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''Digital signature'' has also been used as a broader term encompassing both ''public-key digital signature'' techniques and Message Authentication Code s. Digital signatures differ in some respects from their physical counterparts. The term '' and Telex addresses, as well as FAX transmission of handwritten signatures on a paper document. USES There are three common reasons for applying a digital signature to communications: Authenticity Public-key cryptosystems allow anybody to encrypt a message using their private key. More typically, the message will be sent in plaintext, with the encryption of a shorter Hash appended. By decrypting the hash with the sender's public key, and checking the result against the plaintext, the recipient can confirm that the encryption was done with the sender's private key. This signature allows the recipient to be confident that the sender is indeed who they claim to be. Of course the recipient cannot be 100% ''sure'' that the sender is indeed who they claim to be - the recipient can only be ''confident'' - since the cryptosystem may have been broken. The importance of authenticity is especially obvious in a financial context. For example, suppose a bank sends instructions from its branch offices to the central office in the form ''(a,b)'' where ''a'' is the account number and ''b'' is the amount to be credited to the account. A devious customer may deposit £100, observe the resulting transmission and repeatedly retransmit ''(a,b)''. This is known as a '' Replay Attack ''. Integrity Both parties will always wish to be confident that a message has not been altered during transmission. The encryption makes it difficult for a third party to ''read'' a message, but that third party may still be able to ''alter'' it in a useful way. A popular example to illustrate this is the ''homomorphism attack'': consider the same bank as above which sends instructions from its branch offices to the central office in the form ''(a,b)'' where ''a'' is the account number and ''b'' is the amount to be credited to the account. A devious customer may deposit £100, intercept the resulting transmission and then transmit ''(a,b3)'' to become an instant millionaire. Non-repudiation In a cryptographic context, the word ''repudiation'' refers to the act of denying association with a message (ie claiming it was sent by a third party). The recipient of a message may insist that the sender attach a signature in order to prevent any later repudiation, since the recipient may show the message to a third party to prove its origin. The loss of control of the private key means that all digitally signed communications can still be repudiated. IMPLEMENTATION Digital signature schemes rely on Public-key Cryptography . In public-key cryptography, each user has a pair of keys: one public and one private. The public key is distributed freely, but the private key is kept secret and confidential; another requirement is that it should be infeasible to derive the private key from the public key. A general digital signature scheme consists of three algorithms:
For example, consider the situation in which Bob sends a message to Alice and wants to be able to prove it came from him. Bob sends his message to Alice and attaches a digital signature. The digital signature is generated using Bob's private key, and takes the form of a simple numerical value (normally represented as a string of binary digits). On receipt, Alice can then check whether the message really came from Bob by running the verification algorithm on the message together with the signature and Bob's public key. If they match, then Alice can be confident that the message really was from Bob, because the signing algorithm is designed so that it is very difficult to forge a signature to match a given message (unless one has knowledge of the private key, which Bob has kept secret). More usually, for efficiency reasons, Bob first applies a Cryptographic Hash Function to the message before signing. This makes the signature much shorter and thus saves time since hashing is generally much faster than signing in implementations. However, if the message digest algorithm is insecure (for example, if it is possible to generate Hash Collision s), then it might be feasible to forge digital signatures. SOME DIGITAL SIGNATURE ALGORITHMS
THE CURRENT STATE OF USE — LEGAL AND PRACTICAL Digital signature schemes all have several prior requirements without which no such signature can mean anything, whatever the cryptographic theory or legal provision.
Only if all of these conditions are met will a digital signature actually be evidence of who sent the message. 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 (or limiting) their legal effect. The first appears to have been in Utah , followed closely by Massachusetts and California . Assorted non-US countries have also passed statutes or issued regulations in this area as well and the UN has had an active model law project for some time. These enactments (or proposed enactments) vary from place to place, have typically embodied expectations at variance (optimistically or pessimistically) with the state of the underlying Cryptographic Engineering , and have had the net effect of confusing potential users and specifiers, nearly all of whom are not cryptographically knowledgeable. 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 Length s, etc and so on what the engineering is attempting to provide. See also LEGAL ASPECTS Legislation concerning the effect and validity of digital signatures includes: China
Brazil
European Union and the European Economic Area
India
New Zealand United Nations Commission on International Trade Law
Switzerland Uruguay Uruguay laws include both, electronic and digital signatures:
LEGAL CASES Court decisions discussing the effect and validity of digital signatures or digital signature-related legislation:
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