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[[Image:Cross-Certification.png|thumb|320px|A private key is cross-certified using two other transient-key servers.]]
Through independently operating servers, '''cross-certification''' can provide third-party proof of the validity of a time interval chain and irrefutable evidence of consensus on the current time. Transient-key cryptographic systems display high [[Byzantine fault tolerance]]. A web of interconnected cross-certifying servers in a distributed environment creates a widely witnessed chain of trust that is as strong as its strongest link. By contrast, entire [[Hierarchy|hierarchies]] of traditional public key systems can be compromised if a single private key is exposed.
An individual transient key interval chain can be cross-certified with other transient key chains and server instances. Through cross-certification, Server A signs Server B's interval chain, the signed data of which is the interval definition. In effect, the private keys from Server B are used to sign the public keys of Server A. In the diagram, a server instance is cross-certified with two other server instances (blue and orange). Cross-certification requires that the timestamp for the interval agree with the timestamp of the cross-certifying server within acceptable tolerances, which are user-defined and typically a few hundred milliseconds in duration.
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During an interval, the transient private key is used to sign data concatenated with trusted timestamps and authenticity certificates. To [[File verification|verify]] the data at a later time, a receiver accesses the [[Persistence (computer science)|persistent]] public key for the appropriate time interval. The public key applied to the digital signature can be passed through published cryptographic routines to unpack the [[Cryptographic hash function|hash]] of the original data, which is then compared against a fresh hash of the stored data to verify data integrity. If the signature successfully decrypts using a particular interval's published public key, the receiver can be assured that the signature originated during that time period. If the decrypted and fresh hashes match, the receiver can be assured that the data has not been tampered with since the transient private key created the timestamp and signed the data.
Transient-key cryptography was invented in 1997 by Dr. Michael D. Doyle of Eolas Technologies Inc., while working on the [[Visible Embryo Project]]<ref>{{cite patent |country=US |number=6381696 |title=Method and system for transient key digital time stamps |gdate=2002-04-30 |inventor=MD Doyle}}</ref><ref>{{cite patent |country=US |number=7047415 |title=System and method for widely witnessed proof of time |gdate=2006-05-16 |invent1=MD Doyle |invent2=PF Doyle |invent3=GW Bernsohn |invent4=JD Roberts }}</ref><ref>{{cite patent |country=US |number= 7210035 |title=System and method for graphical indicia for the certification of records |gdate=2007-04-24 |invent1=MD Doyle |invent2=RG Hamilton |invent3=MC Perrone |invent4=PF Doyle}}</ref> and later acquired by and productized by ProofSpace, Inc. It has been adopted as a national standard in the ANSI ASC X9.95 standard for Trusted Timestamping.
==See also==
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