Digital Signature Guidelines Tutorial

In today's commercial environment, establishing a framework for the authentication <1> of computer-based information requires a familiarity with concepts and professional skills from both the legal and computer security fields. Combining these two disciplines is not an easy task. Concepts from the information security field often correspond only loosely to concepts from the legal field, even in situations where the terminology is similar. For example, from the information security point of view, "digital signature" means the result of applying to specific information certain specific technical processes described below. The historical legal concept of "signature" is broader. It recognizes any mark made with the intention of authenticating the marked document. <2> In a digital setting, today's broad legal concept of "signature" may well include markings as diverse as digitized images of paper signatures, typed notations such as "/s/ John Smith," or even addressing notations, such as electronic mail origination headers.

From an information security viewpoint, these simple "electronic signatures" are distinct from the "digital signatures" described in this tutorial and in the technical literature, although "digital signature" is sometimes used to mean any form of computer- based signature. These Guidelines use "digital signature" only as it is used in information security terminology, as meaning the result of applying the technical processes described in this tutorial.

To explain the value of digital signatures in legal applications, this tutorial begins with an overview of the legal significance of signatures. It then sets forth the basics of digital signature technology, and examines how, with some legal and institutional infrastructure, digital signature technology can be applied as a robust computer-based alternative to traditional signatures.

Signatures and the Law
A signature is not part of the substance of a transaction, but rather of its represen tation or form. Signing writings serve the following general purposes:<3>
Evidence: A signature authenticates a writing by identifying the signer with the signed document. When the signer makes a mark in a distinctive manner, the writing becomes attributable to the signer.<4>
Ceremony: The act of signing a document calls to the signer's attention the legal significance of the signer's act, and thereby helps prevent "inconsiderate engagements.<5>
Approval: In certain contexts defined by law or custom, a signature expresses the signer's approval or authorization of the writing, or the signer's intention that it have legal effect.<6>
Efficiency and logistics: A signature on a written document often imparts a sense of clarity and finality to the transaction and may lessen the subsequent need to inquire beyond the face of a document.<7> Negotiable instruments, for example, rely upon formal requirements, including a signature, for their ability to change hands with ease, rapidity, and minimal interruption.<8>
The formal requirements for legal transactions, including the need for signatures, vary in different legal systems, and also vary with the passage of time. There is also variance in the legal consequences of failure to cast the transaction in a required form. The statute of frauds of the common law tradition, for example, does not render a transaction invalid for lack of a "writing signed by the party to be charged," but rather makes it unenforceable in court,<9> a distinction which has caused the practical application of the statute to be greatly limited in case law.

During this century, most legal systems have reduced formal requirements,<10> or at least have minimized the consequences of failure to satisfy formal requirements. Nevertheless, sound practice still calls for transactions to be formalized in a manner which assures the parties of their validity and enforceability.<11> In current practice, formalization usually involves documenting the transaction on paper and signing or authenticating the paper. Traditional methods, however, are undergoing fundamental change. Documents continue to be written on paper, but sometimes merely to satisfy the need for a legally recognized form. In many instances, the information exchanged to effect a transaction never takes paper form. Computer-based information can also be utilized differently than its paper counterpart. For example, computers can "read" digital information and transform the information or take programmable actions based on the information. Information stored as bits rather than as atoms of ink and paper can travel near the speed of light, may be duplicated without limit and with insignificant cost.

Although the basic nature of transactions has not changed, the law has only begun to adapt to advances in technology. The legal and business communities must develop rules and practices which use new technology to achieve and surpass the effects historically expected from paper forms.

To achieve the basic purposes of signatures outlined above, a signature must have the following attributes:<12>

Signer authentication: A signature should indicate who signed a document, message or record,<13> and should be difficult for another person to produce without authorization.
Document authentication: <14> A signature should identify what is signed, <15> making it impracticable to falsify or alter either the signed matter or the signature without detection.
Signer authentication and document authentication are tools used to exclude impersonators and forgers and are essential ingredients of what is often called a "nonrepudiation service" in the terminology of the information security profession. A nonrepudiation service provides assurance of the origin or delivery of data in order to protect the sender against false denial by the recipient that the data has been received, or to protect the recipient against false denial by the sender that the data has been sent. <16> Thus, a nonrepudiation service provides evidence to prevent a person from unilaterally modifying or terminating legal obligations arising out of a transaction effected by computer-based means. <17>

Affirmative act: The affixing of the signature should be an affirmative act which serves the ceremonial and approval functions of a signature and establishes the sense of having legally consummated a transaction.
Efficiency: Optimally, a signature and its creation and verification processes should provide the greatest possible assurance of both signer authenticity and document authenticy, with the least possible expenditure of resources.
Digital signature technology generally surpasses paper technology in all these attributes. <18> To understand why, one must first understand how digital signature technology works.

How Digital Signature Technology Works
Digital signatures are created and verified by cryptography, the branch of applied mathematics that concerns itself with transforming messages into seemingly unintelligible forms and back again. Digital signatures use what is known as "public key cryptography," which employs an algorithm using two different but mathematically related "keys;" one for creating a digital signature or transforming data into a seemingly unintelligible form, and another key for verifying a digital signature or returning the message to its original form. <19> Computer equipment and software utilizing two such keys are often collectively termed an "asymmetric cryptosystem."

The complementary keys of an asymmetric cryptosystem for digital signatures are arbitrarily termed the private key, which is known only to the