SQL Server : ONE-WAY ENCRYPTION (part 2) - Known Vulnerabilities

2/22/2014 3:38:05 AM

3. Known Vulnerabilities

The Payment Card Industry Data Security Standard (PCI DSS), through requirement 3.4, does offer one-way encryption as a valid option in storing the primary account number, which is considered sensitive, in a database. The caveat is that the one-way encryption must use a strong algorithm. Among the algorithm options that are available to one-way encryption in SQL Server, PCI DSS defines the SHA1 algorithm as being considered "... an example of an industry-tested and accepted hashing algorithm.", which is an acknowledgement that SHA1 meets this criteria.

The following sections review a few of the most common known vulnerabilities, when using one-way encryption:

Dictionary Attack Vulnerability

A dictionary attack is one in which a list of values are hashed and then compared to the hash values stored in the target data table. This method is often used in an attempt to reveal passwords that are protected using one-way encryption.

By way of an example, consider an attempted dictionary attack is on the Borrower_Identification table of our HomeLending database, which we've protected using one-way encryption. Within the Identification_Value column are the hash values of Social Security Numbers that are generated through one-way encryption.

The attack, depicted in Figure 2, is executed as follows:

  • The attacker has created an "Attack Dictionary" of hash values that are based upon a sequence of plain text Social Security Numbers, ranging from "555-86-0622" through "555-86-0626".

  • Each of the hash values in the attack dictionary is compared to the hash values stored in the Borrower_Identification table.

  • A match is identified in the Borrower_Identification table with the attack hash value of 0xC36F02D9AC32B2E3813EFF9B 6C23D99D6038FD9A revealing that the plain text value of "555-860625" is a valid Social Security Number within the database.

  • With this knowledge, the attacker gains access to associated information such as the borrower's name, address and birth date.

Figure 2. Dictionary Attack.

A dictionary attack takes advantage of the inherent nature of one-way encryption by performing the same action that is used when a user searches one-way encrypted data, but on a larger scale.

In our example, the attacker knows he is looking for Social Security Numbers which, in their plain text form, have a standard pattern. It is also known to the attacker that Social Security Numbers are commonly stored without the dash ("-") character. Therefore, the attacker has a finite set of base values that will likely return some matches.

If the DBA added a series of characters to the value of the Social Security Number, before it was encrypted, the resulting hash value would be different than the hash value resulting from encrypting the real Social Security Number, and would increase the number of possible character combinations required to return a positive match.

Rainbow Table Attack Vulnerability

Database Administrators are not the only people interested in efficiency. Those who are interested in attacking a database to reveal sensitive data that is protected through one-way encryption are also interested in the efficiency of their efforts. In order to initiate a dictionary attack on a database containing millions of records, the attacker would require a large attack dictionary to cover the possible combinations of plain text and hash values. This would result in a long running attack that requires a lot of resources from the database server, therefore increasing the risk of the attack being detected.

Therefore, the rainbow table attack was developed. The key player in this game is the rainbow table. The rainbow table consists of a series of rows holding two columns of data. The first column contains the plain text values that are being sought, for example a Social Security Number. The second column contains a value that is the ending hash of a reduction chain. A reduction chain is the result of taking the plain text value in the first column of our rainbow table and creating an initial hash; then, a portion of the initial hash, such as its first six digits, is obtained and another hash value is generated. This process continues for a number of iterations until an ending hash is derived.

The ending hash that is stored in the rainbow table represents an array of hash values that can be programmatically derived and iterated in an attack, through the reversal of the reduction chain building process. This approach provides a very efficient means of storing the seed values that are used to mount an attack on one-way encrypted data.

Figure 3. The creation of a rainbow table.

Let's consider an example of how this type of an attack can affect the sensitive data that is protected with one-way encryption. As before, we'll assume that a rainbow table attack is in progress on the Borrower_Identification table of our HomeLending database.

The attacker has created a rainbow table, with a reduction chain represented by each record's last link, based upon a sequence of plain text Social Security Numbers ranging from "555-86-0622" through "555-86-0626", as shown in Table 1.

Table 1. The rainbow table.
Plain TextEnding Hash in the Reduction Chain

Each of the final reduction chain link hash values in the rainbow table is compared to the hash values stored in the Borrower_Identification table. This step is basically identical to a dictionary attack. In our specific example, the result of this stage does not indicate a successful match for any of our ending hashes.

In the next stage of the attack, we revisit the process that created our ending hash: the chain reduction process. In our example, the reduction chain was generated based on the first six digits of each hash. In the execution of this attack we reverse the reduction chain by taking the first six digits of the hashed value that is the subject of our attack. Each subsequent link in the reversed reduction chain is compared to the ending hash that is stored in the rainbow table. In our case, a successful match occurs for the plain text value of "555-86-0625".

Much like a dictionary attack, a rainbow table attack can be reasonably mitigated through the use of a salt on your plain text, prior to applying the one-way encryption. These attacks rely on the perpetrator having anticipated a series of plain text values, hashing these values and then comparing the resulting hashes to the values stored within the database. The use of a salt increases the complexity of the plain text and reduces the likelihood that the anticipated value is among the plain text values sought by the attacker.

Hash Collision Vulnerability

A hash collision occurs when two unique plain text values produce an identical hash value. An example would be both"555-86-1234" and "555-86-5298" returning the identical hash value. Since a value secured using one-way encryption is not decrypted, and its underlying plain text is revealed through the comparison of hash values, a hash collision presents a situation in which the actual plain text value cannot be determined.

The algorithm selected for the encryption process is critical in reducing the likelihood of hash collisions. Algorithms that produce lengthy hashes increase the array of possible values, and so reduce the probability of a hash collision.

Of course, the larger the volume of records to which these algorithms are applied, the higher is the risk of a hash collision. A mathematic problem called "The Birthday Paradox" is commonly referenced as a formula that can be used to determine the probability of hash collisions. While not specific to determining the probability of hash collisions, the Birthday Paradox formula can be modified to provide this information.

For those who are not mathematics or statistics majors, let's boil this issue down to its basics.

The possible unique combination of values for a single bit is 2 since a bit is either a 1 or a 0. The possible unique combination of values for a single byte, which is eight bits, would be 256, represented as 28. The algorithm options that are provided with one-way encryption return either a 128 bit or a 160 bit hash value. The possible unique combination of a 128 bit hash would be 340,282,36 6,920,938,460,000,000,000,000,000,000,000, represented as 2128. The possible unique combination of a 160 bit hash would be 1,461,501,637,330,902,900,000,000,000,000,000,000,000,000,000,000, represented as 2160.

In order for the possibilities of a hash collision to occur in the Identification_Value column of the Borrower_Identification table, using a 128 bit algorithm, to reach a meager 0.1% it would require a volume of 830,000,000,000,000,000 records; each containing a unique plain text value.

There are other factors that come into play that have influence on the actual possibilities of a hash collision, such as the internal processing that takes place within the algorithm. Regardless, the vulnerability for the occurrence of a hash collision is real and should be carefully considered.

With the selection of the hashing algorithm, inclusion of a salt prior to encryption, and by avoiding use of one-way encryption in tables that have an extremely high volume of rows, the potential vulnerabilities of the technique can be mitigated, and it becomes a worthy option to consider when protecting sensitive data.

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  •  SQL Server 2012 : Measuring SQL Server Performance (part 3) - Dynamic Management Objects, Data Collector
  •  SQL Server 2012 : Measuring SQL Server Performance (part 2) - Performance Monitor
  •  SQL Server 2012 : Measuring SQL Server Performance (part 1) - Understanding Performance Counters
  •  SQL Server 2012 Security : How Hackers Attack SQL Server
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