How to break XML encryption

Abstract

XML Encryption was standardized by W3C in 2002, and is implemented in XML frameworks of major commercial and open-source organizations like Apache, redhat, IBM, and Microsoft. It is employed in a large number of major web-based applications, ranging from business communications, e-commerce, and financial services over healthcare applications to governmental and military infrastructures. In this work we describe a practical attack on XML Encryption, which allows to decrypt a ciphertext by sending related ciphertexts to a Web Service and evaluating the server response. We show that an adversary can decrypt a ciphertext by performing only 14 requests per plaintext byte on average. This poses a serious and truly practical security threat on all currently used implementations of XML Encryption. In a sense the attack can be seen as a generalization of padding oracle attacks (Vaudenay, Eurocrypt 2002). It exploits a subtle correlation between the block cipher mode of operation, the character encoding of encrypted text, and the response behaviour of a Web Service if an XML message cannot be parsed correctly.