Understanding Digital Signatures

To understand digital signatures, we need to begin by examining digital signature schemes and their commonalities. All digital signature schemes have a number of prior requirements. Figure 13-7 shows a digital signature.

Figure 13-7 Digital Signature c ra

Signer

Hash Calculation 1AFFF12CF

Private Key Public Key Certificate

Private Key Public Key Certificate

Signed Fingerprint Public Key Certificate

Digital Signature Object Can Be i- l. -i-i -i i -j u. ^ ' Digitally Signed Document

Embedded Inside the Document 3 ' 3

1AFFF12CF

Current Document

Signed Fingerprint

Certificate Public Key Digital Signature

1AFFF12CF

Calculated Fingerprint

1AFFF12CF

Original Fingerprint

Match Results

Invalid Signature

Valid Signature

Invalid Signature

Valid Signature

Hash Calculation 1AFFF12CF

1AFFF12CF

The first requirement is quality algorithms. As we have discussed, some of the available public key algorithms have been called into question with regard to security. Others are known to be insecure based on predictable attacks having been launched against them.

The second requirement is quality implementations. What this means is that even if you have a quality algorithm, if it is implemented incorrectly, it won't help you.

The third requirement is that the private key must remain secret. If this private key is compromised, an attacker can create an exact digital signature of anything he wants.

The fourth requirement is that the distribution of public keys has to be done in a manner that ensures that a public key belonging to a given user actually does belong to that user. Often this is done using a PKI. The public key user association is attested to by the operator of the PKI, the CA. In the case of "open" PKIs—ones in which anyone can request such an attestation—embodied in an identity certificate, the potential for mistaken attestation is not trivial. Unfortunately, commercial PKI providers have suffered a number of publicly known issues. Mistakes such as these could lead to falsely signed, and thus improperly attributed, documents. Maintaining a "closed" PKI system is more costly for organizations but less easily subverted, providing a stronger level of security for those who can take such steps.

Finally, beyond the PKI infrastructure and the steps that administrators must take to provide security, the fifth and final area of concern is users. The users of these PKI systems themselves (and their software) must take care to carry out the signature protocol properly to not compromise the signature.

All the conditions just listed must be met for a digital signature to reliably provide evidence of who sent the message, and therefore of his assent to its contents. Even legal measures cannot alter this reality.

Based on local laws, many countries accord a digital signature the same status as a traditional pen-and-paper signature with regard to its capacity to bind parties in a legal agreement such as a contract. Because of the legally binding nature of digital signatures in some parts of the world, it is generally best to use separate key pairs for encrypting and signing. Use of these key pairs allows an individual to engage in an encrypted conversation about matters that might be legally binding, such as the negotiation of a contract for employment. When the parties involved in the discussion reach an agreement, they can use their signing keys to "sign" the electronic document. At this point they are legally bound by the terms of a specific document. After this signing has taken place, the electronic document can be sent across an encrypted link to complete the transaction.

0 0

Post a comment