cryptography Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...

, signcryption is a public-key primitive that simultaneously performs the functions of both

digital signature A digital signature is a mathematical scheme for verifying the authenticity of digital messages or documents. A valid digital signature, where the prerequisites are satisfied, gives a recipient very high confidence that the message was created b ...

and

encryption In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...

. Encryption and digital signature are two fundamental cryptographic tools that can guarantee the

confidentiality Confidentiality involves a set of rules or a promise usually executed through confidentiality agreements that limits the access or places restrictions on certain types of information. Legal confidentiality By law, lawyers are often required ...

integrity Integrity is the practice of being honest and showing a consistent and uncompromising adherence to strong moral and ethical principles and values. In ethics, integrity is regarded as the honesty and truthfulness or accuracy of one's actions. Inte ...

, and

non-repudiation Non-repudiation refers to a situation where a statement's author cannot successfully dispute its authorship or the validity of an associated contract. The term is often seen in a legal setting when the authenticity of a signature is being challenged ...

. Until 1997, they were viewed as important but distinct building blocks of various cryptographic systems. In public key schemes, a traditional method is to digitally sign a message then followed by an encryption (signature-then-encryption) that can have two problems: Low efficiency and high cost of such summation, and the case that any arbitrary scheme cannot guarantee security. Signcryption is a relatively new cryptographic technique that is supposed to perform the functions of digital signature and encryption in a single logical step and can effectively decrease the computational costs and communication overheads in comparison with the traditional signature-then-encryption schemes. Signcryption provides the properties of both digital signatures and encryption schemes in a way that is more efficient than signing and encrypting separately. This means that at least some aspect of its efficiency (for example the computation time) is better than any hybrid of digital signature and encryption schemes, under a particular model of security. Note that sometimes

hybrid encryption In cryptography, a hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem. Public-key cryptosystems are convenient in that they do not require the sender and receiv ...

can be employed instead of simple encryption, and a single session-key reused for several encryptions to achieve better overall efficiency across many signature-encryptions than a signcryption scheme but the session-key reuse causes the system to lose security under even the relatively weak CPA model. This is the reason why a random session key is used for each message in a hybrid encryption scheme but for a given

level of security In cryptography, security level is a measure of the strength that a cryptographic primitive — such as a cipher or hash function — achieves. Security level is usually expressed as a number of "bits of security" (also security strength ...

(i.e., a given model, say CPA), a signcryption scheme should be more efficient than any simple signature-hybrid encryption combination.

History

The first signcryption scheme was introduced by

Yuliang Zheng Yuliang Zheng is the Chair of the Department of Computer and Information Sciences at the University of Alabama at Birmingham. He is best known for inventing the Signcryption cryptographic primitive that combines the digital signature and encryption ...

in 1997. Zheng also proposed an

elliptic curve In mathematics, an elliptic curve is a smooth, projective, algebraic curve of genus one, on which there is a specified point . An elliptic curve is defined over a field and describes points in , the Cartesian product of with itself. If ...

-based signcryption scheme that saves 58% of computational and 40% of communication costs when it is compared with the traditional elliptic curve-based signature-then-encryption schemes. There are also many other signcryption schemes that have been proposed throughout the years, each of them having its own problems and limitations, while offering different levels of security and computational costs.

Scheme

A signcryption scheme typically consists of three algorithms: Key Generation (Gen), Signcryption (SC), and Unsigncryption (USC). Gen generates a pair of keys for any user, SC is generally a probabilistic algorithm, and USC is most likely deterministic. Any signcryption scheme should have the following properties:M. Tooran
"Cryptanalysis of an Elliptic Curve-based Signcryption Scheme"
International Journal of Network Security, Vol.10, No.1, pp.51–56, Jan. 2010. # ''Correctness'': Any signcryption scheme should be verifiably correct. # ''Efficiency'': The computational costs and communication overheads of a signcryption scheme should be smaller than those of the best known signature-then-encryption schemes with the same provided functionalities. # ''Security'': A signcryption scheme should simultaneously fulfill the security attributes of an encryption scheme and those of a digital signature. Such additional properties mainly include: Confidentiality, Unforgeability, Integrity, and Non-repudiation. Some signcryption schemes provide further attributes such as Public verifiability and

Forward secrecy In cryptography, forward secrecy (FS), also known as perfect forward secrecy (PFS), is a feature of specific key agreement protocols that gives assurances that session keys will not be compromised even if long-term secrets used in the session key ...

of message confidentiality while the others do not provide them. Such properties are the attributes that are required in many applications while the others may not require them. Hereunder, the above-mentioned attributes are briefly described. ::* ''Confidentiality'': It should be computationally infeasible for an adaptive attacker to gain any partial information on the contents of a signcrypted text, without knowledge of the sender's or designated recipient's private key. ::* ''Unforgeability'': It should be computationally infeasible for an adaptive attacker to masquerade as an honest sender in creating an authentic signcrypted text that can be accepted by the unsigncryption algorithm. ::* ''Non-repudiation'': The recipient should have the ability to prove to a third party (e.g. a judge) that the sender has sent the signcrypted text. This ensures that the sender cannot deny his previously signcrypted texts. ::* ''Integrity'': The recipient should be able to verify that the received message is the original one that was sent by the sender. ::* ''Public verifiability'': Any third party without any need for the private key of sender or recipient can verify that the signcrypted text is the valid signcryption of its corresponding message. ::* ''Forward secrecy of message confidentiality'': If the long-term private key of the sender is compromised, no one should be able to extract the plaintext of previously signcrypted texts. In a regular signcryption scheme, when the long-term private key is compromised, all the previously issued signatures will not be trustworthy any more. Since the threat of key exposure is becoming more acute as the cryptographic computations are performed more frequently on poorly protected devices such as mobile phones, forward secrecy seems an essential attribute in such systems.

Applications

Signcryption is seen to have several applications including the following: * Secure and authentic

email Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" meant ...

. *

E-commerce E-commerce (electronic commerce) is the activity of electronically buying or selling of products on online services or over the Internet. E-commerce draws on technologies such as mobile commerce, electronic funds transfer, supply chain manageme ...

and

M-commerce The term mobile commerce was originally coined in 1997 by Kevin Duffey at the launch of the Global Mobile Commerce Forum, to mean "the delivery of electronic commerce capabilities directly into the consumer’s hand, anywhere, via wireless techno ...

applications that often require

authenticity Authenticity or authentic may refer to: * Authentication, the act of confirming the truth of an attribute Arts and entertainment * Authenticity in art, ways in which a work of art or an artistic performance may be considered authentic Music * A ...

, and perhaps

References

{{Reflist Public-key cryptography

History

Scheme

Applications

See also

References