This paper describes new attacks on PKCS#1 v1.5, a deprecated but still widely used RSA encryption standard. The first cryptanalysis is a broadcast attack, allowing the opponent to reveal an identical plaintext sent to different recipients. This is n ontrivial because different randomizers are used for different encryptions (in other words, plaintexts coincide only partially). The second attack predicts, using a single query to a validity checking oracle, which of two chosen plaintexts corresponds to a challenge ciphertext. The attack's success odds are very high. The two new attacks rely on different mathematical tools and underline the need to accelerate the phase out of PKCS#1 v1.5.
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