CN100403326C - Integrity Preservation Method of Digital Evidence Based on Computer Forensics - Google Patents

Abstract

A method for preserving the integrity of digital evidence based on computer forensics. It solves the problem that the existing technology cannot realize the real-time protection of the integrity of the forensic information. The method of the present invention is to copy, sign _and securely store (m _i ') when the newly generated i-th information record m _i is generated, and obtain the digital signature r _i and _si of the forensic computer system S to mi; when After the signature is completed, S sends the signature _ri , s _i and the information record m _i ', the record serial number i and the time t _i when the record was generated to the secure storage object M, and calls the secure hash SHA algorithm to m _i ' ‖i‖t _i ‖SHA _(i-1) performs information summary and completes the preservation process. The method of the present invention can protect the integrity of possible criminal evidence in real time before and during the invasion, avoiding the malicious damage of the intruder, so that the after-the-fact investigation and evidence collection information has objectivity and authenticity, conforms to legal regulations, and has legal evidence required characteristics.

Description

Integrity Preservation Method of Digital Evidence Based on Computer Forensics

【Technical field】：

The invention belongs to the field of security protection of computer evidence. By fixing possible criminal evidence generated in the protected computer system in real time, the original protection of computer criminal evidence is realized.

【Background technique】:

In recent years, computer network crime cases have risen sharply, and network crime has become an international issue of general concern. The key to combating computer network crimes is to find sufficient, reliable and legally effective electronic evidence. Therefore, computer forensics has received more and more attention and has become a research hotspot in the field of computer network security.

Computer forensics is the acquisition, preservation, analysis and presentation of computer crime evidence, mainly including physical evidence and digital evidence. Physical evidence means that legitimate investigators come to the scene of a computer crime or intrusion to find and detain relevant computer hardware; digital evidence means finding electronic evidence from raw data (including files, logs, etc.) evidence. Like other legal evidence, these electronic evidence must be authentic and legal.

Generally speaking, the security protection of physical evidence is relatively easy. However, due to the characteristics of easy modification and deletion of electronic evidence, it is difficult to protect their security. Any change in the electronic evidence from the moment it is generated to when it is submitted to legitimate investigators renders it legally invalid. Therefore, how to ensure that the submitted electronic evidence is completely consistent with the original electronic information has become very important.

At present, in this research field, the theoretical research at home and abroad is mostly follow-up research, with few original innovative ideas, and many research results are stuck in a unified model: forensics machine, evidence machine and analysis machine. Such a research model is almost tracking the framework structure of the intrusion detection system. It does not reflect the specific goal of intrusion forensics. Instead, it inherits some security defects of the intrusion detection system, such as weak self-security protection, high rate of missed and false positives, and resource consumption. Huge, preserved evidence files do not have strict legal effect and are easily tampered with. Different from the progress of specific theoretical research work, there have been many forensics tool software, such as TCT (the Coronor's Toolkit) and Encase and other forensics software. However, the focus of these tool software is how to restore deleted files and how to extract evidence from file information afterwards. With the diversification of network intrusion forms and the emergence of anti-forensics technology, their practicality will be greatly limited. Also, once the data containing intrusion information is deleted or has been maliciously tampered with before being officially submitted, they will lose their legal effect.

【Invention content】：

The purpose of the present invention is to solve the problem that the existing computer forensics technology cannot realize the real-time protection of the integrity of forensic information, and provide a method for ensuring the integrity of digital evidence based on computer forensics.

The method for preserving the integrity of digital evidence based on computer forensics provided by the present invention is to record in real time every piece of key information generated by the host computer that may be related to the intrusion, that is, evidence collection information, during the operation of the host system, and adopt a consistency algorithm to record these information to protect;

Specifically include:

First, secure signature and transfer of evidence collection information

When the newly generated information record m _i is generated, it is copied, signed and stored safely. The detailed steps of information record signature are as follows:

a.S generates prime numbers p and q that meet the conditions, selects h, and calculates g;

b. Since the r value used in the signature process has nothing to do with the specific information, a string of random k values smaller than q can be generated, and the corresponding r value can be calculated. For each k value, ^{k- 1} , where, 0<k<q, .(k ^-1 k)mod q=1, r=(g ^k mod p)mod q;

c. When a new information record m _i is generated, a complete copy m _i ' will be generated. At this time, a set of values is taken from the result of b: (k _i , k ^-1 _i , r ₁ );

dS randomly selects the value x _i as the private key of the i-th information record, and calculates the corresponding public key y _i ;

e. Calculate the corresponding s _i according to the formula s=(k ⁻¹ (SHA(m)+xr)) mod q, where m=m _i '||i||t _i . If moment t _i is included in m _i ', t _i is no longer considered in the expression of m;

f. The r _i and s _i obtained through the previous steps are the digital signatures of the i-th information record m _i by the computer system S to be forensic; when the signature of a new information record is completed, S will sign r _i and s _i and the information record m _i ', the record sequence number i and the time t _i when the record was generated are sent to the secure storage object M;

g. When the signature of the i-th information record and its related information are stored in M, call the secure hash SHA algorithm to perform information digest on m _i '||i||t _i ||SHA _(i-1) , namely: SHA _i ＝SHA(m _i '||i||t _i ||SHA _(i-1) ), since this is a recursive formula, when the relevant information of the i-th information record arrives, the i-1th The information digest SHA _(i-1) of the information record has been generated;

Second, repeat the process of securely signing and transferring the above-mentioned evidence collection information until all the evidence collection documents are recorded;

Among them, symbols and parameters:

"||" indicates the concatenation operation of strings;

"S" means the protected computer system, that is, the forensics machine;

"V" indicates the verifier of the integrity of the forensic information record;

“m _i ” means the i-th information record generated by the computer in the key file;

"m _i '" indicates the object used for safe copy transfer during the process of copying and transferring when the i information record "m _i "is generated, and storing it in a sufficiently safe place;

“t _i ” indicates the moment of the i-th record generated by the computer in the corresponding key file;

System parameters

The method for preserving the integrity of forensic information of the present invention can be regarded as a set of protocols used between the forensic machine S, which is the signer, and user V, which is the verifier. In the forensic information integrity protocol, the same system parameters as DSA are used. , the system parameters of the forensic information integrity protection protocol are:

"p" is a prime number of L bits long, where L is from 512 to 1024 bits and is a multiple of 64;

"q", is a 160-bit long factor that is relatively prime to p-1;

"g", g=h ^(p-1)/q mod p, wherein h is any number smaller than p-1 and satisfying that h ^(p-1)/q mod p is greater than 1;

"x" is a number less than q;

"y", y=g ^x mod p; In addition, a one-way hash function H(m) is also used in the algorithm, and the standard specifies the secure hash algorithm SHA;

Among the above parameters, p, q and g are public and can be shared by all users in the network, x is a private key, and y is a public key.

The above-mentioned key information records include but are not limited to logs generated by the host system, network logs, firewall logs, intrusion detection system logs, etc.

A method for verifying the integrity of forensic information preserved by the above method, the verification method comprising:

First, the verification of forensic information signature and information digest, the specific process is as follows:

a. The information consistency verifier V proves his legal identity to the secure storage object M through the commonly used identity authentication protocol. After obtaining the legal identity, he is allowed to verify the digital signature and information summary of any information record;

b. Verify that the signature of S is valid, and the verification process for the i-th information record signature is:

①Substitute s _i into the formula w=s ^-1 mod q to calculate w _i ;

②u ₁ ＝(SHA(m _i '||i||t _i )×w _i )mod q, u ₂ ＝(r _i w _i )mod q, then v _i ＝((g ^u1 ×y ^u2 )mod p ) mod q;

After calculation, when v _i = _ri , the signature of S is valid, that is: m _i '≡m _i ;

When v _i ≠ r _i is found during the verification process, it means that the signature is invalid, indicating that m _i ' has been modified, and the information recorded in m _i ' is no longer valid and has no legal effect;

c. Verify that the information digest corresponding to the i-th signature is valid:

Substitute the m _i ', i, t _i value corresponding to the i-th signature and the information digest value SHA _(i-1) corresponding to the i-1 signature into the formula SHA(m _i '||i||t _i ||SHA _(i-1) ), get SHA _i ', when SHA _i '=SHA _i , the information digest value corresponding to the i-th signature is valid, which indicates that the difference between the i-th signature and the i-1 signature No records were deleted or added during the period;

Second, repeat the verification process of the above-mentioned evidence collection information until the verification of all the evidence collection document records is completed.

Advantages and positive effects of the present invention: compared with prior art, the present invention has the following beneficial effects:

1. The legitimacy of the information collected. Each piece of forensic information is signed and hashed iteratively through a fixed algorithm, and strict mathematical reasoning can prove that the successfully verified forensic information is legal.

2. The security of the integrity protection process of forensic information. Each information record saved by this method is copied to a safe buffer (rather than stored in a file) at the same time when it is generated, and then it is signed and hashed iteratively to ensure its originality. At the same time, it can prevent other processes from modifying information records, so it has high security.

3. This method has a lower operating cost. Using multi-thread technology, information records and signatures are processed in parallel, requiring less CPU resources and memory resources.

4. Higher real-time performance. The algorithm adopted in the method can protect the integrity and securely transfer the original evidence information in real time under the premise of ensuring the integrity of the evidence collection information. The method of the present invention can protect the integrity of possible criminal evidence in real time in the first two stages, that is, before the invasion and during the invasion (see Figure 1), so as to avoid the malicious destruction of the intruder and make the investigation and evidence collection information after the event objective , authenticity, compliance with the law, and the characteristics required by legal evidence.

[Description of drawings]:

Fig. 1 is a schematic diagram describing the whole process of a computer being invaded;

Figure 2 is a flow chart of secure signature and transfer of evidence collection information records;

Figure 3 is the program testing process of information signature and transfer;

Fig. 4 is the program testing process of digital signature verification;

Figure 5 is the verification result when no information is modified or deleted;

Figure 6 is the verification result when the fourth information record is tampered with;

Fig. 7 is the verification result when the 5th and 6th information records are deleted.

【Detailed ways】:

Example 1

As shown in Figure 1-4, the integrity verification of computer evidence is a key issue in the identification of computer evidence. One of the most difficult aspects of computer forensics is proving that the evidence collected by the forensic investigator has not been altered. Figure 1 describes the whole process of a computer being invaded, which can be divided into three stages: before the intrusion, during the intrusion, and after the intrusion. Among them, in the early stage of the invasion, the intruder first determines the attack target, and uses various scanning tools to remotely scan the target system to obtain the vulnerability of the target system and collect information for the next stage of successful intrusion; during the intrusion, the intruder According to the information collected in the first stage, various intrusion techniques or tools are used to successfully invade the target system; after the computer system is found to have been successfully invaded or is being invaded, the intrusion process enters the third stage. Apparently, ensuring the security and integrity of criminal evidence in the first two stages is as important as after the intrusion. Therefore, the present invention proposes a real-time protection method for the integrity of forensic information. This method is accomplished by the following steps:

The first step, preservation of forensic information

(1) Symbols and basic definitions

"|| indicates the concatenation operation of strings;

"S" means a protected computer system;

"V" indicates the verifier of the integrity of the forensic information record;

"m _i ", in the computer system, some key files such as system logs, network information records and other files contain many information records. The remaining evidence, for the convenience of description, we use "m _i " to represent the i-th information record generated by the computer in the key file;

"m _i '", at present, there is no encryption method that can prevent intruders from deleting or adding malicious behaviors such as key files and information records in key files. To solve this problem, copy and transfer information records when they are generated. And stored in a safe enough place, this place can be a file under safety protection or a safe output machine, where "m _i '" is the object used by "m _i " for safe copy transfer during this process;

“t _i ” indicates the moment of the i-th record generated by the computer in the corresponding key file;

(2) System parameters

The method for preserving the integrity of forensic information of the present invention can be regarded as a set of protocols used between the forensic machine S, which is the signer, and user V, which is the verifier. In the forensic information integrity protocol, the same system parameters as DSA are used. , the system parameters of the forensic information integrity protection protocol are:

"p" is a prime number of L bits long, where L is from 512 to 1024 bits and is a multiple of 64;

"q", is a 160-bit long factor that is relatively prime to p-1;

"g", g=h ^(p-1)/q mod p, where h is any number less than p-1 and satisfying that h ^(p-1)/q mod p is greater than 1; "x" is less than q number;

"y", y=g ^x mod p; In addition, a one-way hash function H(m) is also used in the algorithm, and the standard specifies the secure hash algorithm SHA;

Among the above parameters, p, q and g are public and can be shared by all users in the network, x is the private key, and y is the public key;

(3) Secure signature and transfer of evidence collection information

In order to quickly and securely transfer the newly generated information record _mi , copying, signing and safe storage must be performed when _mi is generated. This process can be described by the process shown in Figure 2. The detailed steps of information record signature are as follows:

(a) S produces prime numbers p and q that satisfy the conditions, calculate g=h ^(p-1)/q mod p, and h<p-1, and h ^(p-1)/q mod p>1, where p , q, g are public;

(b) Since the r value used in the signature process has nothing to do with the specific information, it is possible to generate a series of random k values smaller than q, and calculate the corresponding r value, and for each k value, calculate k ^-1 , where, 0<k<q, (k ^-1 k)mod q=1, r=(g ^k mod p)mod q;

(c) When a new information record m _i is generated, a complete copy m _i ' will be generated. At this time, a set of values is taken from the result of (b): (k _i , k ^-1 _i , r _i );

(d) S randomly selects the value x _i as the private key of the i-th information record, x _i <q, and calculates the corresponding public key y _i according to the formula y=g ^k modp;

(e) Calculate the corresponding _si according to the formula s=(k ⁻¹ (SHA(m)+xr))mod q, where m=m _i '||i ||t _i . If moment t _i is included in m _i ', t _i is no longer considered in the expression of m;

(f) The r _i and _si obtained through the previous steps are the digital signatures of the i-th information record m _i by the computer system S to be forensic; when the signature of a new information record is completed, S will sign r _i , s _i and information record m _i ', record sequence number i and the time t _i when the record was generated are sent to the secure storage object M;

(g) When the signature of the i-th information record and its related information are stored in M, call the secure hash SHA algorithm to perform information digest on m _i '||i||t _i ||SHA _(i-1) , that is : SHA _i ＝SHA(m _i '||i||t _i ||SHA _(i-1) ), since this is a recursive formula, when the relevant information of the i-th information record arrives, the i-1 The information digest SHA _(i-1) of information records has been generated;

(4) Repeat the process of securely signing and transferring the above-mentioned forensic information until the records of all forensic documents are completed.

The second step is the verification of the signature of the forensic information

In M, each information record has been signed and digested through the process shown in Figure 2, and the whole process is completed by S. Then, when the signature and digest are verified correctly, its integrity can be determined. The verification process of the signature and message digest is as follows:

(a) The information consistency verifier V proves his legal identity to the secure storage object M through common identity authentication protocols. After obtaining legal identity, the digital signature and information digest of any information record can be verified. The following is an example of verifying the i-th information record signature;

(b) Verify that the signature of S is valid:

①Substitute s _i into the formula w=s ^-1 mod q to calculate w _i ;

②u ₁ ＝(SHA(m _i '||i||t _i )×w _i )mod q, u ₂ ＝(r _i w _i )mod q, then v _i ＝((g ^u1 ×y ^u2 )mod p ) mod q. After calculation, if v _i = _ri , the signature of S is valid, that is: m _i '≡m _i . If v _i ≠ r _i is found during the verification process, it means that the signature is invalid, and it indicates that m _i ' has been modified, and the information recorded in m _i ' is no longer valid and has no legal effect.

(c) Verify that the information digest corresponding to the i-th signature is valid:

Substitute the m _i ', i, t _i value corresponding to the i-th signature and the information digest value SHA _(i-1) corresponding to the i-1 signature into the formula SHA(m _i '||i||t _i ||SHA _(i-1) ), get SHA _i ', if SHA _i '=SHA _i , then the information digest value corresponding to the i-th signature is valid, which indicates that the difference between the i-th signature and the i-1 signature No records were deleted or added during the period.

Repeat the verification process of the above-mentioned forensic information until the verification of all forensic document records is completed.

Example 2

Test Effects of Forensic Information Integrity Protection Protocol

First, securely sign and transfer the evidence collection information (see Example 1), and its program testing process is shown in Figure 3. The program loop first checks whether there is data to be signed in the buffer, and if so, reads out this record to determine whether it is the first record, if so, calculates its SHA digest, and then calculates its signature, if not, then Add the summary value obtained from the previous record to this information and then hash it, and finally save the information summary, and save this record and its summary, signature, public key and record number safely, and then retrieve the information from the buffer Delete this record.

Next, verify the information records that have been safely signed and saved, and the program testing process is shown in Figure 4. The program reads the first row of data from the database, first verifies the correctness of its signature, and then judges whether it is the first record, if so, calculates its hash value and compares it with the hash value saved in this record, if not, then compares it to The hash value obtained from the previous record is added to the record and then its hash value is calculated, compared with the hash value saved in this row, if they are equal, it can be verified that this record appears in the correct position, that is, this record There are no added or deleted records from the previous record.

Now assume that there are some forensic information as follows: 1111111111, 2222222222, aaaaaaaaaa, bbbbbbbbbb, cccccccccc, 3333333333, ttyyuu1177, 556677hhkk.

When the eight information records have not been changed, the verification result is shown in Figure 5.

When the fourth message record "bbbbbbbbbb" is artificially changed to "bbbbbbbbba", the verification result is shown in Figure 6. It can be seen from Figure 6 that the signature and hash value of the fourth record are invalid, so It can be verified that the fourth record has been tampered with.

When the 5th and 6th information records are deleted, the verification result is shown in Figure 7.

Claims (3)

1. digital evidence integrality preserving method based on computer forensics, it is characterized in that described method is at the host computer system run duration, the information of may each bar key message relevant promptly collecting evidence that writes down in real time that main frame produces with invasion, and take consistency algorithm that these information are write down and protect;

Specifically comprise:

The first, the security signature and the transfer of evidence obtaining information

At the new information record m that produces ₁When generating, duplicate, signature and safe storage, the detailed step of information record signature is as follows:

A.S produces prime number p and the q that satisfies condition, and selects h, calculates g;

B. since in the signature process r value used irrelevant with concrete information, so can produce a string k value at random, and calculate the r value of correspondence with it less than q, to each k value, calculate k ^-1, wherein, 0＜k＜q, (k ^-1K) mod q=1, r=(g ^kMod p) mod q;

C. as a new information record m _iProduce, will produce portion and copy m completely ⁱ', at this moment, from the result of b, take out a class value: (k _i, k ^-1 _i, r _i);

D.S random value x _iAs the private key of i bar information record, and calculate corresponding public-key cryptography y _i

E. according to formula s=(k ^-1(SHA (m)+xr)) mod q calculates corresponding s _i, wherein, m=m _i' || i||t _i', if m _i' in comprise t constantly _i, then no longer consider t in the expression formula of m _i

F. respectively go on foot resulting r through the front _iAnd s _iBe exactly to i bar information record m by evidence obtaining computer system S _iDigital signature; When the signature of a new information record is finished, the S r that just will sign _i, s _iWith information record m _i', records series i and the time t that should record generates _iSend among the safe storage object M;

G. when the signature of i bar information record stores M into its relevant information, call secure hash SHA algorithm to mi ' || i||t _i|| SHA _(i-1)Carry out informative abstract, that is: SHA _i=SHA (m _i' || i||t _i|| SHA _(i-1), because this is a stepping type, when the relevant information of i bar information record arrives, the informative abstract SHA of i-1 bar information record _(i-1)Generate;

The second, security signature and transfer process to the file logging of all collecting evidence that repeats above-mentioned evidence obtaining information finished; Wherein symbol and parameter:

" || ", the attended operation of expression character string;

" S " represents protected computer system;

" V ", expression evidence obtaining information record integrity verification person;

" m _i", the i bar information record that the expression computing machine is produced in key file;

" m _i' " be illustrated in i bar information record " m _i" duplicate transfer when producing, and leave in this process of place of safe enough and be used for the object that secure copy shifts;

" t _i" moment of the i bar record that in corresponding key file, produced of expression computing machine;

" p " is the long prime number in L position, wherein from 512 to 1024 of L and be 64 multiple;

" q " is 160 long and coprime with p-1 factors;

" g ", g=h ^(p-1)/qMod p, wherein h is less than p-1 and satisfies h ^(p-1)/qMod p is greater than any number of 1;

" x " is the number less than q;

" y ", y=g ^xMod p; In addition, also use an one-way hash function H (m) in the algorithm, standard has been specified Secure Hash Algorithm SHA;

In the above-mentioned parameter, p, q and g are disclosed, and are that all users in the network are publicly-owned, and x is a private key, and y is a public-key cryptography.

2. according to the store method of claim 1, it is characterized in that the key message record includes daily record and the network log that host computer system produces, fire wall daily record, intruding detection system daily record.

3. the verification method of an evidence obtaining information integrity of preserving with the described method of claim 1 is characterized in that this verification method comprises:

The first, the checking of evidence obtaining Information Signature and informative abstract, detailed process is as follows:

A. consistency on messaging verifier V, permits the digital signature and the informative abstract of any information record are verified by authentication agreement commonly used legal identity to safe storage object M proof oneself after obtaining legal identity;

B. the signature of verifying S is effective, to i bar information record signature verification process is:

1. s _iSubstitution formula w=s ^-1Mod q calculates w _i

2. u ₁=(SHA (m _i' || i||t _i) * w _i) mod q, u ₂=(r _iw _i) mod q, then v _i=((g ^U1* y ^U2) mod p) mod q;

Through calculating, work as v _i=r _i, then the signature of S is effective, that is: m _i' ≡ m _i

In proof procedure, find v _i≠ r _i, illustrate that signature is invalid, show m _i' be modified m _i' in the information that write down no longer valid, null;

C. verify whether the i bar pairing informative abstract of signing is effective:

The i bar pairing m that signs _i', i, t _iValue and the i-1 bar pairing informative abstract value SHA that signs _(i-1)Substitution formula S HA (m _i' || i||t _i|| SHA _(i-1)), obtain SHA _i', work as SHA _i'=SHA _i, then the i bar pairing informative abstract value of signing is effective, and this shows the deleted or interpolation without any record between i bar signature and the i-1 bar signature;

The second, proof procedure to the file logging checking of all collecting evidence that repeats above-mentioned evidence obtaining information is finished.

Images