Attacks on cryptographic systems are nothing new. Whenever someone has information to hide, there is usually someone who would like to reveal it. William Frederick Friedman is considered one of the best cryptologists of all time. He helped break the encryption scheme used by the Japanese Purple Machine. Many of his inventions and cryptographic systems were never patented because they were considered so significant that the release of any information about them might aid an enemy. Following are some common attacks that an enemy might use to attack a cryptographic system:
- Known plain-text attack This attack requires the attacker to have the plain text and cipher text of one or more messages.
- Cipher textonly attack This attack requires the attacker to obtain several encrypted messages that have been encrypted using the same encryption algorithm. The attacker does not have the associated plain text; he attempts to crack the code by looking for patterns and using statistical analysis.
- The birthday attack The birthday attack gets its name from the birthday paradox, which states that for a given number of individuals, two or more will share the same birthday. This same logic is applied to calculate collisions in hash functions. Digital signatures can be susceptible to birthday attacks if the output of the hash function is not large enough to avoid collisions.
- Man-in-the middle attack This attack is carried out when attackers place themselves between two users. Whenever the attackers can place themselves in the communications path, the possibility exists that they can intercept and modify communications.
- Chosen cipher text The chosen cipher text attack is carried out when the attacker can decrypt portions of the cipher-text message. The decrypted portion of the message can then be used to discover the key.
- Chosen plain text The chosen plain text attack is carried out when a attacker can have the plain text messages encrypted and can then analyze the cipher text output of the event.
- Replay attack This form of attack occurs when the attacker can intercept cryptographic keys and reuse them later to either encrypt or decrypt messages the attacker should not have access to.