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Chosen plaintext attack

(also CPA)

Chosen plaintext attack definition

A chosen plaintext attack is a method hackers use to break a secret code or encryption system in order to gain unauthorized access to information. In this attack, hackers can choose specific messages (plaintexts) to be encrypted using the target’s encryption algorithm. Doing so produces corresponding ciphertexts (encrypted versions of plaintext). Hackers analyze the ciphertexts to reveal all or part of the secret encryption key. This attack aims to gather information that can be used to decrypt future messages and compromise the system's security.

See also: cryptanalysis, reverse engineering

How a chosen plaintext attack happens

  • The attacker chooses plaintexts they want to encrypt using the target encryption algorithm or system.
  • The chosen plaintexts are encrypted using the target encryption algorithm, producing the corresponding ciphertexts. The attacker may find out the target encryption algorithm through public knowledge or reverse engineering.
  • The attacker observes and records the generated ciphertexts for the chosen plaintexts.
  • They then analyze the relationship between the chosen plaintexts and their corresponding ciphertexts to identify patterns or vulnerabilities in the encryption algorithm.
  • Based on the analysis, the attacker may attempt to learn more about the secret key or exploit any weaknesses discovered in the encryption process.
  • If necessary, the attacker can repeat the steps above with additional chosen plaintexts to gather more information.

Chosen plaintext attack types

  • Batch chosen plaintext attack. Batch chosen plaintext attack allows the attacker to process multiple plaintexts simultaneously.
  • Adaptive chosen plaintext attack. In this attack, the adversary can dynamically adjust their chosen plaintexts based on the responses received from the encryption system.

Where chosen plaintext attacks could happen

  • Cryptographic protocols. Chosen plaintext attacks can target cryptographic protocols (e.g., secure communication, key exchange, or authentication). By manipulating chosen plaintexts during the protocol's execution, an attacker may gain insights into the protocol's security vulnerabilities or access sensitive information.
  • Network security. Chosen plaintext attacks can target cryptographic systems that protect network communications, such as VPNs (virtual private networks).

How to prevent chosen plaintext attacks

  • Use trusted encryption algorithms. Well-vetted encryption algorithms will have undergone extensive testing to withstand known attacks, including chosen plaintext attacks.
  • Secure key management. Use strong, random, and sufficiently long encryption keys and ensure only authorized users can access them.