Chosen Ciphertext Attacks on RSA
Last Updated :
29 Jan, 2024
In the domain of cryptography, a chosen ciphertext attack (CCA) is a significant threat, particularly to public-key cryptosystems. This article aims to resolve the complexities of chosen-ciphertext attacks, exploring their implications and impact on various cryptographic schemes.
Terminology on Cipher Text Attacks
Chosen-Ciphertext Attack (CCA): A chosen ciphertext attack allows an adversary to select a piece of ciphertext and attempt to reveal its corresponding decrypted plaintext. This model is particularly relevant to public-key cryptosystems.
Cryptanalyst: A cryptanalyst is an individual who analyzes and attempts to break cryptographic systems to expose their vulnerabilities.
Explaining Chosen-Ciphertext Attacks
In a chosen ciphertext attack, the cryptanalyst collects information by selecting a ciphertext and obtaining its decryption under an unknown key. The opponent can input known ciphertexts into the system, aiming to conclude the hidden secret key used for decryption.
Certain secure schemes can be compromised under a chosen ciphertext attack. For instance, the El Gamal cryptosystem, initially semantically secure, becomes vulnerable in this context. Early RSA padding used in SSL protocols also faced challenges from worldly-wise chosen-ciphertext attacks.
Categories of Cryptanalytic Attacks
According to RSA, cryptanalytic attacks are categorized based on the information available to the cryptanalyst. These include ciphertext-only attacks, known-plaintext attacks, and chosen-plaintext attacks. Chosen-plaintext attacks are particularly relevant to public-key cryptography, where encryption keys are public.
Batch and Adaptive Chosen-Plaintext Attacks
Two forms of chosen-plaintext attacks are batch chosen-plaintext attacks and adaptive chosen-plaintext attacks. The former involves choosing all plaintexts before encryption, while the latter dynamically selects plaintext samples based on previous encryption results.
Probabilistic Encryption
Public key encryption algorithms vulnerable to "dictionary"-type attacks necessitate probabilistic encryption to enhance security. This involves randomized encryption to prevent straightforward decryption table exploitation.
Adaptive Chosen-Ciphertext Attack (CCA2)
An adaptive chosen-ciphertext attack is an interactive form where an attacker sends ciphertexts to be decrypted and uses the results to choose subsequent ciphertexts. This attack aims to gradually reveal information about an encrypted message or the decryption key.
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