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7,401 | With a digital signature scheme, the central office can arrange beforehand to have a public key on file whose private key is known only to the branch office.
The branch office can later sign a message and the central office can use the public key to verify the signed message was not a forgery before acting on it.
A for... |
7,402 | Encryption can hide the content of the message from an eavesdropper, but encryption on its own may not let recipient verify the message's authenticity, or even detect selective modifications like changing a digit—if the bank's offices simply encrypted the messages they exchange, they could still be vulnerable to forger... |
7,403 | Replays.
A digital signature scheme on its own does not prevent a valid signed message from being recorded and then maliciously reused in a replay attack.
For example, the branch office may legitimately request that bank transfer be issued once in a signed message.
If the bank doesn't use a system of transaction ids in... |
7,404 | Uniqueness and malleability of signatures.
A signature itself cannot be used to uniquely identify the message it signs—in some signature schemes, every message has a large number of possible valid signatures from the same signer, and it may be easy, even without knowledge of the private key, to transform one valid sign... |
7,405 | Authenticating a public key.
Prior knowledge of a public key can be used to verify authenticity of a signed message, but not the other way around—prior knowledge of a signed message cannot be used to verify authenticity of a public key.
In some signature schemes, given a signed message, it is easy to construct a public... |
7,406 | Non-repudiation, or more specifically non-repudiation of origin, is an important aspect of digital signatures. By this property, an entity that has signed some information cannot at a later time deny having signed it. Similarly, access to the public key only does not enable a fraudulent party to fake a valid signature. |
7,407 | Note that these authentication, non-repudiation etc. properties rely on the secret key not having been revoked prior to its usage. Public revocation of a key-pair is a required ability, else leaked secret keys would continue to implicate the claimed owner of the key-pair. Checking revocation status requires an "online"... |
7,408 | In their foundational paper, Goldwasser, Micali, and Rivest lay out a hierarchy of attack models against digital signatures: |
7,409 | They also describe a hierarchy of attack results: |
7,410 | The strongest notion of security, therefore, is security against existential forgery under an adaptive chosen message attack. |
7,411 | All public key / private key cryptosystems depend entirely on keeping the private key secret. A private key can be stored on a user's computer, and protected by a local password, but this has two disadvantages: |
7,412 | A more secure alternative is to store the private key on a smart card. Many smart cards are designed to be tamper-resistant . In a typical digital signature implementation, the hash calculated from the document is sent to the smart card, whose CPU signs the hash using the stored private key of the user, and then return... |
7,413 | Entering a PIN code to activate the smart card commonly requires a numeric keypad. Some card readers have their own numeric keypad. This is safer than using a card reader integrated into a PC, and then entering the PIN using that computer's keyboard. Readers with a numeric keypad are meant to circumvent the eavesdroppi... |
7,414 | Smart card design is an active field, and there are smart card schemes which are intended to avoid these particular problems, despite having few security proofs so far. |
7,415 | One of the main differences between a digital signature and a written signature is that the user does not "see" what they sign. The user application presents a hash code to be signed by the digital signing algorithm using the private key. An attacker who gains control of the user's PC can possibly replace the user appl... |
7,416 | To protect against this scenario, an authentication system can be set up between the user's application and the signing application. The general idea is to provide some means for both the user application and signing application to verify each other's integrity. For example, the signing application may require all req... |
7,417 | One of the main differences between a cloud based digital signature service and a locally provided one is risk. Many risk averse companies, including governments, financial and medical institutions, and payment processors require more secure standards, like FIPS 140-2 level 3 and FIPS 201 certification, to ensure the... |
7,418 | Technically speaking, a digital signature applies to a string of bits, whereas humans and applications "believe" that they sign the semantic interpretation of those bits. In order to be semantically interpreted, the bit string must be transformed into a form that is meaningful for humans and applications, and this is d... |
7,419 | An ink signature could be replicated from one document to another by copying the image manually or digitally, but to have credible signature copies that can resist some scrutiny is a significant manual or technical skill, and to produce ink signature copies that resist professional scrutiny is very difficult. |
7,420 | Digital signatures cryptographically bind an electronic identity to an electronic document and the digital signature cannot be copied to another document. Paper contracts sometimes have the ink signature block on the last page, and the previous pages may be replaced after a signature is applied. Digital signatures can... |
7,421 | RSA |
7,422 | DSA |
7,423 | ECDSA |
7,424 | EdDSA |
7,425 | RSA with SHA |
7,426 | ECDSA with SHA |
7,427 | ElGamal signature scheme as the predecessor to DSA, and variants Schnorr signature and Pointcheval–Stern signature algorithm |
7,428 | Rabin signature algorithm |
7,429 | Pairing-based schemes such as BLS |
7,430 | NTRUSign is an example of a digital signature scheme based on hard lattice problems |
7,431 | Undeniable signatures |
7,432 | Aggregate signatureru – a signature scheme that supports aggregation: Given n signatures on n messages from n users, it is possible to aggregate all these signatures into a single signature whose size is constant in the number of users. This single signature will convince the verifier that the n users did indeed sign t... |
7,433 | Signatures with efficient protocols – are signature schemes that facilitate efficient cryptographic protocols such as zero-knowledge proofs or secure computation. |
7,434 | Most digital signature schemes share the following goals regardless of cryptographic theory or legal provision: |
7,435 | Only if all of these conditions are met will a digital signature actually be any evidence of who sent the message, and therefore of their assent to its contents. Legal enactment cannot change this reality of the existing engineering possibilities, though some such have not reflected this actuality. |
7,436 | Legislatures, being importuned by businesses expecting to profit from operating a PKI, or by the technological avant-garde advocating new solutions to old problems, have enacted statutes and/or regulations in many jurisdictions authorizing, endorsing, encouraging, or permitting digital signatures and providing for the... |
7,437 | Adoption of technical standards for digital signatures have lagged behind much of the legislation, delaying a more or less unified engineering position on interoperability, algorithm choice, key lengths, and so on what the engineering is attempting to provide. |
7,438 | Some industries have established common interoperability standards for the use of digital signatures between members of the industry and with regulators. These include the Automotive Network Exchange for the automobile industry and the SAFE-BioPharma Association for the healthcare industry. |
7,439 | In several countries, a digital signature has a status somewhat like that of a traditional pen and paper signature, as in the
1999 EU digital signature directive and 2014 EU follow-on legislation. Generally, these provisions mean that anything digitally signed legally binds the signer of the document to the terms there... |
7,440 | For technical reasons, an encryption scheme usually uses a pseudo-random encryption key generated by an algorithm. It is possible to decrypt the message without possessing the key but, for a well-designed encryption scheme, considerable computational resources and skills are required. An authorized recipient can easily... |
7,441 | Historically, various forms of encryption have been used to aid in cryptography. Early encryption techniques were often used in military messaging. Since then, new techniques have emerged and become commonplace in all areas of modern computing. Modern encryption schemes use the concepts of public-key and symmetric-key.... |
7,442 | One of the earliest forms of encryption is symbol replacement, which was first found in the tomb of Khnumhotep II, who lived in 1900 BC Egypt. Symbol replacement encryption is “non-standard,” which means that the symbols require a cipher or key to understand. This type of early encryption was used throughout Ancient Gr... |
7,443 | Around 800 AD, Arab mathematician Al-Kindi developed the technique of frequency analysis – which was an attempt to systematically crack Caesar ciphers. This technique looked at the frequency of letters in the encrypted message to determine the appropriate shift. This technique was rendered ineffective after the creatio... |
7,444 | Around 1790, Thomas Jefferson theorized a cipher to encode and decode messages in order to provide a more secure way of military correspondence. The cipher, known today as the Wheel Cipher or the Jefferson Disk, although never actually built, was theorized as a spool that could jumble an English message up to 36 charac... |
7,445 | A similar device to the Jefferson Disk, the M-94, was developed in 1917 independently by US Army Major Joseph Mauborne. This device was used in U.S. military communications until 1942. |
7,446 | In World War II, the Axis powers used a more advanced version of the M-94 called the Enigma Machine. The Enigma Machine was more complex because unlike the Jefferson Wheel and the M-94, each day the jumble of letters switched to a completely new combination. Each day's combination was only known by the Axis, so many th... |
7,447 | Today, encryption is used in the transfer of communication over the Internet for security and commerce. As computing power continues to increase, computer encryption is constantly evolving to prevent eavesdropping attacks. With one of the first "modern" cipher suites, DES, utilizing a 56-bit key with 72,057,594,037,927... |
7,448 | In the context of cryptography, encryption serves as a mechanism to ensure confidentiality. Since data may be visible on the Internet, sensitive information such as passwords and personal communication may be exposed to potential interceptors. The process of encrypting and decrypting messages involves keys. The two mai... |
7,449 | Many complex cryptographic algorithms often use simple modular arithmetic in their implementations. |
7,450 | In symmetric-key schemes, the encryption and decryption keys are the same. Communicating parties must have the same key in order to achieve secure communication. The German Enigma Machine utilized a new symmetric-key each day for encoding and decoding messages. |
7,451 | In public-key encryption schemes, the encryption key is published for anyone to use and encrypt messages. However, only the receiving party has access to the decryption key that enables messages to be read. Public-key encryption was first described in a secret document in 1973; beforehand, all encryption schemes were s... |
7,452 | RSA is another notable public-key cryptosystem. Created in 1978, it is still used today for applications involving digital signatures. Using number theory, the RSA algorithm selects two prime numbers, which help generate both the encryption and decryption keys. |
7,453 | A publicly available public-key encryption application called Pretty Good Privacy was written in 1991 by Phil Zimmermann, and distributed free of charge with source code. PGP was purchased by Symantec in 2010 and is regularly updated. |
7,454 | Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. For example, the Computer Security Institute reported that in 2007, 71% of companies surveyed utilized encryption for some of their dat... |
7,455 | Encryption is also used to protect data in transit, for example data being transferred via networks , mobile telephones, wireless microphones, wireless intercom systems, Bluetooth devices and bank automatic teller machines. There have been numerous reports of data in transit being intercepted in recent years. Data shou... |
7,456 | Conventional methods for permanently deleting data from a storage device involve overwriting the device's whole content with zeros, ones, or other patterns – a process which can take a significant amount of time, depending on the capacity and the type of storage medium. Cryptography offers a way of making the erasure a... |
7,457 | Encryption is used in the 21st century to protect digital data and information systems. As computing power increased over the years, encryption technology has only become more advanced and secure. However, this advancement in technology has also exposed a potential limitation of today's encryption methods. |
7,458 | The length of the encryption key is an indicator of the strength of the encryption method. For example, the original encryption key, DES , was 56 bits, meaning it had 2^56 combination possibilities. With today's computing power, a 56-bit key is no longer secure, being vulnerable to brute force attacks. |
7,459 | Quantum computing utilizes properties of quantum mechanics in order to process large amounts of data simultaneously. Quantum computing has been found to achieve computing speeds thousands of times faster than today's supercomputers. This computing power presents a challenge to today's encryption technology. For example... |
7,460 | While quantum computing could be a threat to encryption security in the future, quantum computing as it currently stands is still very limited. Quantum computing currently is not commercially available, cannot handle large amounts of code, and only exists as computational devices, not computers. Furthermore, quantum co... |
7,461 | Encryption is an important tool but is not sufficient alone to ensure the security or privacy of sensitive information throughout its lifetime. Most applications of encryption protect information only at rest or in transit, leaving sensitive data in clear text and potentially vulnerable to improper disclosure during pr... |
7,462 | In response to encryption of data at rest, cyber-adversaries have developed new types of attacks. These more recent threats to encryption of data at rest include cryptographic attacks, stolen ciphertext attacks, attacks on encryption keys, insider attacks, data corruption or integrity attacks, data destruction attacks,... |
7,463 | The question of balancing the need for national security with the right to privacy has been debated for years, since encryption has become critical in today's digital society. The modern encryption debate started around the '90s when US government tried to ban cryptography because, according to them, it would threaten ... |
7,464 | Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code or a digital signature usually done by a hashing algorithm or a PGP signature. Authenticated encry... |
7,465 | Integrity protection mechanisms such as MACs and digital signatures must be applied to the ciphertext when it is first created, typically on the same device used to compose the message, to protect a message end-to-end along its full transmission path; otherwise, any node between the sender and the encryption agent coul... |
7,466 | Even when encryption correctly hides a message's content and it cannot be tampered with at rest or in transit, a message's length is a form of metadata that can still leak sensitive information about the message. For example, the well-known CRIME and BREACH attacks against HTTPS were side-channel attacks that relied on... |
7,467 | Padding a message's payload before encrypting it can help obscure the cleartext's true length, at the cost of increasing the ciphertext's size and introducing or increasing bandwidth overhead. Messages may be padded randomly or deterministically, with each approach having different tradeoffs. Encrypting and padding mes... |
7,468 | Read/write conflicts, commonly termed interlocking in accessing the same shared memory location simultaneously are resolved by one of the following strategies: |
7,469 | Here, E and C stand for 'exclusive' and 'concurrent' respectively. The read causes no discrepancies while the concurrent write is further defined as: |
7,470 | Several simplifying assumptions are made while considering the development of algorithms for PRAM. They are: |
7,471 | These kinds of algorithms are useful for understanding the exploitation of concurrency, dividing the original problem into similar sub-problems and solving them in parallel. The introduction of the formal 'P-RAM' model in Wyllie's 1979 thesis had the aim of quantifying analysis of parallel algorithms in a way analogous... |
7,472 | PRAM algorithms cannot be parallelized with the combination of CPU and dynamic random-access memory because DRAM does not allow concurrent access to a single bank ; but they can be implemented in hardware or read/write to the internal static random-access memory blocks of a field-programmable gate array , it can be d... |
7,473 | However, the test for practical relevance of PRAM algorithms depends on whether their cost model provides an effective abstraction of some computer; the structure of that computer can be quite different than the abstract model. The knowledge of the layers of software and hardware that need to be inserted is beyond the... |
7,474 | This is an example of SystemVerilog code which finds the maximum value in the array in only 2 clock cycles. It compares all the combinations of the elements in the array at the first clock, and merges the result at the second clock. It uses CRCW memory; m <= 1 and maxNo <= data are written concurrently. The concurrency... |
7,475 | While the variety of existing process calculi is very large , there are several features that all process calculi have in common: |
7,476 | To define a process calculus, one starts with a set of names whose purpose is to provide means of communication. In many implementations, channels have rich internal structure to improve efficiency, but this is abstracted away in most theoretic models. In addition to names, one needs a means to form new processes f... |
7,477 | Parallel composition of two processes
P
{\displaystyle {\mathit {P}}}
and
Q
{\displaystyle {\mathit {Q}}}
, usually written
... |
7,478 | Channels may be synchronous or asynchronous. In the case of a synchronous channel, the agent sending a message waits until another agent has received the message. Asynchronous channels do not require any such synchronization. In some process calculi channels themselves can be sent in messages through channels, allow... |
7,479 | Interaction can be a directed flow of information. That is, input and output can be distinguished as dual interaction primitives. Process calculi that make such distinctions typically define an input operator
{\displaystyle x}
) and an output operator , both of which name an interaction point tha... |
7,480 | Should information be exchanged, it will flow from the outputting to the inputting process. The output primitive will specify the data to be sent. In
x
⟨
y
⟩
{\displaystyle x\langle y\rangle }
, this data is
y
{\d... |
7,481 | Sometimes interactions must be temporally ordered. For example, it might be desirable to specify algorithms such as: first receive some data on
x
{\displaystyle {\mathit {x}}}
and then send that data on
... |
7,482 | The key operational reduction rule, containing the computational essence of process calculi, can be given solely in terms of parallel composition, sequentialization, input, and output. The details of this reduction vary among the calculi, but the essence remains roughly the same. The reduction rule is: |
7,483 | The interpretation to this reduction rule is: |
7,484 | The class of processes that
P
{\displaystyle {\mathit {P}}}
is allowed to range over as the continuation of the output operation substantially influences the properties of the calculus. |
7,485 | Processes do not limit the number of connections that can be made at a given interaction point. But interaction points allow interference . For the
synthesis of compact, minimal and compositional systems, the ability to restrict interference is crucial. Hiding operations allow control of the connections made between in... |
7,486 | The operations presented so far describe only finite interaction and are consequently insufficient for full computability, which includes non-terminating behaviour. Recursion and replication are operations that allow finite descriptions of infinite behaviour. Recursion is well known from the sequential world. Replicat... |
7,487 | Process calculi generally also include a null process which has no interaction points. It is utterly inactive and its sole purpose is to act as the inductive anchor on top of which more interesting processes can be generated. |
7,488 | Process algebra has been studied for discrete time and continuous time . |
7,489 | In the first half of the 20th century, various formalisms were proposed to capture the informal concept of a computable function, with μ-recursive functions, Turing machines and the lambda calculus possibly being the best-known examples today. The surprising fact that they are essentially equivalent, in the sense th... |
7,490 | Research on process calculi began in earnest with Robin Milner's seminal work on the Calculus of Communicating Systems during the period from 1973 to 1980. C.A.R. Hoare's Communicating Sequential Processes first appeared in 1978, and was subsequently developed into a full-fledged process calculus during the early 198... |
7,491 | Various process calculi have been studied and not all of them fit the paradigm sketched here. The most prominent example may be the ambient calculus. This is to be expected as process calculi are an active field of study. Currently research on process calculi focuses on the following problems. |
7,492 | The ideas behind process algebra have given rise to several tools including: |
7,493 | CADP |
7,494 | Concurrency Workbench |
7,495 | mCRL2 toolset |
7,496 | The history monoid is the free object that is generically able to represent the histories of individual communicating processes. A process calculus is then a formal language imposed on a history monoid in a consistent fashion. That is, a history monoid can only record a sequence of events, with synchronization, but doe... |
7,497 | The use of channels for communication is one of the features distinguishing the process calculi from other models of concurrency, such as Petri nets and the actor model . One of the fundamental motivations for including channels in the process calculi was to enable certain algebraic techniques, thereby making it easier... |
7,498 | Like industry standards such as UML activity diagrams, Business Process Model and Notation, and event-driven process chains, Petri nets offer a graphical notation for stepwise processes that include choice, iteration, and concurrent execution. Unlike these standards, Petri nets have an exact mathematical definition of ... |
7,499 | The German computer scientist Carl Adam Petri, after whom such structures are named, analyzed Petri nets extensively in his 1962 dissertation Kommunikation mit Automaten. |
7,500 | A Petri net consists of places, transitions, and arcs. Arcs run from a place to a transition or vice versa, never between places or between transitions. The places from which an arc runs to a transition are called the input places of the transition; the places to which arcs run from a transition are called the output p... |
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