Trusted timestamping

Trusted timestamping is the process of securely keeping track of the creation and modification time of a document. Security here means that no one—not even the owner of the document—should be able to change it once it has been recorded provided that the timestamper's integrity is never compromised.

The administrative aspect involves setting up a publicly available, trusted timestamp management infrastructure to collect, process and renew timestamps.

History
The idea of timestamping information is centuries old.  For example, when Robert Hooke discovered Hooke's law in 1660, he did not want to publish it yet, but wanted to be able to claim priority.  So he published the anagram ceiiinosssttuv and later published the translation ut tensio sic vis (Latin for "as is the extension, so is the force").  Similarly, Galileo first published his discovery of the phases of Venus in the anagram form.

Sir Isaac Newton, in responding to questions from Leibniz in a letter in 1677, concealed the details of his "fluxional technique" with an anagram:

The foundations of these operations is evident enough, in fact; but because I cannot proceed with the explanation of it now, I have preferred to conceal it thus: 6accdae13eff7i3l9n4o4qrr4s8t12ux. On this foundation I have also tried to simplify the theories which concern the squaring of curves, and I have arrived at certain general Theorems.

Trusted digital timestamping has first been discussed in literature by Haber and Stornetta.

Classification
There are many timestamping schemes with different security goals:

 PKI-based – timestamp token is protected using PKI digital signature.
 Linking-based schemes – timestamp is generated such a way that it is related to other timestamps.
 Distributed schemes – timestamp is generated in cooperation of multiple parties.
 Transient key scheme – variant of PKI with short-living signing keys.
 MAC – simple secret key based scheme, found in ANSI ASC X9.95 Standard.
 Database – document hashes are stored in trusted archive; there is online lookup service for verification.
 Hybrid schemes – the linked and signed method is prevailing, see X9.95.

Coverage in standards:

   
For systematic classification and evaluation of timestamping schemes see works by Masashi Une.

Trusted (digital) timestamping

According to the RFC 3161 standard, a trusted timestamp is a timestamp issued by a Trusted Third Party (TTP) acting as a Time Stamping Authority (TSA). It is used to prove the existence of certain data before a certain point (e.g. contracts, research data, medical records, ...) without the possibility that the owner can backdate the timestamps.  Multiple TSAs can be used to increase reliability and reduce vulnerability.

The newer ANSI ASC X9.95 Standard for trusted timestamps augments the RFC 3161 standard with data-level security requirements to ensure data integrity against a reliable time source that is provable to any third party. This standard has been applied to authenticating digitally signed data for regulatory compliance, financial transactions, and legal evidence.

Creating a timestamp
The technique is based on digital signatures and hash functions. First a hash is calculated from the data. A hash is a sort of digital fingerprint of the original data: a string of bits that is practically impossible to duplicate with any other set of data. If the original data is changed then this will result in a completely different hash. This hash is sent to the TSA. The TSA concatenates a timestamp to the hash and calculates the hash of this concatenation. This hash is in turn digitally signed with the private key of the TSA. This signed hash + the timestamp is sent back to the requester of the timestamp who stores these with the original data (see diagram).

Since the original data cannot be calculated from the hash (because the hash function is a one way function), the TSA never gets to see the original data, which allows the use of this method for confidential data.

Checking the timestamp

Anyone trusting the timestamper can then verify that the document was not created after the date that the timestamper vouches. It can also no longer be repudiated that the requester of the timestamp was in possession of the original data at the time given by the timestamp. To prove this (see diagram) the hash of the original data is calculated, the timestamp given by the TSA is appended to it and the hash of the result of this concatenation is calculated, call this hash A.

Then the digital signature of the TSA needs to be validated.  This can be done by checking that the signed hash provided by the TSA was indeed signed with their private key by digital signature verification.  The hash A is compared with the hash B inside the signed TSA message to confirm they are equal, proving that the timestamp and message is unaltered and was issued by the TSA. If not, then either the timestamp was altered or the timestamp was not issued by the TSA.

Decentralized timestamping on the blockchain
With the advent of cryptocurrencies like bitcoin, it has become possible to securely timestamp information in a decentralized and tamper-proof manner. Digital data can be hashed and the hash can be incorporated into a transaction stored in the blockchain, which serves as a secure proof of the exact time at which that data existed. The proof is due to a tremendous amount of computational effort performed after the hash was submitted to the blockchain. Tampering with the timestamp would require more computational resources than the rest of the network combined, and cannot be done unnoticed.

The decentralized timestamping approach using the blockchain has also found applications in other areas, such as in dashboard cameras, to secure the integrity of video files at the time of their recording, or to prove priority for creative content and ideas shared on social media platforms.

See also
 Timestamp
 Timestamping (computing)
 Cryptography
 Computer security
 Digital signature
 Digital postmark
 Smart contract
 CAdES – CMS Advanced Electronic Signature
 PAdES – PDF Advanced Electronic Signature
 XAdES – XML Advanced Electronic Signature

References

External links
 RFC 3161 Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)
 RFC 3628 Policy Requirements for Time-Stamping Authorities (TSAs)
 Decentralized Trusted Timestamping (DTT) using the Crypto Currency Bitcoin
 ANSI ASC X9.95 Standard for Trusted Time Stamps
 ETSI TS 101 861 V1.4.1 Electronic Signatures and Infrastructures (ESI); Time stamping profile
 ETSI TS 102 023 V1.2.2 Electronic Signatures and Infrastructures (ESI); Policy requirements for time-stamping authorities
 Analysis of a Secure Time Stamp Device (2001) SANS Institute
 Implementation of TSP Protocol CMSC 681 Project Report, Youyong Zou

Category:Computer security
Category:Time
Category:Authentication methods