Passive Attacks

Active Attacks

• Modifying the information in an unauthorized manner.
• Initiating unintended or unauthorized transmission of information.
• Alteration of authentication data such as originator name or timestamp associated with information
• Unauthorized deletion of data.
• Denial of access to information for legitimate users (denial of service).

Assumptions of Attacker

Environment around Cryptosystem

Details of the Encryption Scheme

• Public Algorithms − With this option, all the details of the algorithm are in the public domain, known to everyone.
• Proprietary algorithms − The details of the algorithm are only known by the system designers and users.

Availability of Ciphertext

Availability of Plaintext and Ciphertext

• The attacker influences the sender to convert plaintext of his choice and obtains the ciphertext.
• The receiver may divulge the plaintext to the attacker inadvertently. The attacker has access to corresponding ciphertext gathered from open channel.
• In a public-key cryptosystem, the encryption key is in open domain and is known to any potential attacker. Using this key, he can generate pairs of corresponding plaintexts and ciphertexts.

Cryptographic Attacks

• Ciphertext Only Attacks (COA) − In this method, the attacker has access to a set of ciphertext(s). He does not have access to corresponding plaintext. COA is said to be successful when the corresponding plaintext can be determined from a given set of ciphertext. Occasionally, the encryption key can be determined from this attack. Modern cryptosystems are guarded against ciphertext-only attacks.
• Known Plaintext Attack (KPA) − In this method, the attacker knows the plaintext for some parts of the ciphertext. The task is to decrypt the rest of the ciphertext using this information. This may be done by determining the key or via some other method. The best example of this attack is linear cryptanalysis against block ciphers.
• Chosen Plaintext Attack (CPA) − In this method, the attacker has the text of his choice encrypted. So he has the ciphertext-plaintext pair of his choice. This simplifies his task of determining the encryption key. An example of this attack is differential cryptanalysis applied against block ciphers as well as hash functions. A popular public key cryptosystem, RSA is also vulnerable to chosen-plaintext attacks.
• Dictionary Attack − This attack has many variants, all of which involve compiling a ‘dictionary’. In simplest method of this attack, attacker builds a dictionary of ciphertexts and corresponding plaintexts that he has learnt over a period of time. In future, when an attacker gets the ciphertext, he refers the dictionary to find the corresponding plaintext.
• Brute Force Attack (BFA) − In this method, the attacker tries to determine the key by attempting all possible keys. If the key is 8 bits long, then the number of possible keys is 2⁸ = 256. The attacker knows the ciphertext and the algorithm, now he attempts all the 256 keys one by one for decryption. The time to complete the attack would be very high if the key is long.
• Birthday Attack − This attack is a variant of brute-force technique. It is used against the cryptographic hash function. When students in a class are asked about their birthdays, the answer is one of the possible 365 dates. Let us assume the first student's birthdate is 3^rd Aug. Then to find the next student whose birthdate is 3^rd Aug, we need to enquire 1.25^*√365 ≈ 25 students.
  Similarly, if the hash function produces 64 bit hash values, the possible hash values are 1.8x10¹⁹. By repeatedly evaluating the function for different inputs, the same output is expected to be obtained after about 5.1x10⁹ random inputs.
  If the attacker is able to find two different inputs that give the same hash value, it is a collision and that hash function is said to be broken.
• Man in Middle Attack (MIM) − The targets of this attack are mostly public key cryptosystems where key exchange is involved before communication takes place.
  • Host A wants to communicate to host B, hence requests public key of B.
  • An attacker intercepts this request and sends his public key instead.
  • Thus, whatever host A sends to host B, the attacker is able to read.
  • In order to maintain communication, the attacker re-encrypts the data after reading with his public key and sends to B.
  • The attacker sends his public key as A’s public key so that Btakes it as if it is taking it from A.
• Side Channel Attack (SCA) − This type of attack is not against any particular type of cryptosystem or algorithm. Instead, it is launched to exploit the weakness in physical implementation of the cryptosystem.
• Timing Attacks − They exploit the fact that different computations take different times to compute on processor. By measuring such timings, it is be possible to know about a particular computation the processor is carrying out. For example, if the encryption takes a longer time, it indicates that the secret key is long.
• Power Analysis Attacks − These attacks are similar to timing attacks except that the amount of power consumption is used to obtain information about the nature of the underlying computations.
• Fault analysis Attacks − In these attacks, errors are induced in the cryptosystem and the attacker studies the resulting output for useful information.