-10700 CPU and 16GB of memory. The functional code of each smart contract was developed based on Matlab, simulating the smart contract operation process of each node and the communication process between each node, and comparing the aggregation efficiency of each method.

[0040] In this embodiment, it is assumed that each encrypted aggregation group contains 7 encrypted representatives, and the threshold value is q = 3. The example assumes that there are malicious representatives in encrypted aggregation groups 3 and 4, and the proportion of malicious representatives exceeds 1 / 3 in both groups. It is also assumed that the communication delay between any two nodes, including charging pile nodes, representative nodes, data aggregation centers, and power grid companies, follows an exponential distribution with a mean of 0.02 seconds.

[0041] In this embodiment, malicious representatives in encryption aggregation groups 3 and 4 tamper with the encrypted charging plan aggregation results they calculate. At the decryption submission layer, the main decryption representative receives the tampered results from encryption aggregation groups 3 and 4, and receives the correct results from other encryption aggregation groups.

[0042] like Figure 5 The image shows the enumeration of aggregated charging loads in region 4 and the actual aggregated load situation under the aggregated charging load data decryption algorithm in this embodiment. Figure 5 It can be seen that in the aggregated charging load data decryption algorithm, each decryption representative can be enumerated to obtain The system ensures that the correct decryption result is obtained only once, as all other incorrect decryption results are only enumerated once. Therefore, non-malicious decryption representatives will receive the correct result. Since the number of malicious representatives does not exceed one-third of all decryption representatives, the power grid company can receive a sufficient number of correct aggregated charging load results, thus guaranteeing the correctness of the aggregation result. In contrast, if an aggregation method based on the PBFT consensus blockchain is used, the accuracy of the charging data aggregation result in Region 4 cannot be guaranteed because malicious representatives account for more than one-third of the regional aggregation group. Furthermore, if a direct aggregation method is used, it is impossible to prevent tampering by the data aggregation center in Region 4.

[0043] like Figure 6 The image shows the actual charging plan for a charging station in area 3 and the encrypted charging plan received by the representatives of aggregation groups 3 and 4. Figure 6 It is evident that malicious representatives in aggregation groups 3 and 4 can only receive encrypted charging plans that significantly differ from the actual charging plans of the charging piles. Furthermore, because the malicious representatives cannot obtain a sufficient number of encrypted charging plans, they are also unable to decrypt and obtain the charging pile's charging plan information. In contrast, if an aggregation method based on the PBFT consensus blockchain is used, any malicious representative in the regional aggregation group of region 3 can obtain the actual charging plan of the charging pile. If a direct aggregation method is used, the data aggregation center of region 3 can directly obtain the actual charging plan information of the charging pile, leading to potential privacy exposure risks.

[0044]

[0045] In summary, when the proportion of malicious representatives in at least (2K+q+1) / 3 encrypted aggregation groups is no more than 1 / 3 and the proportion of malicious representatives in decryption groups is no more than 1 / 3, malicious representatives cannot tamper with the data aggregation results of electric vehicle charging loads; when there are no malicious representatives in at least Kq encrypted aggregation groups, malicious representatives cannot steal charging pile privacy information by sharing information.

[0046] Compared with existing technologies, this method balances privacy protection in the calculation process of regional net charging load with the reliability of the calculation results. On the one hand, vehicle owner privacy information cannot be obtained by any third party, including power grid companies, data aggregation centers, and representative nodes. Existing direct aggregation technologies cannot prevent privacy theft by data aggregation centers, and PBFT consensus-based aggregation technologies cannot prevent privacy theft by representative nodes. On the other hand, power grid companies can ensure the accuracy of regional net charging load results without having to bear a large amount of data storage and calculation tasks.

[0047] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. A privacy-preserving method for trusted aggregation of electric vehicle charging load data, characterized in that, A multi-level aggregation architecture for electric vehicle charging data is established to aggregate charging load data within the distribution network in a multi-level, multi-blockchain collaborative manner. By uploading only encrypted charging plan data at the charging pile layer through local charging plan sharing, performing encrypted aggregation of charging data based on practical Byzantine fault-tolerant consensus at the encrypted aggregation layer, and decrypting the aggregated charging load data based on improved practical Byzantine fault-tolerant consensus at the decryption and submission layer, the charging load data of each area within the distribution network is decrypted in a trustworthy manner, thereby achieving trusted aggregation of electric vehicle charging data and ensuring the privacy protection and trustworthiness of the aggregation process. The aforementioned multi-level structure refers to: the charging pile layer, the encryption aggregation layer, and the decryption submission layer. Specifically: the charging pile layer includes all charging pile nodes in each area of ​​the distribution network; the encryption aggregation layer contains several encryption aggregation groups, each group consisting of an encryption master representative and several encryption slave representatives; and the decryption submission layer contains a decryption master representative and several decryption slave representatives. The aforementioned encrypted aggregation of charging data based on practical Byzantine fault-tolerant consensus refers to: when the encryption master representative DLk of the encryption aggregation group k receives the encrypted charging plan {L} from all charging piles... i } k At that time, the cryptographic master representative will reach an agreement with the cryptographic slave representatives of the group on the aggregation results of the ciphertext charging plan for each region based on the practical Byzantine fault-tolerant consensus, ensuring the reliability of the aggregation results, specifically including: a) The encrypted master representative DLk forwards all received encrypted charging plans for charging stations to the encrypted slave representatives of the encrypted aggregation group; b) Encryption representatives verify the legitimacy of the encrypted charging plan; and forward the encrypted charging plan to other encryption representatives if and only if each encrypted charging plan contains the correct digital signature of the charging station. c) When the encrypted representative receives the complete encrypted charging plan for the charging pile from the encrypted master representative, and from no less than 2f k When one encrypted representative sends a commitment message to another encrypted representative when the same encrypted charging plan is being implemented, the encrypted representative will send the commitment message to the other encrypted representatives. d) When the encrypted representative receives not less than 2f k When a cryptographic representative sends a commitment message, it is assumed that the cryptographic representative has reached an agreement on the encrypted charging plan for each charging station, based on... Calculate the aggregated results of the encrypted charging plan for each region and submit them to the decryption master representative, where: {L k R } n For the cryptographic representative D in cryptographic aggregation group k kn The calculated aggregated results of the encrypted charging plan for region k, Ω k R Let k be the set of charging stations in region k.

2. The method for trusted aggregation of electric vehicle charging load data with privacy protection according to claim 1, characterized in that, In the aforementioned encrypted aggregation layer, each encrypted aggregation group maintains its own encrypted aggregation blockchain, collects encrypted charging plans for charging stations, and submits the aggregated results of the encrypted charging plans to the decryption submission layer.

3. The method for trusted aggregation of electric vehicle charging load data with privacy protection according to claim 2, characterized in that, In the aforementioned decryption submission layer, the primary decryption representative and the secondary decryption representative jointly maintain the decryption submission blockchain, collect the aggregated results of the encrypted charging plan submitted by the encryption aggregation layer, decrypt and calculate the total charging load of each region, and submit it to the power grid company.

4. The method for trusted aggregation of electric vehicle charging load data with privacy protection according to claim 3, characterized in that, The participants in the charging data aggregation scenario are only charging pile nodes and power grid companies within the distribution network area. Under the multi-level aggregation architecture, each charging pile node only submits an encrypted local charging plan, i.e., a predicted charging load curve, to ensure that its privacy information is not leaked. The charging data aggregation result is jointly decrypted, calculated, and verified by representative nodes of the charging pile. When some representative nodes tamper with the result, other representative nodes can still calculate the correct aggregation result, ensuring the reliability of the aggregation result.

5. The privacy-preserving trusted aggregation method for electric vehicle charging load data according to claim 4, characterized in that, When the distribution network is divided into K areas, each encrypted aggregation group contains N encrypted representatives; In addition, there are no more than Each representative node is a malicious representative, and there are f nodes in the encrypted aggregation group k. k Malicious representatives may collude to alter the accuracy of data aggregation or infringe upon the rights of charging piles by stealing privacy information. .

6. The method for trusted aggregation of electric vehicle charging load data with privacy protection according to claim 1, characterized in that, The aforementioned local charging plan sharing refers to: charging stations formulating local charging plans and generating K encrypted charging plans based on the Shamir secret sharing mechanism, which are then submitted to each encrypted master representative, specifically including: 1) A set of polynomials is randomly constructed for each charging pile: , where: p it (x) is the polynomial constructed for charging pile i, L it Let T be the charging power of charging pile i during time period t. min and T max For the start and end periods during which a charging plan must be submitted, a itl Let q be the coefficient of the l-th power of the polynomial randomly generated for charging pile i during time period t, and q be the highest power of the polynomial. 2) The charging station calculates the encrypted charging plan, adds a digital signature, and submits it to each encryption master representative: , where: L i Let {L} be the charging power vector of charging pile i during the start and end time periods. i } k The encrypted charging power vector submitted by charging pile i to the encrypted master representative DLk of the encrypted aggregation group k.

7. The privacy-preserving trusted aggregation method for electric vehicle charging load data according to claim 1, characterized in that, The aforementioned decryption of aggregated charging load data based on improved practical Byzantine fault-tolerant consensus refers to: all representatives jointly decrypting the aggregated charging load of each region based on the ciphertext information submitted by the encryption aggregation layer, specifically including: i) The decryption master representative collects the aggregation results of the regional ciphertext charging plans submitted by the encryption aggregation layer; for the encryption aggregation group k, the decryption master representative receives no less than f k +1 encryption represents the aggregated result of the same ciphertext sent, that is At that time, the ciphertext aggregation result is considered to have reached a consensus within the encryption aggregation group and is therefore a trustworthy aggregation result, where: {L k R } k R The consensus ciphertext aggregation result for the charging load in region k, Ω k D Let k be the set of cryptographic representatives of the cryptographic aggregation group; ii) If the decryption master representative receives no less than (2K+q+1) / 3 consensus ciphertext aggregation results, then forward all consensus ciphertext aggregation results to all decryption slave representatives; otherwise, the encryption master representatives of the encryption aggregation group that has not reached a consensus are regarded as malicious representatives, and a list of malicious encryption master representatives is sent to all charging pile nodes, proceeding to step vii). iii) Decrypt the representative to verify the legitimacy of the ciphertext aggregation result; forward the consensus ciphertext aggregation result to other decryption representatives if and only if no less than (2K+q+1) / 3 ciphertext aggregation results have been confirmed to have reached a consensus; iv) When a decryption representative receives (2K+q+1) / 3 consensus ciphertext aggregation results from the primary decryption representative and the same ciphertext aggregation results from no less than 2f secondary decryption representatives, it sends a commitment message to the other decryption representatives. v) When a decryption representative receives commitment messages from no fewer than 2f decryption representatives, it is considered that the decryption representatives have reached a consensus on the aggregation results of no fewer than (2K+q+1) / 3 consensus ciphertexts. Based on the aggregated charging load of each region, the decryption is then sent to the power grid company, specifically as follows: , , , where: Ω l G For any q+1 encrypted aggregation groups, Ω G For the set of cryptographic aggregation groups that have submitted consensus ciphertext aggregation results, s l,n G For set Ω l G The nth cryptographic aggregation group in the group, k,l R For the set Ω l G The obtained decryption result of aggregated charging load data, L k R The final confirmed area k aggregates the charging load, and m is an auxiliary parameter; The decryption representative will enumerate the decryption results calculated from the aggregation results of any q+1 consensus ciphertexts, and regard Cq+1(K+2q+2) / 3 identical decryption results as the aggregated charging load of region k; vi) If the power grid company receives no less than f F If the same aggregated charging load result is obtained from +1 decryption representatives, the result is considered reliable; otherwise, a message to change the decryption master representative is sent to all charging pile nodes, and the next representative is appointed as the decryption master representative in sequence, and then proceed to step viii). vii) After receiving the list of malicious encryption master representatives, the encryption aggregation group on the list will take the next encryption representative as the new encryption master representative in sequence; all charging pile nodes will resend the ciphertext charging power vector to the new encryption master representative and restart the charging data encryption aggregation process. viii) After receiving the message to change the decryption master representative, each encryption representative resends the regional ciphertext charging plan aggregation results to the new decryption master representative and returns to step i).

Images