Transient-key cryptography

Transient-key cryptography is a form of public-key cryptography wherein keypairs are generated and assigned to brief intervals of time instead of to individuals or organizations. In a transient-key system, private keys are used briefly and then destroyed, which is why it is sometimes nicknamed “disposable crypto.” Data encrypted with a private key associated with a specific time interval can be irrefutably linked to that interval, making transient-key cryptography particularly useful for digital trusted timestamping.

Both public-key and transient-key systems can be used to generate digital signatures that assert that a given piece of data has not changed since it was signed. But the similarities end there. In a traditional public key system, the public/private keypair is typically assigned to an individual, server, or organization. Data signed by a private key asserts that the signature came from the indicated source. Keypairs persist for years at a time, so the private component must be carefully guarded against disclosure; in a public-key system, anyone with access to a private key can counterfeit that person’s digital signature. In transient-key systems, however, the keypair is assigned to a brief interval of time, not to a particular person or entity. Data signed by a specific private key becomes associated with a specific time and date. A keypair is active only for a few minutes, after which the private key is permanently destroyed. Therefore, unlike public-key systems, transient-key systems do not depend upon the long-term security of the private keys.

In a transient-key system, the source of time must be a consistent standard understood by all senders and receivers. Since a local system clock may be changed by a user, it is never used as a source of time. Instead, data is digitally signed with a time value derived from Universal Coordinated Time (UTC) accurate to within a millisecond, in accordance with the ANSI ASC X9.95 standard for Trusted Timestamping. Whenever a time interval in a transient-key system expires, a new public/private keypair is generated, and the private key from the previous interval is used to digitally certify the new public key. The old private key is then destroyed. For the new interval, time values are obtained from a trusted third-party source, and specific moments in time can be interpolated in between received times by using a time-biasing method based on the internal system timer. If a trusted time source cannot be obtained or is not running within specified tolerances, transient private keys are not issued. In that case, the time interval chain is terminated, and a fresh one is begun. The old and new chains are connected through network archives, which enable all servers to continue to verify the data integrity through time of protected data, regardless of how often the chain must be restarted. The start times of the chain and of each interval can be coupled together to form an unbroken sequence of public keys, which can be used for the following:

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