CAPEC-112 Brute Force

In this attack, some asset (information, functionality, identity, etc.) is protected by a finite secret value. The attacker attempts to gain access to this asset by using trial-and-error to exhaustively explore all the possible secret values in the hope of finding the secret (or a value that is functionally equivalent) that will unlock the asset.

Examples of secrets can include, but are not limited to, passwords, encryption keys, database lookup keys, and initial values to one-way functions. The key factor in this attack is the attackers' ability to explore the possible secret space rapidly. This, in turn, is a function of the size of the secret space and the computational power the attacker is able to bring to bear on the problem. If the attacker has modest resources and the secret space is large, the challenge facing the attacker is intractable. Assuming a finite secret space, a brute force attack will eventually succeed. The defender must rely on making sure that the time and resources necessary to do so will exceed the value of the information.

1. [Explore] [Determine secret testing procedure] Determine how a potential guess of the secret may be tested. This may be accomplished by comparing some manipulation of the secret to a known value, use of the secret to manipulate some known set of data and determining if the result displays specific characteristics (for example, turning cryptotext into plaintext), or by submitting the secret to some external authority and having the external authority respond as to whether the value was the correct secret. Ideally, the attacker will want to determine the correctness of their guess independently since involvement of an external authority is usually slower and can provide an indication to the defender that a brute-force attack is being attempted. 2. [Explore] [Reduce search space] Find ways to reduce the secret space. The smaller the attacker can make the space they need to search for the secret value, the greater their chances for success. There are a great many ways in which the search space may be reduced. 3. [Explore] [Expand victory conditions] It is sometimes possible to expand victory conditions. For example, the attacker might not need to know the exact secret but simply needs a value that produces the same result using a one-way function. While doing this does not reduce the size of the search space, the presence of multiple victory conditions does reduce the likely amount of time that the attacker will need to explore the space before finding a workable value. 4. [Exploit] [Gather information so attack can be performed independently.] If possible, gather the necessary information so a successful search can be determined without consultation of an external authority. This can be accomplished by capturing cryptotext (if the goal is decoding the text) or the encrypted password dictionary (if the goal is learning passwords).

• The attacker must be able to determine when they have successfully guessed the secret. As such, one-time pads are immune to this type of attack since there is no way to determine when a guess is correct.

• [Low] The attack simply requires basic scripting ability to automate the exploration of the search space. More sophisticated attackers may be able to use more advanced methods to reduce the search space and increase the speed with which the secret is located.

• Confidentiality: Read Data • Confidentiality, Access Control, Authorization: Gain Privileges

• Select a provably large secret space for selection of the secret. Provably large means that the procedure by which the secret is selected does not have artifacts that significantly reduce the size of the total secret space. • Use a secret space that is well known and with no known patterns that may reduce functional size. • Do not provide the means for an attacker to determine success independently. This forces the attacker to check their guesses against an external authority, which can slow the attack and warn the defender. This mitigation may not be possible if testing material must appear externally, such as with a transmitted cryptotext.