CN101980471A - Digital signature method, digital signature verification method, device and system - Google Patents

Abstract

The invention relates to a digital signature method, a digital signature verification method, a device and a system. The digital signature method includes: processing the message to be transmitted to obtain a first message digest value; selecting at least one message fragment from the message; processing the message fragments respectively to obtain at least one corresponding second A message digest value: using a digital signature algorithm to encrypt the first message digest value and the at least one second message digest value to obtain a final digital signature. The present invention selects at least one message segment from the message, and calculates the message digest value of the message and the message segment respectively, so that the attacker must find at least two matching messages with intrinsic relations at the same time, respectively pointing to the message digest of the message value and the message digest value of at least one message segment, thereby increasing the difficulty of downgrade attacks, thereby improving the security of digital signatures.

Description

Digital signature method, digital signature verification method, device and system

technical field

The invention relates to a network information encryption technology, in particular to a digital signature method capable of effectively preventing downgrade attacks, a digital signature verification method, device and system.

Background technique

With the rapid development of network technology, network security issues are becoming more and more important, and information encryption technology is the core technology of network security technology. Digital Signatures (Digital Signatures) are widely used among many information encryption technologies. The solution is an important part of e-commerce security.

In the prior art, there are many digital signature schemes, such as RSA (Ron Rivest, Adi Shamir&Leonard Adleman) signature, digital signature based on digital envelope, digital signature scheme based on message digest, etc. Among them, the digital signature scheme based on the message digest includes: first using the message digest algorithm to form a message digest value for the message to be encrypted, and then using the RSA encoding method to calculate the message digest value to form a digital signature. This method ensures the integrity of the message data through the message digest value, and ensures the confidentiality of the message data through the RSA encoding method. At the same time, because the length of the message digest value is much smaller than the length of the message data, the workload of RSA encoding is greatly reduced. .

Message Digest (MD) in the above-mentioned message digest-based digital signature scheme is an encoding method, which uses a one-way hash (Hash) function to form a message of any length to be encrypted into a string of fixed length The message digest value of different messages is different, but the message digest value of the same message must be the same. An important property of message digests is irreversibility (irreversibility), that is, given a message digest value, it should be extremely difficult to calculate its corresponding message. Another important property of message digests is that it is difficult to generate two messages M and M' with the same message digest value, which is called collision resistance. In fact, the strength of any message digest against collisions is only half of the message digest value, so a 128-bit message digest value is only 64 bits strong to avoid collisions, which means that it takes about 2 ⁶⁴ operations to generate a collision , which produces another message with the same message digest value, thus ensuring the integrity of the message.

The message digest values obtained by different algorithms have different lengths and different security strengths. For example: the length of the message digest value obtained by MD5 is 128 bits, and the length of the message digest value obtained by SHA-1 is 160 bits. Therefore, it takes 2 ⁶⁴ operations to find a collision of MD5, while for SHA-1 it is 2 ⁸⁰ operations.

The defect of the existing technology is that the information encrypted by the existing message digest-based digital signature scheme is vulnerable to downgrade attacks when transmitted in the network, and the attacker replaces the advanced message digest algorithm with a low security level message digest algorithm to achieve the purpose of the attack.

FIG. 11 is a schematic diagram of a downgrade attack on a message using an existing digital signature technology. As shown in Figure 11, in order to improve the security of data transmission, the sender upgrades the original message digest algorithm MD5 algorithm to SHA-1 algorithm. During data transmission, the sender first calculates the message A to be sent through the SHA-1 algorithm to obtain the message digest value h=SHA-1(A), then calculates the digital signature RSA(h), and then sends the message in the format: The data of text A+digital signature RSA(h) is sent to the receiver.

The attacker has obtained the verified message A1 and the digital signature RSA(h1) before the message digest algorithm is upgraded at the sender, where h1 is obtained by calculating the message A1 with the low-level message digest algorithm MD5 before the upgrade, that is, h1 =MD5(A1), the attacker performs a collision operation according to the message digest value, and obtains a forged message B, so that MD5(B)=MD5(A1).

The attacker intercepts the data sent by the sender, replaces the digital signature RSA(h) in the data with RSA(h1), and then replaces its message A with message B. The tampered data format is: message B+ Digitally sign RSA(h1), then the attacker sends this data to the receiver.

After receiving the falsified data, the receiving end first decrypts RSA(h1) to obtain the message digest value h1, and then calculates the message B with the MD5 algorithm carried in it to obtain MD5(B), because MD5(B)= MD5(A1)=h1, so the receiving end cannot judge that the message has been tampered with. Therefore, the information encrypted by the existing message digest-based digital signature method is vulnerable to downgrade attacks.

Contents of the invention

Aiming at the defects of the prior art, the present invention provides a digital signature method, a digital signature verification method, device and system, which can effectively prevent downgrade attacks.

The invention provides a digital signature method, comprising:

Process the message to be transmitted to obtain the first message digest value;

selecting at least one message segment from the message;

Process the message fragments respectively to obtain at least one corresponding second message digest value;

Encrypting the first message digest value and the at least one second message digest value using a digital signature algorithm to obtain a final digital signature.

The present invention also provides a verification method of a digital signature, comprising:

Select at least one message segment from the received message according to the method agreed with the message sender;

Decrypt the received final digital signature according to the decryption method and digital signature algorithm agreed with the message sender, and obtain a first message digest value and at least one second message digest value, the first message digest value and the received Corresponding to the received message, the at least one second message digest value corresponds to the at least one message fragment;

Process the received message to obtain a third message digest value;

Process the message fragments respectively to obtain at least one corresponding fourth message digest value;

comparing the first message digest value with the third message digest value;

comparing the at least one second message digest value with the at least one fourth message digest value;

When the first message digest value is equal to the third message digest value, and the at least one second message digest value is equal to the at least one fourth message digest value, it is determined that the received The packet is not subject to a downgrade attack;

Otherwise, it is determined that the received packet is subject to a downgrade attack.

The present invention also provides a digital signature device, comprising:

The first message digest processing module is configured to process the message to be transmitted to obtain the first message digest value;

A segment selection module, configured to select at least one message segment from the message;

A second message digest processing module, configured to process the message fragments to obtain at least one corresponding second message digest value;

A digital signature module, configured to use a digital signature algorithm to encrypt the first message digest value and the at least one second message digest value to obtain a final digital signature.

The present invention further provides a digital signature verification device, comprising:

A fragment acquisition module, configured to select at least one message fragment from the received message according to the method agreed with the message sender;

A decryption module, configured to decrypt the received final digital signature according to the decryption method and digital signature algorithm agreed with the message sender, and obtain a first message digest value and at least one second message digest value, the first message digest value The value corresponds to the received message, and the at least one second message digest value corresponds to the at least one message fragment;

A third message digest processing module, configured to process the received message to obtain a third message digest value;

A fourth message digest processing module, configured to process the message fragments respectively to obtain at least one corresponding fourth message digest value;

a first comparison module, configured to compare the first message digest value with the third message digest value;

a second comparison module, configured to compare the at least one second message digest value with the at least one fourth message digest value;

A judging module, configured to judge when the first message digest value is equal to the third message digest value, and the at least one second message digest value is all equal to the at least one fourth message digest value The received message is not subject to a downgrade attack; otherwise, it is determined that the received message is subject to a downgrade attack.

The present invention also provides a digital signature system, including: the above-mentioned digital signature device and the above-mentioned digital signature verification device.

As can be seen from the above technical solution, the present invention selects at least one message segment from the message, and calculates the message digest values of the message and the message segment respectively, so that the attacker must simultaneously find at least two matching messages with intrinsic connections, respectively Pointing to the message digest value of the message and the message digest value of at least one message fragment, thereby increasing the difficulty of downgrade attacks, thereby improving the security of the digital signature.

The technical solutions of the present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

Description of drawings

Fig. 1 is a flowchart of a digital signature method provided by an embodiment of the present invention;

FIG. 2 is a schematic diagram of a message format provided by an embodiment of the present invention;

Fig. 3 is a schematic diagram of the final digital signature format provided by an embodiment of the present invention;

FIG. 4 is a flowchart of a digital signature verification method provided by an embodiment of the present invention;

Fig. 5 is a schematic diagram of the final digital signature format provided by another embodiment of the present invention;

FIG. 6 is a block diagram of a digital signature device provided by an embodiment of the present invention;

FIG. 7 is a block diagram of a digital signature verification device provided by an embodiment of the present invention;

FIG. 8 is a block diagram of a digital signature system provided by an embodiment of the present invention;

Fig. 9 is a block diagram of a digital signature module provided by an embodiment of the present invention;

Fig. 10 is a block diagram of a digital signature module provided by another embodiment of the present invention;

FIG. 11 is a schematic diagram of a downgrade attack on a message using an existing digital signature technology.

Detailed ways

Fig. 1 is a flowchart of a digital signature method provided by an embodiment of the present invention.

As shown in Figure 1, digital signature methods include:

Step 101: Process the message to be transmitted to obtain the first message digest value;

Specifically, the message sender first needs to specify a message digest algorithm according to security requirements, and calculate the entire message according to the message algorithm to obtain the first message digest value.

Step 102: Select at least one message segment from the message;

Specifically, one message segment or multiple message segments may be selected from the message, and each message segment may be a certain field or several bits negotiated in advance between the message sender and the message receiver.

The selected message fragments should minimize conflicts, so that when an attacker performs a downgrade attack on the message, it is not easy to find the message text that matches the original message fragment and obtain the same message digest value.

The message fragment can select the field representing the time information of the message to form a digital signature. This digital signature containing the time information of the message is called a digital time stamp (digital time-stamp), which is provided by a special organization on the network. stamp service (digital time-stamp service, referred to as DTS) to form. It is very difficult for an attacker to find the message text that matches the digital timestamp to get the same message digest. It needs to satisfy the time format characteristics and range (maybe only one minute), so the probability of meeting is very low.

When multiple message segments are selected, each message segment can contain the same field or the same bit, that is, each message segment has overlapping parts, or some message segments in multiple message segments have mutual overlapping parts.

It should be noted that the message fragments are not limited to a certain field or a certain number of bits or the field representing the time information of the message described in this embodiment, and the overlapping manner of multiple message fragments is not limited to the above-mentioned manner. Any method of selecting message segments known to those skilled in the art can be used to implement the present invention.

Step 103: Process the message fragments respectively to obtain at least one corresponding second message digest value;

Specifically, when a message segment is selected, the sender calculates the selected message segment to obtain a second message digest value; when multiple message segments are selected, the sender calculates each message segment Calculations are performed respectively to obtain a plurality of corresponding second message digest values.

The algorithm used to calculate the message digest value of each message fragment can be the same or different, and when the same algorithm is used to calculate all message fragments, this algorithm can also be used when calculating the message digest value of the message algorithm is different.

In the existing technology, information about the algorithm used to calculate the digest of each message is usually inserted into the message before the message is sent. Therefore, when the receiver receives the message and the digital signature, it also receives the information contained in the digest value of each message. The corresponding message digest algorithm.

Step 104: Encrypt the first message digest value and the at least one second message digest value using a digital signature algorithm to obtain a final digital signature.

Specifically, the digital signature algorithm is an algorithm agreed upon by the sender and the receiver. When this algorithm is used to encrypt the first message digest value and at least one second message digest value, the first message digest value and at least one second message digest value can be encrypted. The two message digest values are respectively encrypted to obtain a corresponding first digital signature and at least one second digital signature, and then the first digital signature and at least one second digital signature are combined to obtain a final digital signature. Wherein, at least one second digital signature is connected behind the first digital signature, and the arrangement order of the at least one second digital signature is consistent with the order of the selected message segments.

When using this algorithm to encrypt the first message digest value and at least one second message digest value, the first message digest value and at least one second message digest value can also be combined first, and then the combined first message digest value value and at least one second message digest value are encrypted as a whole to obtain a final digital signature. Wherein, at least one second message digest value is connected behind the first message digest value, and the arrangement order of the at least one second message digest value is consistent with the order of the selected message segments.

It should be noted that the manner of encrypting the first message digest value and at least one second message digest value is not limited to the above two methods, and some message digest values may be encrypted as a whole and other message digest values may be encrypted separately, Then the obtained digital signatures are combined to obtain the final digital signature. In addition to the above methods, any method of encrypting the first message digest value and at least one second message digest value can be used to implement the present invention.

In the digital signature method of this embodiment, not only the message digest is calculated for the whole message to obtain the first message digest value, but also the message digest is calculated for one or some fragments in the message to obtain at least one The second message digest value makes it necessary for the attacker to simultaneously find at least two matching messages with intrinsic connections pointing to the first message digest value and at least one second message digest value respectively. Since the second message digest value corresponds to a part of a specific position in the message, the matching message found by the attacker must meet this point before the attack can be carried out, which increases the attack difficulty of the attacker and reduces the risk of the message being attacked , which effectively prevents downgrade attacks and meets the higher security requirements of messages and digital signatures.

In addition, in the digital signature method of this embodiment, there are multiple ways to encrypt the message digest value, and the sender and receiver can agree on a way according to needs, which further improves the security of the digital signature.

Fig. 2 is a schematic diagram of a message format provided by an embodiment of the present invention. Fig. 3 is a schematic diagram of the final digital signature format provided by an embodiment of the present invention. The final digital signature shown in Fig. 3 is obtained by selecting a message segment, which contains the first digital signature corresponding to the message and a second digital signature corresponding to the message segment. As shown in FIG. 2 , the message 201 and the final digital signature 202 constitute a complete data.

FIG. 4 is a flowchart of a digital signature verification method provided by an embodiment of the present invention. Referring to Figure 4, the verification methods for digital signatures include:

Step 401: Select at least one message segment from the received message according to the method agreed with the message sender.

Specifically, the receiver selects one or more message segments from the received message according to a method agreed upon by the sender and the receiver.

Step 402: Decrypt the received final digital signature according to the decryption method and digital signature algorithm agreed with the message sender, and obtain the first message digest value corresponding to the message and at least one second message corresponding to the message fragment summary value.

Specifically, the receiver decrypts the received final digital signature through the digital signature algorithm according to the decryption method agreed with the message sender. The digital signature algorithm is agreed by the sender and the receiver. The encryption methods in the embodiments correspond, that is, the sender adopts one encryption method, and the receiver uses this method to decrypt the received final digital signature. When the selected message segment in step 401 is one, obtain a first message digest value corresponding to the message and a second message digest value corresponding to the message segment after deciphering; when the selected message in step 401 When there are multiple fragments, one first message digest value corresponding to the message and multiple second message digest values corresponding to multiple message fragments are obtained after decryption.

Step 403: Process the message to obtain a third message digest value, and respectively process at least one message fragment to obtain at least one fourth message digest value.

Specifically, the message is calculated to obtain the third message digest value, and the selected message segment is calculated. When a message segment is selected, a fourth message digest value is obtained after calculation; when multiple messages are selected When fragments are used, multiple fourth message digest values are obtained after calculation.

When calculating the message digest value of a message or a certain message fragment, the message digest algorithm corresponding to the message or the message fragment obtained by deciphering in step 402 is taken out, and the message or the message fragment is processed with the algorithm calculate.

Step 404: Compare the first message digest value with the third message digest value.

Specifically, the first message digest value is compared with the third message digest value. When the comparison result is not equal, it means that the received message is a message that has been tampered with, and then go to step 407 to determine that the message has been attacked. When the comparison result is equal, go to step 405 .

Step 405: Compare the second message digest value with the fourth message digest value.

When a message fragment is selected, the second message digest value and the fourth message digest value are respectively one, and the second message digest value and the fourth message digest value are compared. When the comparison result is not equal, it means that the If the message is a tampered message, go to step 407 to determine that the message has been attacked. When the comparison results are equal, it means that the correct message has been received, and then go to step 406 to determine that the message has not been attacked.

When a plurality of message fragments are selected, the second message digest value and the fourth message digest value are multiple respectively, and according to the order of selecting message fragments, each fourth message digest value corresponding to the message fragments is sequentially compared with each A second message digest value is compared. When there is an inequality between the second message digest value and the fourth message digest value, it means that the received message is a tampered message, then stop the subsequent comparison, go to step 407, and determine that the message has been attacked. When the comparison results are all equal, it means that the correct message has been received, and then go to step 406 to determine that the message has not been attacked.

Through the above verification method, the receiver can correctly determine whether the received message is subjected to a downgrade attack, thereby ensuring the integrity and reliability of the received data.

It should be noted that there is no limitation on the order of execution between the above step 404 and step 405, that is, first compare the first message digest value and the third message digest value, or first compare the second message digest value and the fourth message digest value. Appropriate selection can be made according to actual needs.

Fig. 6 is a block diagram of a digital signature device provided by an embodiment of the present invention. Referring to FIG. 6 , the digital signature device 601 includes: a first message digest processing module 603 , a fragment selection module 602 , a second message digest processing module 604 , and a digital signature module 605 .

Wherein, the first message digest processing module 603 is configured to calculate the message to be transmitted to obtain the first message digest value. The segment selection module 602 is used for selecting at least one message segment from the message to be transmitted. The second message digest processing module 604 is configured to calculate at least one message segment to obtain at least one second message digest value. The digital signature module 605 is configured to use a digital signature algorithm to encrypt the first message digest value and at least one second message digest value to obtain a final digital signature. For the method of selecting message segments, refer to the descriptions in the above embodiments for details, and will not be repeated here.

FIG. 9 is a block diagram of a digital signature module provided by an embodiment of the present invention. As shown in FIG. 9 , the digital signature module 605 includes a first digital signature unit 6051 , a second digital signature unit 6052 and a first combination unit 6053 . Wherein, the first digital signature unit 6051 is used to encrypt the first message digest value using a digital signature algorithm to obtain the first digital signature; the second digital signature unit 6052 is used to use a digital signature algorithm to at least one second message digest value respectively Perform encryption to obtain at least one second digital signature; the first combining unit 6053 is configured to combine the first digital signature and at least one second digital signature to obtain a final digital signature. At least one second digital signature is connected behind the first digital signature, and the sequence of the at least one second digital signature is consistent with the sequence of the selected message segments.

When n message segments are selected, the format of the final digital signature is shown in FIG. 5 . Among them, the first digital signature represents the digital signature obtained after calculating and encrypting the received message as a whole; the second digital signature 1 represents the digital signature obtained after calculating and encrypting the selected first message segment; the second digital signature Signature 2 represents the digital signature obtained after calculation and encryption of the second selected message segment, and so on, and the second digital signature n represents the digital signature obtained after calculation and encryption of the selected nth message segment. The second digital signature 1, the second digital signature 2, . For example, if the order of the selected n message fragments is: the first message fragment, the second message fragment, ... the nth message fragment, the final digital signature is shown in FIG. 5 .

The message digest value is generated by calculating the selected message fragments separately, so that the attacker must find multiple matching messages to point to the corresponding message digest value at the same time, which further increases the difficulty of the downgrade attack, and the selected message fragments The larger the number, the lower the possibility of the message being attacked, thereby further improving the security of the digital signature method.

Fig. 10 is a block diagram of a digital signature module provided by another embodiment of the present invention. As shown in FIG. 10 , the digital signature module 605 includes a second combination unit 6054 and a third digital signature unit 6055 . Wherein, the second combination unit 6054 is used to combine the first message digest value with at least one second message digest value; the third digital signature unit 6055 is used to use a digital signature algorithm to combine the first message digest value and at least one second message digest value The two message digest values are encrypted as a whole to obtain the final digital signature. At least one second message digest value is connected behind the first message digest value, and the sequence of the at least one second message digest value is consistent with the sequence of the selected message segments.

Fig. 7 is a block diagram of a digital signature verification device provided by an embodiment of the present invention. Referring to Fig. 7, the verification device 701 of the digital signature includes: a segment acquisition module 702, a decryption module 703, a third message digest processing module 704, a fourth message digest processing module 705, a first comparison module 706, a second comparison module 707 and a judgment Module 708.

Among them, the fragment acquisition module 702 selects at least one message fragment from the received message according to the method agreed with the message sender; the decryption module 703 adopts a digital signature algorithm to decrypt the received final message according to the decryption method agreed with the message sender. The digital signature is decrypted to obtain the first message digest value and at least one second message digest value. The digital signature algorithm is an algorithm agreed with the message sender. The first message digest value corresponds to the received message. At least one The second message digest value corresponds to at least one packet fragment. The decryption method corresponds to the encryption method used by the sender.

The third message digest processing module 704 calculates the received message to obtain a third message digest value; the fourth message digest processing module 705 separately calculates at least one message fragment to obtain at least one fourth message digest value.

When the third message digest processing module 704 calculates the message digest value of a message, or when the fourth message digest processing module 705 calculates the message digest value of a certain message segment, it first decrypts the corresponding message obtained by decrypting the decryption module 703. The message digest algorithm of the text or the message fragment is taken out, and the algorithm is used to calculate the message or the message fragment.

The first comparison module 706 compares the first message digest value with the third message digest value; the second comparison module 707 compares at least one second message digest value with at least one fourth message digest value; the judging module 708 according to the first The comparison result of the comparison module 706 and the second comparison module 707 determines whether the received packet is subject to a downgrade attack.

In this embodiment, the judging module 708 is specifically configured to determine that the received The packets received are not subjected to downgrade attacks; otherwise, it is determined that the received packets are subjected to downgrade attacks.

Fig. 8 is a block diagram of a digital signature system provided by an embodiment of the present invention. Referring to FIG. 8 , a digital signature system 801 includes a digital signature device 802 and a digital signature verification device 803 . Wherein, the digital signature device 802 and the digital signature verification device 803 have been described in detail in the above embodiments, and will not be repeated here.

It should be noted that the message digest algorithm and digital signature algorithm of the above-mentioned embodiments are not limited to the algorithms listed in the embodiments, and any message digest algorithm and digital signature algorithm known to those skilled in the art can be used to implement the present invention .

Those of ordinary skill in the art can understand that all or part of the steps for realizing the above-mentioned method embodiments can be completed by hardware related to program instructions, and the aforementioned program can be stored in a computer-readable storage medium. When the program is executed, the It includes the steps of the above method embodiments; and the aforementioned storage medium includes: ROM, RAM, magnetic disk or optical disk and other various media that can store program codes.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: it still Modifications or equivalent replacements can be made to the technical solutions of the present invention, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present invention.

Claims (15)

1. a digital signature method is characterized in that, comprising:

Message waiting for transmission is handled, obtained first message digest value;

From described message, choose at least one message fragment;

Described message fragment is handled respectively, obtained at least one corresponding second message digest value;

Adopt Digital Signature Algorithm that described first message digest value and described at least one second message digest value are encrypted, obtain final digital signature.

2. digital signature method according to claim 1 is characterized in that, adopts Digital Signature Algorithm that described first message digest value and described at least one second message digest value are encrypted, and obtains final digital signature and comprises:

Adopt described Digital Signature Algorithm that described first message digest value is encrypted, obtain first digital signature;

Adopt described Digital Signature Algorithm that described at least one second message digest value is encrypted respectively, obtain at least one second digital signature;

Described first digital signature and described at least one second digital signature are made up.

3. digital signature method according to claim 1 is characterized in that, adopts Digital Signature Algorithm that described first message digest value and described at least one second message digest value are encrypted, and obtains final digital signature and comprises:

Described first message digest value and described at least one second message digest value are made up;

Adopt described Digital Signature Algorithm that described first message digest value and described at least one second message digest value integral body of combination are encrypted, obtain described final digital signature.

4. according to each described digital signature method of claim 1-3, it is characterized in that, from described message, choose at least one message fragment and comprise:

From described message, choose at least one field or at least one position.

5. according to each described digital signature method of claim 1-3, it is characterized in that, from described message, choose at least one message fragment and comprise:

From described message, choose a plurality of message fragments that comprise same field or identical bits.

6. digital signature method according to claim 4 is characterized in that, chooses at least one field and comprise from described message:

From described message, choose the field that at least one is used to represent the temporal information of described message.

7. the verification method of a digital signature is characterized in that, comprising:

According to the message that receives being chosen at least one message fragment with the mode of message transmitting party agreement;

According to manner of decryption and Digital Signature Algorithm the final digital signature that receives is decrypted with the message transmitting party agreement, obtain first message digest value and at least one second message digest value, described first message digest value is corresponding with the described message that receives, and described at least one second message digest value is corresponding with described at least one message fragment;

The described message that receives is handled, obtained the 3rd message digest value;

Described message fragment is handled respectively, obtained at least one corresponding the 4th message digest value;

Described first message digest value and described the 3rd message digest value are compared;

Described at least one second message digest value and described at least one the 4th message digest value are compared;

Equate with described the 3rd message digest value in described first message digest value, and under described at least one second message digest value and the situation that described at least one the 4th message digest value all equates, judge that the described message that receives is not subjected to downgrade attacks;

Otherwise, judge that the described message that receives is subjected to downgrade attacks.

8. a digital signature device is characterized in that, comprising:

The first eap-message digest processing module is used for message waiting for transmission is handled, and obtains first message digest value;

Fragment is chosen module, is used for choosing at least one message fragment from described message;

The second eap-message digest processing module is used for described message fragment is handled, and obtains at least one corresponding second message digest value;

The digital signature module is used to adopt Digital Signature Algorithm that described first message digest value and described at least one second message digest value are encrypted, and obtains final digital signature.

9. digital signature device according to claim 8 is characterized in that, described digital signature module comprises:

The first digital signature unit is used for adopting described Digital Signature Algorithm to encrypt to described first message digest value, obtains first digital signature;

The second digital signature unit is used for adopting described Digital Signature Algorithm to encrypt respectively to described at least one second message digest value, obtains at least one second digital signature;

First assembled unit is used for described first digital signature and described at least one second digital signature are made up, and obtains final digital signature.

10. digital signature device according to claim 8 is characterized in that, described digital signature module comprises:

Second assembled unit is used for described first message digest value and described at least one second message digest value are made up;

The 3rd digital signature unit is used to adopt described Digital Signature Algorithm that described first message digest value and described at least one second message digest value integral body of combination are encrypted, and obtains described final digital signature.

11. each described digital signature device is characterized in that according to Claim 8-10, described fragment is chosen module and is used for choosing at least one field or at least one position from described message.

12. each described digital signature device is characterized in that according to Claim 8-10, described fragment is chosen module and is used for choosing a plurality of message fragments that comprise same field or identical bits from described message.

13. each described digital signature device is characterized in that according to Claim 8-10, described fragment is chosen module and is used for choosing the field that at least one is used to represent the temporal information of described message from described message.

14. the demo plant of a digital signature is characterized in that, comprising:

The fragment acquisition module is used for according to the mode of message transmitting party agreement the message that receives being chosen at least one message fragment;

Deciphering module, be used for the final digital signature that receives being decrypted according to manner of decryption and Digital Signature Algorithm with the message transmitting party agreement, obtain first message digest value and at least one second message digest value, described first message digest value is corresponding with the described message that receives, and described at least one second message digest value is corresponding with described at least one message fragment;

The 3rd eap-message digest processing module is used for the described message that receives is handled, and obtains the 3rd message digest value;

The 4th eap-message digest processing module is used for described message fragment is handled respectively, obtains at least one corresponding the 4th message digest value;

First comparison module is used for described first message digest value and described the 3rd message digest value are compared;

Second comparison module is used for described at least one second message digest value and described at least one the 4th message digest value are compared;

Determination module, be used for equating with described the 3rd message digest value in described first message digest value, and under described at least one second message digest value and the situation that described at least one the 4th message digest value all equates, judge that the described message that receives is not subjected to downgrade attacks; Otherwise, judge that the described message that receives is subjected to downgrade attacks.

15. a digital signature system is characterized in that, comprising: aforesaid right requires each described digital signature device of 8-13 and aforesaid right to require the demo plant of 14 described digital signature.

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