Public Key Certificates
To verify a digital signature, the verifier must have access to the signer's public key and have assurance that it corresponds to the signer's private key. However, a public and private key pair has no intrinsic association with any person; it is simply a pair of numbers. Some convincing strategy is necessary to reliably associate a particular person or entity to the key pair.
In a transaction involving only two parties, each party can simply communicate (by a relatively secure "out-of-band" channel such as a courier or a secure voice telephone) the public key of the key pair each party will use. Such an identification strategy is no small task, especially when the parties are geographically distant from each other, normally conduct communication over a convenient but insecure channel such as the Internet, are not natural persons but rather corporations or similar artificial entities, and act through agents whose authority must be ascertained. As electronic commerce increasingly moves from a bilateral setting to the many-on-many architecture of the World Wide Web on the Internet, where significant transactions will occur among strangers who have no prior contractual relationship and will never deal with each other again, the problem of authentication/nonrepudiation becomes not merely one of efficiency, but also of reliability. An open system of communication such as the Internet needs a system of identity authentication to handle this scenario.
To that end, a prospective signer might issue a public statement, such as: "Signatures verifiable by the following public key are mine." However, others doing business with the signer may for good reason be unwilling to accept the statement, especially where there is no prior contract establishing the legal effect of that published statement with certainty. A party relying upon such an unsupported published statement in an open system would run a great risk of trusting a phantom or an imposter, or of attempting to disprove a false denial of a digital signature ("nonrepudiation") if a transaction should turn out to prove disadvantageous for the purported signer.
The solution to these problems is the use of one or more trusted third parties to associate an identified signer with a specific public key. <31> That trusted third party is referred to as a "certification authority" in most technical standards and in these Guidelines.
To associate a key pair with a prospective signer, a certification authority issues a certificate, an electronic record which lists a public key as the "subject" of the certificate, and confirms that the prospective signer identified in the certificate holds the corresponding private key. The prospective signer is termed the "subscriber. <32> A certificate's principal function is to bind a key pair with a particular subscriber. A "recipient" of the certificate desiring to rely upon a digital signature created by the subscriber named in the certificate (whereupon the recipient becomes a "relying party") can use the public key listed in the certificate to verify that the digital signature was created with the corresponding corresponding private key. <33> If such verification is successful, this chain of reasoning provides assurance that the corresponding private key is held by the subscriber named in the certificate, and that the digital signature was created by that particular subscriber.
To assure both message and identity authenticity of the certificate, the certification authority digitally signs it. The issuing certification authority's digital signature on the certificate can be verified by using the public key of the certification authority listed in another certificate by another certificate authority (which may but need not be on a higher level in a hierarchy) <34>, and that other certificate can in turn be authenticated by the public key listed in yet another certificate, and so on, until the person relying on the digital signature is adequately assured of its genuineness. In each case, the issuing certification authority must digitally sign its own certificate during the operational period of the other certificate used to verify the certification authority's digital signature.
A digital signature, whether created by a subscriber to authenticate a message or by a certification authority to authenticate its certificate (in effect a specialized message) should be reliably time-stamped to allow the verifier to determine reliably whether the digital signature was created during the "operational period" stated in the certificate, which is a condition upon verifiability of a digital signature under these Guidelines. <35>
To make a public key and its identification with a specific subscriber readily available for use in verification, the certificate may be published in a repository or made available by other means. Repositories are on-line databases of certificates and other information available for retrieval and use in verifying digital signatures. Retrieval can be accomplished automatically by having the verification program directly inquire of the repository to obtain certificates as needed.
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