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 adv ...

, a group key is a

cryptographic key A key in cryptography is a piece of information, usually a string of numbers or letters that are stored in a file, which, when processed through a cryptographic algorithm, can encode or decode cryptographic data. Based on the used method, the key ...

that is shared between a group of users. Typically, group keys are distributed by sending them to individual users, either physically, or encrypted individually for each user using either that user's pre-distributed private key. A common use of group keys is to allow a group of users to decrypt a broadcast message that is intended for that entire group of users, and no one else. For example, in the

Second World War World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...

, group keys (known as "iodoforms", a term invented by a classically educated non-chemist, and nothing to do with the chemical of the same nameRichard Clayton, ''Hiding: Anonymity Systems'', http://www.cl.cam.ac.uk/~rnc1/notes/AT02_hiding.pdf, lecture notes, 2002.) were sent to groups of agents by the

Special Operations Executive The Special Operations Executive (SOE) was a secret British World War II organisation. It was officially formed on 22 July 1940 under Minister of Economic Warfare Hugh Dalton, from the amalgamation of three existing secret organisations. Its p ...

. These group keys allowed all the agents in a particular group to receive a single coded message. ''Between Silk and Cyanide – a Codemaker’s War 1941-1945'',

Leo Marks Leopold Samuel Marks, (24 September 1920 – 15 January 2001) was an English writer, screenwriter, and cryptographer. During the Second World War he headed the codes office supporting resistance agents in occupied Europe for the secret Special ...

, HarperCollins 1998. Ross Anderson, ''Security Engineering: A Guide to Building Dependable Distributed Systems'', chapter 20, page 428, Wiley 2001, {{ISBN, 0-471-38922-6 (paperback, 641pp.) Available online at http://www.cl.cam.ac.uk/~rja14/book.html In present-day applications, group keys are commonly used in

conditional access Conditional access (CA) is a term commonly used in relation to software and to digital television systems. Conditional access is that ‘just-in-time’ evaluation to ensure the person who is seeking access to content is authorized to access the c ...

systems, where the key is the common key used to decrypt the broadcast signal, and the group in question is the group of all paying subscribers. In this case, the group key is typically distributed to the subscribers' receivers using a combination of a physically distributed secure cryptoprocessor in the form of a

smartcard A smart card, chip card, or integrated circuit card (ICC or IC card) is a physical electronic authentication device, used to control access to a resource. It is typically a plastic credit card-sized card with an embedded integrated circuit (IC) c ...

and encrypted over-the-air messages.

References

Cryptography