Cryptography: Cryptanalytic Attacks

•Ciphertext only attack

–In this method, the attacker has access to a set of ciphertext(s). One ciphertext or many?

–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

–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.

–Let’s say for example that Alice sends Bob a message everyday starting with “hello” messages…. If Darth analyzes those ciphertext to get the message.

 

•Chosen plaintext attack

–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.

–A popular public key cryptosystem, RSA is also vulnerable to chosen-plaintext attacks.

–Chosen-plaintext attacks were used by Allied cryptanalysts in World War II for breaking of Enigma ciphers.

 

•Chosen ciphertext attack

–A cryptanalyst can analyze chosen ciphertexts together with corresponding plaintexts. His goal is to acquire a secret key or to get as many information about the attacked system, as possible.

–The attacker has capability to make the victim (who knows the secret key) decrypt a selected ciphertext and send him the result. Analyzing the chosen ciphertext and corresponding received plaintext, the intruder tries to guess the secret key which was used by the victim.

–Chosen-ciphertext attacks are usually used for breaking systems with public key encryption. For example, early versions of RSA cipher were vulnerable to such attacks. They are used less often for attacking systems protected by symmetric ciphers. Some self-synchronizing stream ciphers were attacked in that way.

 

Types of Attacks	Known to Cryptanalyst
Ciphertext Only	·Encryption algorithm ·Ciphertext
Known Plaintext	·Encryption algorithm ·Ciphertext ·One or more plaintext–ciphertext pairs formed with the secret key
Chosen Plaintext	·Encryption algorithm ·Ciphertext ·Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key

 

Types of Attacks	Known to Cryptanalyst
Chosen Ciphertext	·Encryption algorithm ·Ciphertext ·Ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key
Chosen Text	·Encryption algorithm ·Ciphertext ·Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key ·Ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key

•Chosen text attack
–This is a combination of Chosen plaintext attack and chosen ciphertext attack