Optimal asymmetric encryption padding

About: Optimal asymmetric encryption padding is a research topic. Over the lifetime, 299 publications have been published within this topic receiving 43101 citations. The topic is also known as: OAEP.

Optimal asymmetric encryption and signature paddings

Abstract: Strong security notions often introduce strong constraints on the construction of cryptographic schemes: semantic security implies probabilistic encryption, while the resistance to existential forgeries requires redundancy in signature schemes. Some paddings have thus been designed in order to provide these minimal requirements to each of them, in order to achieve secure primitives. A few years ago, Coron et al. suggested the design of a common construction, a universal padding, which one could apply for both encryption and signature. As a consequence, such a padding has to introduce both randomness and redundancy, which does not lead to an optimal encryption nor an optimal signature. In this paper, we refine this notion of universal padding, in which a part can be either a random string in order to introduce randomness or a zero-constant string in order to introduce some redundancy. This helps us to build, with a unique padding, optimal encryption and optimal signature: first, in the random-permutation model, and then in the random-oracle model. In both cases, we study the concrete sizes of the parameters, for a specific security level: The former achieves an optimal bandwidth.

On all-or-nothing transforms and password-authenticated key exchange protocols

Abstract: This thesis provides a formal analysis of two kinds of cryptographic objects that used to be treated with much less rigor: All-or-Nothing Transforms (AONTs) and Password-Authenticated Key Exchange protocols. For both, novel formal definitions of security are given, and then practical and efficient constructions are proven secure. The constructions for password-authenticated key exchange are novel, and the AONT construction is an application of an existing scheme to a new area. AONTs have been proposed by Rivest as a mode of operation for block ciphers. An AONT is an unkeyed, invertible, randomized transformation, with the property that it is hard to invert unless all of the output is known. Applications of AONTs include improving the security and efficiency of encryption. We give several strong formal definitions of security for AONTs. We then prove that Optimal Asymmetric Encryption Padding (OAEP) satisfies these definitions (in the random oracle model). This is the first construction of an AONT that has been proven secure in the strong sense. We also show that no AONT can achieve substantially better security than OAEP. The second part of this thesis is about password-authenticated key exchange protocols. We present a new protocol called PAK which is the first such Diffie-Hellman-based protocol to provide a formal proof of security (in the random oracle model) against active adversaries. In addition to the PAK protocol that provides mutual explicit authentication, we also show a more efficient protocol called PPK that is provably secure in the implicit-authentication model. We then extend PAK to a protocol called PAK-X, in which one side (the client) stores a plaintext version of the password, while the other side (the server) only stores a verifier for the password. We formally prove security of PAK-X, even when the server is compromised. Our formal model for password-authenticated key exchange is new, and may be of independent interest. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)

Side Channel Attacks on CBC Encrypted Messages in the PKCS#7 Format

Abstract: Vaudenay has shown in [5] that a CBC encryption mode ([2], [9]) combined with the PKCS#5 padding [3] scheme allows an attacker to invert the underlying block cipher, provided she has access to a valid-padding oracle which for each input ciphertext tells her whether the corresponding plaintext has a valid padding or not. Having on mind the countermeasures against this attack, different padding schemes have been studied in [1]. The best one is referred to as the ABYT-PAD. It is designed for byte-oriented messages. It removes the validpadding oracle, thereby defeating Vaudenay's attack, since all deciphered plaintexts are valid in this padding scheme. In this paper, we try to combine the well-known cryptographic message syntax standard PKCS#7 [8] with the use of ABYT-PAD instead of PKCS#5. Let us assume that we have access to a PKCS#7CONF oracle that tells us for a given ciphertext (encapsulated in the PKCS#7 structure) whether the deciphered plaintext is correct or not according to the PKCS#7 (v1.6) syntax. This is probably a very natural assumption, because applications usually have to reflect this situation in its behavior. It could be a message for the user, an API error message, an entry in the log file, different timing behavior, etc. We show that access to such an oracle again enables an attacker to invert the underlying block cipher. The attack requires single captured ciphertext and approximately 128 oracle calls per one ciphertext byte. It shows that we cannot hope to fully solve problems with side channel attacks on the CBC encryption mode by using a “magic” padding method or an obscure message-encoding format. Strong cryptographic integrity checks of ciphertexts should be incorporated instead.

The modification of RSA algorithm to adapt fully homomorphic encryption algorithm in cloud computing

Abstract: RSA is Partially homomorphic cryptosystem, based on the features of the RSA algorithm, we design a encryption system, this encryption system firstly discriminates whether the values of the public key and private key generated during the encryption process contain prime number, then combines with the Pascal's triangle theorem and RSA algorithm model and inductive methods to construct a new cryptosystem that meets homomorphic computation of some operations on cihpertexts(e.g., additions, multiplications), Thus the new cryptosystem satisfies fully homomorphic encryption in cloud computing.