Earticle

개인 의료정보는 개인의 존엄성과 가치를 소중히 여기는 개인정보의 한 부분으로서, 만약 불법적인 유출이 되었을 때 한 개인에게 심각한 사회적 편견과 불이익이 돌아올 수 있다. 또한, 그 의료정보는 쓰임새가 많아 그 가치가 상대적으로 높아 내, 외부적인 위협이 지속되고 있는 것이 현실이다. 이에 본 논문에서는 암호문 정책 속성중심 프락시 재암호 기법을 사용하여 블 록체인 기반 환자 프라이버시가 보장되는 의료정보공유 프레임워크를 제안한다. 제안하는 프레임워크는 먼저 블록체인을 사 용하여 의무기록의 무결성과 투명성을 보장하고, 스텔스 주소를 사용하여 의사-환자의 연계불가성을 제안한다. 또한 암호문 기반 속성중심 암호를 이용하여 세밀한 접근제어가 가능하고, 환자 미동의 및 응급 상황에서의 정보공유가 가능하다.

Medical record is part of the personal information that values the dignity and value of an individual, and can lead to serious social prejudice and disadvantage to an individual when it is breached illegally. In addition, the medical record has been highly threatened because its value is relatively high, and external threats are continuing. In this paper, we propose a medical record sharing framework that guarantees patient's privacy based on blockchain using ciphertext policy-based attribute based proxy re-encryption scheme. The proposed framework first uses the blockchain technology to ensure the integrity and transparency of medical records, and uses the stealth address to build the unlinkability between physician and patient. Besides, the ciphertext policy attribute-based proxy re-encryption scheme is used to enable fine-grained access control, and it is possible to share information in emergency situations without patient’s agreement.

In Distributed systems, the users with a certain set of attributes can only be able to access the data. At present this process can be done through a trusted server where we will store the data and there will be certain constraints on the access of the data. In this case there will be a possibility to compromise the data and so the confidentiality of the data is lost. An Attribute-Based Encryption (ABE) is an encryption scheme, where users with some attributes can decrypt ciphertexts associated with these attributes. Now this is our turn to develop a system with a more complex policy of access of the encrypted data and which can be called as Ciphertext Policy Attribute-based Encryption (CP-ABE). By using this method the information can't be traded off even through the trusted server where the information is put away. These methods are also secure against the collusion attack. In this method attributes are generally assigned in the form of access trees. The attributes are placed at the leaf nodes of this access tree. In older Attribute-based encryption strategies encrypted data is described by the attributes and policies are given to the user’s keys, while in our system users credentials are described by the attributes and there will be a policy where it tells us about who should access or decrypt the data. So, this type of access method is very much closer to the Role-based attribute-based encryption.

In this work, we design a method for efficient revocation within Ciphertext-Policy Attribute-Based Encryption scheme. Our main technical innovation is based on linear secret sharing and binary tree techniques, every user in system is assigned with both a set of attributes and a unique identifier. Any user can also be efficiently revoked by using this identifier. Furthermore, this technique resulted in two key contributions: the size of the cryptographic key material is smaller and encryption/decryption cannot be affected with an unbounded number of revoked users. Then, the scheme is proved to be secure under the q-MEBDH assumption in the standard model. The efficiency is also optimized that the size of user’s private key has only a constant increase. The revocation information is embedded in the ciphertext so that the fine-grained access control is more flexible.

Green and Ateniese introduced the notion of identity-based proxy re-encryption (IBPRE), whereby the proxy can covert a ciphertext encrypted under the delegator's identity to an encryption under the delegatee's identity of the same message. In some situations, biometric, such as dactylogram, was used as identities. However, these biometric identities will inherently have some noise when they are sampled each time. To make identity-based proxy re-encryption flexible on identities, we introduced a new primitive called fuzzy identity-based proxy re-encryption (FIB-PRE), in which an identity is viewed as a set of descriptive attributes. In a fuzzy identity-based proxy re-encryption scheme, an identity Wll can decrypt a ciphertext re-encrypted under another identity W, if and only if W and W l are close to each other as measured by the “set overlap” distance metric. In this work, we first formulate the security model of a FIB-PRE scheme. Finally, we present a construction of FIB-PRE and prove its CCA security under the decisional bilinear Diffie-Hellman (DBDH) assumption in the random model.

Although current online social networks (OSNs) schemes propose to encrypt data before sharing, the enforcement of access policies over encrypted data has become a challenging task, and the OSNs currently do not provide any mechanism to allow users to update access policies. In this paper, we propose a ciphertext sharing scheme in cloud-based OSNs, which allows the users to outsource encrypted data to the OSNs service provider for sharing. In order to meet the authorization requirement, we present a multiparty access control model based on ciphertext-policy attribute-based proxy re-encryption, which enables the access control of encrypted data associated with multiple users. On the basis of ciphertext-policy attribute-based encryption, the owners can customize the access policy of their own data. Based on proxy re-encryption, the disseminators such as friends and group members can further customize the access policy of the owners’ data upon existing access policy. Besides, we achieve immediate user revocation based on secret sharing without issuing new attribute secret keys to unrevoked users. The security and performance analysis show that our proposed scheme is secure, efficient and practical.

As Hierarchical Identity Based Encryption (HIBE) system usually maps the true institutional structure of an organization or entity relationship between objects in real world, It is important that computation & communication complexity of private key, ciphertext, cryptographic computations and so on related to an entity in the hierarchy is independent to the hierarchy depth of the entity. Moreover, key escrow problem that any non-leaf entity in a hierarchical identity based cryptosystem can derive private keys for its descendants with use of its private key should be resolved, in order to prevent any entity from behaving on behalf of its descendants. In this paper, a new technique is introduced for composing a private key for each individual entity in HIBE system by differentiating between non-local identifiers and local identifiers of the identity of the entity. That we call Identifier Discrimination. With the technique, A selective identity secure HIBE system is constructed under Decisional Bilinear Diffie-Hellman (DBDH) assumption without using random oracles, where the private key and the ciphertext consist of constant number of group elements, and decryption requires only three bilinear map computations, regardless of the identity hierarchy depth. Moreover, in contrast to previous HIBE constructions, where private key for an entity can be derived by its ancestors with direct use of their private keys, key escrow problem inherent in identity based cryptosystems is resolved in our HIBE construction. Privilege of deriving private keys for an entity can be delegated to any of its ancestors through authorization by distributing specifically crafted values to the ancestor in our HIBE system, that we call Authorized Delegation.

In this work, an oblivious transfer with complex access control scheme that is constructed based on ciphertext policy attribute based encryption (CP-ABE) scheme is proposed. In this scheme, the database server can enforce fine grained access control for each record where the authorized user is allowed to access, but the unauthorized user cannot, whereas it learns neither which record a user accesses, nor which attributes a user has. This scheme has the advantages as follows: First, it allows the expressive access control policies where access structures are based on linear secret sharing scheme that directly supports AND, OR and Threshold gates. Second, the communication complexity in this scheme is constant in the numbers of records which have been accessed. Third, this scheme is constructed in prime order bilinear group. Fourth, this scheme is secure in the standard model. To the best of our knowledge, this scheme is the first to obtain these features simultaneously.

Nowadays, more and more users outsource their data to third party cloud storage servers for the purpose of sharing, so cloud data sharing becomes one of the popular services offered by cloud service providers. However, the third party storage servers in cloud data sharing systems, which are not fully trusted by data owners, make access control to the shared data a challenging issue. Although Ciphertext-Policy Attribute-Based Encryption (CP-ABE) is an emerging cryptographic solution for this issue, dealing with dynamic changes to users' access privileges (attribute revocation) in its practical applications as cloud data sharing systems is a real challenge. To overcome this challenge, we propose a fine-grained access control scheme for cloud data sharing systems by designing secure and efficient attribute-revocable CP-ABE scheme. Our scheme only allows non-revoked users in the attribute group to update their secret key by themselves using their unique key-update keys and the ciphertexts are updated by minimally trusted cloud server using a ciphertext-update key. Compared with the existing access controls achieved by attribute-revocable CP-ABE schemes, our proposed access control scheme reduces the trust degree of the cloud server in the attribute revocation mechanism. Furthermore, the analysis indicates that our access control scheme is more secure and efficient to apply to practical scenarios.

본 논문에서는 Bethencourt등의 CP-ABE에서 효율적인 속성값 철회 기법에 대해 알아본다. 기존에 제안된 속성값 철회 기법은 대부분 KP-ABE에 대한 것이며, CP-ABE에서 속성값 철회는 철회를 위한 메시지 크기가 철회자에 비례해 커지고 NOT연산을 필요로 한다는 측면에서 효율적이지 못하다. 이에 대해 Bethencourt등의 CP-ABE와 기존의 속성값 철회 기법에 대해 알아본 후 Bethencourt등의 CP-ABE에서 효율적인 속성값 철회 기법에 대해 제시하고자 한다.

본 논문에서는 Bethencourt등의 CP-ABE에서 효율적인 속성값 철회 기법에 대해 알아본다. 기존에 제안된 속성값 철회 기법은 대부분 KP-ABE에 대한 것이며, CP-ABE에서 속성값 철회는 철회를 위한 메시지 크기가 철회자에 비례해 커지고 NOT연산을 필요로 한다는 측면에서 효율적이지 못하다. 이에 대해 Bethencourt등의 CP-ABE와 기존의 속성값 철회 기법에 대해 알아본 후 Bethencourt등의 CP-ABE에서 효율적인 속성값 철회 기법에 대해 제시하고자 한다.

CP-ABE 방식은 신뢰된 서버 없이 접근 제어 메카니즘을 구현할 수 있다. 본 논문에서는 권한을 부여받은 사용자가 민감한 데이터에 접근할 수 있도록 CP-ABE 방식으로 속성기반 접근 제어 메카니즘을 제안한다. CP-ABE 개념은 암호문에서 접근 제어 방법을 포함하는 것이다. 만약 사용자가 암호문의 접근 구조를 통해 속성을 가진다면 암호문은 복호될 수 있다. 본 논문에서는 제안한 CP-ABE 방식이 비단조 접근 구조로 표현됨을 증명하고 다른 CP-ABE 방식들과 성능 비교한다.

A ciphertext policy-attribute based encryption(CP-ABE) scheme can be used to realize access control mechanism without a trusted server. We propose an attribute-based access control mechanism by incorporating a CP-ABE scheme to ensure only authorized users can access the sensitive data. The idea of CP-ABE is to include access control policy in the ciphertexts, in which they can only be decrypted if a user possesses attributes that pass through the ciphertext's access structure. In this paper, we prove a secure CP-ABE scheme where the policy can be expressed in non-monotonic access structures. We further compare the performance of our scheme with the existing CP-ABE schemes.

It has been widely believed that one can obtain $\iota$-Hierarchical Identity Based Encryption (HIBE) scheme secure against chosen ciphetext attacks from ($\iota$+1)-HIBE scheme secure against chosen plaintext attacks. In this paper, however, we show that when applying two concrete HIBE schemes that Boneh et al. [1, 2] proposed, chosen ciphertext secure $\iota$-HIBE schemes are directly derived from chosen plaintext secure $\iota$-HIBE schemes. Our constructions are based on a one-time signature-based transformation that Canetti et at.[3] proposed. The security of our schemes is proved in the selective-ID suity model without using random oracles.

In classical encryption schemes, data is encrypted under a single key that is associated with a user or group. In Ciphertext-Policy Attribute-Based Encryption(CP-ABE) keys are associated with attributes of users, given to them by a central trusted authority, and data is encrypted under a logical formula over these attributes. We extend this idea to the case where an arbitrary number of independent parties can be present to maintain attributes and their corresponding secret keys. We present a scheme for multi-authority CP-ABE, propose the first two constructions that fully implement the scheme, and prove their security against chosen plaintext attacks.

An access control system is needed to ensure only authorized users can access a sensitive resource. We propose a secure access control based on a fully secure and fine grained ciphertext-policy attribute-based encryption scheme. The access control for a sensitive resource is ensured by encrypting it with encryption algorithm from the CP-ABE scheme parameterized by an access control policy. Furthermore, the proposed access control supports non-monotone type access control policy. The ciphertext only can be recovered by users whose attributes satisfy the access control policy. We also implement and measure the performance of our proposed access control. The results of experiments show that our proposed secure access control is feasible.

격자 기반 암호는 양자 컴퓨터 공격에 대응 가능한 포스트 양자 암호 중 하나로 알려져 있다. 그 중 ring-LWE 문제는 LWE의 대수적 변종으로 벡터 대신 환(ring)의 원소를 이용한다. 포스트 양자 암호라 할지라도 실제 디바이스에 이를 적용 할 때 부채널 분석 취약점이 존재한다는 것은 이미 알려져 있다. 실제 2016년 Park 등은 Roy 등이 제안한 NTT를 이용한 ring-LWE 기반 공개키 암호시스템의 SPA 취약점을 보고했으며, Reparaz 등은 Roy 암호에 대한 DPA 공격 및 대응법을 2015년과 2016년에 제안하였다. 본 논문에서는 Roy 암호에 대하여 Park 등이 제안한 선택 암호문 SPA 공격이 NTT를 적용하지 않은 Lyubashevsky 암호의 경우에도 동일하게 적용 가능함을 보인다. 또한 선택 암호문 SPA 공격에 안전한 대응기법을 제안한고 실험적으로 안전성을 검증한다.

A lattice-based cryptography is known as one of the post-quantum cryptographies. Ring-LWE problem is an algebraic variant of LWE, which operates over elements of polynomial rings instead of vectors. It is already known that post-quantum cryptography has side-channel analysis vulnerability. In 2016, Park et al. reported a SPA vulnerability of the public key cryptosystem, which is proposed by Roy et al., based on the ring-LWE problem. In 2015 and 2016, Reparaz et al. proposed DPA attack and countermeasures against Roy cryptosystem. In this paper, we show that the chosen ciphertext SPA attack is also possible for Lyubashevsky cryptosystem which does not use NTT. And then we propose a countermeasure against CCSPA(Chosen Ciphertext SPA) attack and we also show through experiment that our proposed countermeasure is secure.

본 논문에서는 잘못된 의미의 내용을 전달할 수 있는 문제점 해결을 위하여 부정확한 암호문의 복호화를 방지한 암호화 알고리즘을 설계하였다. 알고리즘은 복호화시 암호문의 에러를 체크하여 한 비트 이상의 에러가 포함되었을 경우 현재 진행중인 복호화를 중단함과 동시에 이전에 복호화 되어있는 복호문까지 모두 제거하여 수신자에게 아무런 내용도 전달하지 않게 된다. 실험은 비밀키의 대표적인 알고리즘인 DES알고리즘과 비교하였다. 실험결과에서와 같이 에러 비트가 포함되어 있는 부분은 원문과 다른 내용으로 복호화가 되었지만 100%의 복호률을 보였다. 제안한 알고리즘에서는 에러 체크를 통한 복호화로 에러 발생시 0%의 복호률을 나타내었다.

In this paper, we designed the encryption algorithm which protects ciphertext from deciphering solve the problem that it can pass the incorrect meaning. The algorithm checks the error of ciphertext during deciphering, and then if there are some errors of more than one bit. It stops the deciphering sequence and it doesn't pass any contents to receiver by removing the deciphering contents which was already done. The experiment compared with DES algorithm that is representative algorithm of secret key. As the result of experiment, the part that error bit was included with was deciphered to one that was different from plaintext, but it showed 100% decipher_rate. by the algorithm showed decipher_rate of 0% with deciphering through the error check.