## public domain

(PD) The total absence of copyright protection. If something is "in the public domain" then anyone can copy it or use it in any way they wish. The author has none of the exclusive rights which apply to a copyright work.

The phrase "public domain" is often used incorrectly to refer to freeware or shareware (software which is copyrighted but is distributed without (advance) payment). Public domain means no copyright -- no exclusive rights. In fact the phrase "public domain" has no legal status at all in the UK.

See also archive site, careware, charityware, copyleft, crippleware, guiltware, postcardware and -ware. Compare payware.

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## public domain software

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public domain ♦ **public domain software** ♦ public-key cryptography

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## public-key cryptography

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public domain software ♦ **public-key cryptography** ♦ Public-Key Cryptography Standards

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## Public-Key Cryptography Standards

(PKCS) A set of standards for public-key cryptography, developed by RSA Data Security, Inc. in cooperation with an informal consortium, originally including Apple, Microsoft, DEC, Lotus, Sun and MIT. The PKCS have been cited by the OSI Implementers' Workshop (OIW) as a method for implementation of OSI standards.

PKCS includes both algorithm-specific and algorithm-independent implementation standards. Many algorithms are supported, including RSA and Diffie-Hellman key exchange, however, only the latter two are specifically detailed. PKCS also defines an algorithm-independent syntax for digital signatures, digital envelopes, and extended digital certificates; this enables someone implementing any cryptographic algorithm whatsoever to conform to a standard syntax, and thus achieve interoperability.

E-mail: [email protected]

Last updated: 1999-02-16

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## public-key encryption

(PKE, Or "public-key cryptography") An encryption scheme, introduced by Diffie and Hellman in 1976, where each person gets a pair of keys, called the public key and the private key. Each person's public key is published while the private key is kept secret. Messages are encrypted using the intended recipient's public key and can only be decrypted using his private key. This is often used in conjunction with a digital signature.

The need for sender and receiver to share secret information (keys) via some secure channel is eliminated: all communications involve only public keys, and no private key is ever transmitted or shared.

Public-key encryption can be used for authentication, confidentiality, integrity and non-repudiation.

RSA encryption is an example of a public-key cryptosystem.

See also knapsack problem.

Last updated: 1995-03-27

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## Public Key Infrastructure

<*cryptography, communications*>

(PKI) A system of public key encryption using digital certificates from Certificate Authorities and other registration authorities that verify and authenticate the validity of each party involved in an electronic transaction.

PKIs are currently evolving and there is no single PKI nor even a single agreed-upon standard for setting up a PKI. However, nearly everyone agrees that reliable PKIs are necessary before electronic commerce can become widespread.

Last updated: 1999-11-30

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## Public Switched Telephone Network

(PSTN, T.70) The collection of interconnected systems operated by the various telephone companies and administrations (telcos and PTTs) around the world. Also known as the Plain Old Telephone System (POTS) in contrast to xDSL and ISDN (not to mention other forms of PANS).

The PSTN started as human-operated analogue circuit switching systems (plugboards), progressed through electromechanical switches. By now this has almost completely been made digital, except for the final connection to the subscriber (the "last mile"): The signal coming out of the phone set is analogue. It is usually transmitted over a twisted pair cable still as an analogue signal. At the telco office this analogue signal is usually digitised, using 8000 samples per second and 8 bits per sample, yielding a 64 kb/s data stream (DS0). Several such data streams are usually combined into a fatter stream: in the US 24 channels are combined into a T1, in Europe 31 DS0 channels are combined into an E1 line. This can later be further combined into larger chunks for transmission over high-bandwidth core trunks. At the receiving end the channels are separated, the digital signals are converted back to analogue and delivered to the received phone.

While all these conversions are inaudible when voice is transmitted over the phone lines it can make digital communication difficult. Items of interest include A-law to mu-law conversion (and vice versa) on international calls; robbed bit signalling in North America (56 kbps <--> 64 kbps); data compression to save bandwidth on long-haul trunks; signal processing such as echo suppression and voice signal enhancement such as AT&T TrueVoice.

Last updated: 2000-07-09

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