Limited Entropy Dot Com Not so random thoughts on security featured by Eloi Sanfèlix

Crypto Series: Classification of Attacks

As a quick note on the cryptographic systems description on the previous post, I'd like to mention that atacks to cryptosystems are usually classified based on the information known to the cryptanalyst. The basic types of attacks are:ásicos son:

• Ciphertext-only: The cryptanalyst knows only the ciphertext, and often also some information about the context of the message.
• Known-Plaintext: The cryptanalyst knows pairs of plaintexts and corresponding ciphertexts.
• Chosen-Plaintext: The cryptanalyst is able to choose plain texts and obtain their corresponding ciphertexts.
• Chosen-Ciphertext: The cryptanalyst can choose any ciphertext and obtain its corresponding plaintext.

Although the final two kinds could seem to be identical, there is a big difference mainly when applied to public key algorithms. In these algorithms, it is usually very easy to encrypt any plaintext. Thus, these algorithms need to withstand chosen-plaintext attacks. However, a chosen-ciphertext attack would require a decryption oracle, which would return any ciphertext decrypted without exposing the decryption key.