Signature creation

Although other schemes exist, we assume here that creating a digital signature value consists in the encryption of a hash computed on the signed data.

The standard ETSI EN 319 102-1 clause 4 ([R09]) provides a complete conceptual model for the creation of “AdES digital signatures”, but for the sake of simplicity we can extract from this model the following steps:

• Receiving a (set of) document(s) or a (set of) hash(es) representing those documents, together with other inputs (such as so-called “signed attribute” values e.g. signer’s location, and constraints driving the creation of the signature such as the cryptographic algorithms to be used for the creation of the signature value);

• Composing the “data to be signed” (DTBS) which is the data object that will be covered by the signature value (including thus the document(s) and attributes to be signed), and the associated “data to be signed formatted” (DTBSF) which can be taken as the format-specific byte-stream on which the signature value will be computed;

• Creating the “data to be signed representation” (DTBSR) by applying the appropriate hash algorithm on the DTBSF obtained in the previous step;

• Computing the signature value by encrypting the DTBSR using the appropriate algorithm (this is usually done by activating the private key within a “Signature creation device” (SCDev), that will perform the operation);

• Formatting the result into a “signed data object” (SDO) complying with the desired signature format (e.g. XAdES, PAdES, etc).

As mentioned above, the activation of the private key and the operation of creating the signature value is assumed to be performed by a specific device. It is in general desirable that this device is a secure (e.g. tamper-proof) device that requires authentication for the activation of the key (e.g. using PIN codes).

When the private key contained in that device is controlled by an end-entity, this device is usually called a “signature creation device” or SCDev. This can be a local SCDev such as a smartcard, but it can also be a remote SCDev managed by a CA or TSP.

When the private key is used by a CA for signing certificates, this device is usually called a “hardware security module” or HSM.

Frequently, when the private key is under the control of a legal entity (such as when the key is used to create electronic seals) the device is also called an HSM.