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I. Introduction
A. Asymmetric Cryptography
Public key cryptography is the security discipline employing asymmetric cryptography that enables key establishment of symmetric keys and digital signatures. The intrinsic modularity of asymmetric cryptographic algorithms limits their applicability to relatively short data lengths such that they are typically used to establish symmetric cryptographic keys versus operating on the data itself. The established symmetric keys are then used in streaming or block ciphers with practically unlimited data lengths. Key establishment algorithms come in two flavors—key agreement and key transport [X9.44]:
- Key agreement is a key establishment protocol whose secret key is a function of information contributed by two or more participants, so that no party can predetermine the value of the key.
- Key transport is a key establishment protocol under which the secret key is determined by the initiating party.
B. Digital Signatures
With regard to digital signatures, a hashing function is applied to very long data strings to produce a hash value; and the asymmetric private is applied to the hash value to generate a digital signature. The digital signature can be verified by a relying party using the corresponding asymmetric public key. In a classical public key infrastructure the identity of the signer is provided to the relying party via a public key certificate [X.509] issued by a certification authority (CA). The CA’s digital signature on the signer’s certificate cryptographically binds the signer’s identity to the public key. Similarly, the CA’s public key may likewise be encapsulated in another certificate issued by another CA such that a chain of certificates may exist; leading to a root CA. The underlying assumption is that the relying party has and trusts one or more of the certification authorities in the certificate chain.
C. Time Stamping
Most networks provide a system time such that data can be time stamped with the year, month, day, hour, minute, and second. Figure 2 – System Time Stamps shows the relationship between the system clock and the time stamp associated with each piece of signed data.
Presumably the time stamp indicates a sequence to the relying party and implies when the digital signatures were generated. However, note that the system generated time stamp is not independent of the data generation, or the digital signature generation processes.
The system clock, via the local or wide area network, may be synchronized to a national measurement institute whose clock is calibrated to the international time authority Bureau International des Poids et Mesures (BIPM) located in France. In the United States, the recognized national measurement institutes are the National Institute of Standards and Technology (NIST) and the United States Naval Observatory (USNO).
A. Time-Insensitive Digital Signatures
Figure 4 – Digital Signature Paradox below shows the same sequence of signed data; however the system clock has been reset such that the same time stamp is generated at three different times (T1, T2, and T3) for different versions of the same data. The time paradox is that a relying party now has three versions of the same data with the same time stamp; and despite the presence of a legitimate digital signature the relying party can no longer have confidence in the data. Thus digital signatures are time-insensitive.
The relying party has no practical method to distinguish between the three data versions, has no method to prioritize the data versions and has no option but to distrust all three versions. In order for the relying party to distinguish between the data versions and continue to trust the digital signatures, the signer needs to implement a verifiable mechanism such that the time stamp generation is independent of the signature generation. This method is referred to as a trusted time stamp.
B. Trusted Time Stamp
In a trusted time stamp scheme, there are five entities: time source entity, time stamp authority, requestor, verifier, and relying party. The relying party can be the requestor or any other third party. The time stamp authority (TSA) calibrates its clock with an upstream time source entity such as Master Clock (MC) or directly with a national measurement institute. The TSA provides a trusted time stamp token to the requestor. The time stamp token can be verified by a third party verifier.
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