# Patent

## Title
Method for accessing data safely suitable for electronic tag

## Abstract

A method for accessing data safely, which is suitable for the electronic tag with low performance, is provided. The method comprises the following steps: when performing a data writing process, the first read-write device encrypts the message MSG and then writes the message in the electronic tag; when performing a data reading process, the second read-write device sends a data request packet to the electronic tag; the electronic tag sends a data response packet to the second read-write device according to the data request packet; the second read-write device sends a key request packet to a trusted third party; the trusted third party verifies the validity of the identity of the second read-write device according to the key request packet, and sends a key response packet to the second read-write device upon the verification is passed; the second read-write device obtains the plain text of the electronic tag message MSG according to the key response packet. This invention can realize the safe access of the data of the electronic tag with low performance.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a 371 U.S. National Stage of International Application No. PCT/CN2009/072826, filed Jul. 20, 2009. This application claims the priority to Chinese Patent Application No. 200810150433.X, filed with the Chinese Patent Office on Jul. 23, 2008 and entitled “METHOD FOR ACCESSING DATA SAFELY SUITABLE FOR ELECTRONIC TAG”, which is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to a secured data access method for an electronic tag.
BACKGROUND OF THE INVENTION
The security issue of a wireless network, e.g., a wireless local area network, a wireless metropolitan area network, etc., is far more serious than a wired Ethernet. A Radio Frequency Identification (RFID) tag also suffers the security issue, and it is also necessary to address effectively the issue of privilege verification between a reader-writer and an electronic tag for safe communication in the RFID. If the electronic tag with a high performance is somewhat capable of calculation and processing, an existing secured access protocol of the wireless network may be used or referred to so as to address the issue of authentication and privilege verification between the electronic tag and the reader-writer. However, if the electronic tag is only capable of storing data but can not process any data by itself, it may be difficult in the traditional security protocol to ensure the security of the stored data.
In view of the use scenarios and application environments of electronic tags, the electronic tags can roughly be divided into three categories: (1) an advanced electronic tag, which is readable and writable and is somewhat provided with a memory space and a calculation capability; (2) an intermediate electronic tag, which is slightly inferior in performance but functionally similar to the advanced electronic tag; and (3) a low-grade electronic tag, which is only intended to record some data information to ensure that the information can be read or written by a reader-writer and is typically provided with neither data processing function nor calculation capability, e.g., a pre-charged billing card.
For the former two categories of electronic tags, the security of channels between the electronic tags and the readers-writers can be ensured in an authentication protocol. The third category of electronic tags is merely a medium for storing information, and is provided with neither calculation capability nor memory capability for calculation and without identity information, etc. Despite the low performance of the third category of electronic tags, there may be a high security requirement on the data information stored therein to ensure that the stored information can be read and decrypted by only an intended legal reader-writer; and alike information written by only the intended legal reader-writer can be accepted by another reader-writer. It is obviously infeasible to accommodate this security demand in the existing authentication solution.
In existing security solutions of wireless networks, e.g., IEEE802.11i for wireless local area networks, IEEE802.16e for wireless metropolitan area networks, the Chinese national standard WAPI for wireless local networks, there are fundamental performance requirements such as calculation and data processing, for respective parties involved in the protocols. The existing security solutions are more or less applicable to the former two categories of electronic tags but can not be applicable to the third category of electronic tags.
Therefore, there is a need of devising a new security solution to verify the identity and privilege of a reader-writer to ensure the security of data information for the third category of electronic tags.
SUMMARY OF THE INVENTION
The present invention provides a secured data access method for a low-performance electronic tag to solve the technical problem in the prior art of failing to secure information written to and read from the low-performance electronic tag.
A technical solution of the present invention lies in a secured data access method for a low-performance electronic tag. The method includes the following steps of:
in a process of writing data, encrypting and writing by a first reader-writer a message, MSG, into the electronic tag;
in a process of reading the data, transmitting by a second reader-writer a data request packet to the electronic tag, the data request packet is adapted to trigger the electronic tag to transmit a data response packet to the second reader-writer;
transmitting by the electronic tag to the second reader-writer the data response packet including an ID1 field, a TTP field, a PKeyID field, a CP field, a CMSG field and an MIC field, wherein the ID1 field is the identity of the first reader-writer writing the data, the TTP field is the identity of a trusted third party, the PKeyID field is a key identifier selected and written by the first reader-writer into the electronic tag, the CP field is a CP value equal to r·P calculated and written by the first reader-writer into the electronic tag with r being a secret random number selected by the first reader-writer and P being a generation element of G1 in a q-order cyclic group, the CMSG field is a cipher text of the MSG written by the first reader-writer into the electronic tag, and the MIC field is an integrity check value, MIC, equal to h(ID1∥S1∥PKeyID∥CMSG∥CP) calculated and written by the first reader-writer into the electronic tag with S1 being a private key of the first reader-writer and h(x) being a unidirectional hash function;
transmitting by the second reader-writer to the trusted third party a key request packet comprising an ID2 field, the TTP field, a Nonce field, the ID1 field, the PKeyID field, the CP field, the CMSG field, the MIC field and an MIC1 field, wherein the ID2 field is the identity of the second reader-writer reading the data, the Nonce field is a one-time random number selected by the second reader-writer, and the MIC1 field is a message integrity check value, MIC1, calculated by the second reader-writer as h(ID2|S2|TTP|Nonce|MIC) with S2 being a private key of the second reader-writer;
verifying by the trusted third party the identities of the second reader-writer and the first reader-writer for validity respectively in response to the key request packet, and after the verification is passed, transmitting to the second reader-writer a key response packet comprising the ID2 field, the TTP field, a CSkeyID field, a CP1 field, the Nonce field and an MIC2 field, wherein the CP1 field is a CP1 value equal to r1·P calculated and transmitted by the trusted third party to the second reader-writer with r1 being a secret random number selected by the trusted third party; and
deriving by the second reader-writer a plain text of the MSG of the electronic tag from the key response packet.
Preferably, before the process of writing the data, the method further includes: creating system parameters by the trusted third party;
wherein the system parameters comprise two q-order cyclic groups (G1, +) and (G2, ·), the generation element P of G1, a value e resulting from bilinear transform on G1 and G2, i.e., e:G1×G1→G2, a private key STTPεZ*q selected randomly by the trusted third party for itself and a corresponding public key QTTP=STTP·PεG1, an encryption key K to encrypt a message m in a symmetric encryption algorithm, and the unidirectional hash function h(x).
Preferably, encrypting and writing the MSG by the first reader-writer into the electronic tag includes:
selecting by the first reader-writer randomly the key identifier PKeyID as a public key;
selecting by the first reader-writer the secret random number r and calculating K=r·QTTP·PKeyID;
encrypting by the first reader-writer the MSG by CMSG=EK(MSG) into a cipher text CMSG, and calculating CP=r·P and the message integrity check value MIC=h(ID1∥S1∥TTP∥PKeyID∥CMSG∥CP); and
writing by the first reader-writer ID1, TTP, CP, PKeyID, CMSG and MIC together into the electronic tag.
Preferably, verifying by the trusted third party the identities of the second reader-writer and the first reader-writer for validity respectively in response to the key request packet includes:
verifying the identity of the second reader-writer for validity, and if the identity of the second reader-writer is invalid, terminating the protocol;
if the identity of the second reader-writer is valid, calculating the private key S2 of the second reader-writer, recalculating the MIC1, and comparing the recalculated MIC1 with the received MIC1;
if the recalculated MIC1 is unequal to the received MIC1, terminating the protocol;
if the recalculated MIC1 is equal to the received MIC1, verifying the identity of the first reader-writer for validity;
if the identity of the first reader-writer is invalid, terminating the protocol; and
if the identity of the first reader-writer is valid, calculating the private key S1 of the first reader-writer, recalculating the MIC and comparing the recalculated MIC with the received MIC, and if the recalculated MIC is equal to the received MIC, terminating the protocol; if the recalculated MIC is unequal to the received MIC, transmitting the key response packet to the second reader-writer.
Preferably, deriving by the second reader-writer the plain text of the MSG of the electronic tag from the key response packet comprises:
verifying by the second reader-writer whether Nonce is the random number selected by the second reader-writer upon reception of the key response packet, and if Nonce is not the random number selected by the second reader-writer, stopping the protocol due to an error;
if Nonce is the random number selected by the second reader-writer, recalculating K1=CP1·S2, deriving an encryption key EK and an integrity check key IK from K1, recalculating the MIC2 from IK, and comparing the recalculated MIC2 with the received MIC2 to verify the key response packet for validity, and if the key response packet is valid, decrypting CSKeyID using EK into a plain text of SKeyID, calculating K=CP·SKeyID from the contents of the data response packet, and decrypting the cipher text CMSG using K as a key into the plain text of MSG.
The present invention has the following advantages:
1. An identity-based public mechanism is offered without maintenance of any PKI as done for a traditional public key.
2. No digital certificate will be transported during authentication to thereby save a communication overhead.
3. An identity verification function is added to obviate the difficulty of verifying an identity for validity in the identity-based public mechanism.
4. Identity authentication and privilege verification is performed between respective reader-writers so that data stored in only a legal reader-writer can be accepted by another reader-writer, and alike only the legal reader-writer can be authorized to read and decrypt data information stored in the other reader-writer.
5. A bilinear pair on an elliptical curve is adopted to thereby shorten the length of secured data without degrading the security thereof, thus greatly improving the performance of calculation and communication.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a schematic flow chart of an embodiment of a secured data access method for an electronic tag according to the present invention.



DETAILED DESCRIPTION OF THE INVENTION
The method according to the present invention is performed by a Trusted Third Party (TTP), e.g., an authentication server or another authentication-enabled device, which is responsible for physical verification of the identity of a user entity, generation of system parameters, creation of user parameters, etc.
Referring to FIG. 1, an embodiment of the method according to the present invention is implemented as follows:
1) Firstly, the trusted third party creates system parameters including two q-order cyclic groups (G1, +) and (G2, ·); a generation element P of G1, a value e resulting from bilinear transform on G1 and G2, i.e., e:G1×G1→G2, a private key STTPεZ*q selected randomly by the trusted third party for itself and a corresponding public key QTTP=STTP·PεG1, EK(m) which represents encryption of a message m in a symmetric encryption algorithm with an encryption key K, and h(x) which represents a unidirectional hash function.
Identifiers ID1 and ID2 of a first reader-writer and a second reader-writer are their public keys respectively, and their private keys are S1=STTP·ID1 and S2=STTP·ID2 respectively. No pair of public and private keys will be calculated for an electronic tag ID0.
This step is performed only in an initial application to create the system parameters and will not be performed in subsequent repeated applications after the creation.
2) A process of writing data: either of the reader-writer, e.g., the first reader-writer, encrypts a message MSG and then writes the encrypted MSG into the electronic tag, where the MSG is encrypted as follows:
Firstly, the first reader-writer selects randomly a key identifier PKeyID as a public key, then selects a secret random number r and calculates K=r·QTTP·PKeyID, next encrypts the MSG using K into a cipher text, that is, calculates CMSG=EK(MSG), and also calculates CP=r·P and a message integrity check value MIC=h(ID1∥S1∥TTP∥PKeyID∥CMSG∥CP), and finally writes ID1, TTP, CP, PKeyID, CMSG and MIC together into the electronic tag.
Thus, the first reader-writer ID1 securely writes the MSG into the electronic tag.
3) A process of reading data, which includes two sub-processes of retrieving data and of retrieving a key. The sub-process of retrieving data involves two packets of a data request and a data response, and the sub-process of retrieving a key involves two packets of a key request and a key response. Detailed description is given below:
3.1) The second reader-writer transmits the data request packet to the electronic tag, where the data request packet is null.
3.2) The electronic tag transmits the data response packet to the second reader-writer in response to the data request packet;
Where the data response packet includes the following contents:




















ID1
TTP
PKeyID
CP
CMSG
MIC










Where:
The ID1 field: the identity of the first reader-writer ID1 writing the data information;
The TTP field: the identity of the trusted third party;
The PKeyID field: the PKeyID value selected and written by the first reader-writer ID1 into the electronic tag;
The CP field: the CP value equal to r·P calculated and written by the first reader-writer ID1 into the electronic tag;
The CMSG field: the cipher text of the MSG written by the first reader-writer ID1 into the electronic tag; and
The MIC field: the MIC value equal to h(ID1∥S1∥PKeyID∥CMSG∥CP) calculated and written by the first reader-writer ID1 into the electronic tag.
2.3) The second reader-writer transmits the key request packet to the trusted third party;
Where the data request packet includes the following contents:




















ID2
TTP
Nonce
ID1
PKeyID
CP
CMSG
MIC
MIC1









Where:
The ID2 field: the identity of the second reader-writer ID2 reading the data information;
The TTP field: the identity of the trusted third party;
The Nonce field: a one-time random number selected by the second reader-writer ID2;
The ID1 field: the identity of the first reader-writer ID1 writing the data information;
The PKeyID field: the PKeyID value selected and written by the first reader-writer ID1 into the electronic tag;
The CP field: the CP value calculated and written by the first reader-writer ID1 into the electronic tag;
The CMSG field: the cipher text of the MSG written by the first reader-writer ID1 into the electronic tag;
The MIC field: the MIC value calculated and written by the first reader-writer ID1 into the electronic tag; and
The MIC1 field: a message integrity check value calculated by the second reader-writer ID2 as h(ID2∥S2∥TTP∥Nonce∥MIC);
The trusted third party can verify the identity of the second reader-writer for validity upon reception of the key request packet from the second reader-writer. If the identity is invalid, the protocol is terminated; otherwise, the TTP calculates a private key S2 of the second reader-writer, recalculates the MIC1 and compares the recalculated MIC1 value with the received MIC1 value. If the recalculated MIC1 value and the received MIC1 value are unequal, the protocol is terminated; otherwise, the TTP verifies the identity of the first reader-writer for validity. If the identity of the first reader-writer is invalid, the protocol is terminated; otherwise, the TTP calculates a private key S1 of the first reader-writer, recalculates the MIC value and compares the MIC value with the received MIC value. If the recalculated MIC value and the received MIC value are unequal, the protocol is terminated; otherwise, the TTP constructs the key response packet and transmits the key response packet to the second reader-writer.
2.4) The trusted third party transmits the key response packet to the second reader-writer in response to the key request packet.
Where the data response packet includes the following contents:




















ID2
TTP
CSKeyID
CP1
Nonce
MIC2










Where:
The ID2 field: the identity of the second reader-writer ID2 reading the data information;
The TTP field: the identity of the trusted third party;
The CSkeyID field: a cipher text of a key SKeyID as requested, which is calculated as follows: the trusted third party firstly calculates SKeyID=STTP·PKeyID, then selects a secret random number r1, calculates K1=r1·QTTP·ID2 and derives an encryption key EK and an integrity check key IK from K1, and then encrypts SKeyID using EK into the cipher text CSkeyID, that is, calculates CSKeyID=EEK(SKeyID);
The CP1 field: CP1=r1·P;
The Nonce field: the one-time random number selected by the second reader-writer; and
The MIC2 field: a message integrity check value calculated on all the fields preceding this field using the integrity check key IK derived from K1;
The second reader-writer verifies whether the Nonce in the key response packet is the random number selected by itself. If the Nonce in the key response packet is not the random number selected by itself, the protocol is stopped due to an error; otherwise, the second reader-writer recalculates K1=CP1·S2 and derives therefrom the encryption key EK and the integrity check key IK and can recalculate from IK the MIC2 value and compare the recalculated MIC2 value with received MIC2 to verify the packet for validity. If it is verified as being valid, the second reader-writer decrypts CSKeyID using EK into a plain text of SKeyID, calculates K=CP. SKeyID from the contents of the data response packet after deriving the SKeyID, and finally decrypts CMSG using K as a key into a plain text of MSG.
With the foregoing protocol, the function of secured writing into the electronic tag by the reader-writer can be performed in the step 2), and the function of secured reading from the electronic tag by the reader-writer can be performed in the step 3). Also the function of verifying the reader-writer for both validity of its identity and its privilege can be preformed in the verification function of the trusted third party.
The step 2) can be performed if the second reader-writer as necessary needs to write the processed data information into the electronic tag after deriving the plain text information and correspondingly processing the plain text information on the data information of the electronic tag.
REFERENCE LIST


    ID0: identity information of the electronic tag
    ID1: identity information of the first reader-writer;
    ID2: identity information of the second reader-writer;
    Nonce: a one-time random number;
    PKeyID: a key identifier used as a public key;
    SKeyID: a private key corresponding to PKeyID; and
    MSG: a message written into the electronic tag.

## Claims

1. A secured data access method for a low-performance electronic tag, comprising the steps of:
in a process of writing data, encrypting and writing by a first reader-writer a message, MSG, into the electronic tag;
in a process of reading the data, transmitting by a second reader-writer a data request packet to the electronic tag, the data request packet is adapted to trigger the electronic tag to transmit a data response packet to the second reader-writer;
transmitting by the electronic tag to the second reader-writer the data response packet comprising an ID1 field, a TTP field, a PKeyID field, a CP field, a CMSG field and an MIC field, wherein the ID1 field is the identity of the first reader-writer writing the data, the TTP field is the identity of a trusted third party, the PKeyID field is a key identifier selected and written by the first reader-writer into the electronic tag, the CP field is a CP value equal to r·P calculated and written by the first reader-writer into the electronic tag with r being a secret random number selected by the first reader-writer and P being a generation element of G1 in a q-order cyclic group, the CMSG field is a cipher text of the MSG written by the first reader-writer into the electronic tag, and the MIC field is an integrity check value, MIC, equal to h(ID1∥S1∥PKeyID∥CMSG∥CP) calculated and written by the first reader-writer into the electronic tag with S1 being a private key of the first reader-writer and h(x) being a unidirectional hash function;
transmitting by the second reader-writer to the trusted third party a key request packet comprising an ID2 field, the TTP field, a Nonce field, the ID1 field, the PKeyID field, the CP field, the CMSG field, the MIC field and an MIC1 field, wherein the ID2 field is the identity of the second reader-writer reading the data, the Nonce field is a one-time random number selected by the second reader-writer, and the MIC1 field is a message integrity check value, MIC1, calculated by the second reader-writer as h(ID2∥S2∥TTP∥Nonce∥MIC) with S2 being a private key of the second reader-writer;
verifying by the trusted third party the identities of the second reader-writer and the first reader-writer for validity respectively in response to the key request packet, and after the verification is passed, transmitting to the second reader-writer a key response packet comprising the ID2 field, the TTP field, a CSkeyID field, a CP1 field, the Nonce field and an MIC2 field, wherein the CP1 field is a CP1 value equal to r1·P calculated and transmitted by the trusted third party to the second reader-writer with r1 being a secret random number selected by the trusted third party; and
deriving by the second reader-writer a plain text of the MSG of the electronic tag from the key response packet.

2. The method according to claim 1, wherein before the process of writing the data, the method further comprises: creating system parameters by the trusted third party;
wherein the system parameters comprise two q-order cyclic groups (G1, +) and (G2, ·), the generation element P of G1, a value e resulting from bilinear transform on G1 and G2, i.e., e:G1×G1→G2, a private key STTPεZ*q selected randomly by the trusted third party for itself and a corresponding public key QTPP=STTP·PεG1, encryption key K to encrypt a message m in a symmetric encryption algorithm, and the unidirectional hash function h(x).

3. The method according to claim 1, wherein encrypting and writing the MSG by the first reader-writer into the electronic tag comprises:
selecting by the first reader-writer randomly the key identifier PKeyID as a public key;
selecting by the first reader-writer the secret random number r and calculating K=r·QTTP·PKeyID;
encrypting by the first reader-writer the MSG by CMSG=EK(MSG) into a cipher text CMSG, and calculating CP=r·P and the message integrity check value MIC=h(ID1∥S1∥TTP∥PKeyID∥CMSG∥CP); and
writing by the first reader-writer ID1, TTP, CP, PKeyID, CMSG and MIC together into the electronic tag.

4. The method according to claim 1, wherein verifying by the trusted third party the identities of the second reader-writer and the first reader-writer for validity respectively in response to the key request packet comprises:
verifying the identity of the second reader-writer for validity, and if the identity of the second reader-writer is invalid, terminating the protocol;
if the identity of the second reader-writer is valid, calculating the private key S2 of the second reader-writer, recalculating the MIC1, and comparing the recalculated MIC1 with the received MIC1;
if the recalculated MIC1 is unequal to the received MIC1, terminating the protocol;
if the recalculated MIC1 is equal to the received MIC1, verifying the identity of the first reader-writer for validity;
if the identity of the first reader-writer is invalid, terminating the protocol; and
if the identity of the first reader-writer is valid, calculating the private key S1 of the first reader-writer, recalculating the MIC and comparing the recalculated MIC with the received MIC, and if the recalculated MIC is equal to the received MIC, terminating the protocol; if the recalculated MIC is unequal to the received MIC, transmitting the key response packet to the second reader-writer.

5. The method according to claim 1, wherein deriving by the second reader-writer the plain text of the MSG of the electronic tag from the key response packet comprises:
verifying by the second reader-writer whether Nonce is the random number selected by the second reader-writer upon reception of the key response packet, and if Nonce is not the random number selected by the second reader-writer, stopping the protocol due to an error;
if Nonce is the random number selected by the second reader-writer, recalculating K1=CP1·S2, deriving an encryption key EK and an integrity check key IK from K1, recalculating the MIC2 from IK, and comparing the recalculated MIC2 with the received MIC2 to verify the key response packet for validity, and if the key response packet is valid, decrypting CSKeyID using EK into a plain text of SKeyID, calculating K=CP·SKeyID from the contents of the data response packet, and decrypting the cipher text CMSG using K as a key into the plain text of MSG.

6. The method according to claim 2, wherein encrypting and writing the MSG by the first reader-writer into the electronic tag comprises:
selecting by the first reader-writer randomly the key identifier PKeyID as a public key;
selecting by the first reader-writer the secret random number r and calculating K=r·QTTP·PKeyID;
encrypting by the first reader-writer the MSG by CMSG=EK(MSG) into a cipher text CMSG, and calculating CP=r·P and the message integrity check value MIC=h(ID1∥S1∥TTP∥PKeyID∥CMSG∥CP); and
writing by the first reader-writer ID1, TTP, CP, PKeyID, CMSG and MIC together into the electronic tag.