Determining the Purpose of Cryptographic Material

Once key material has been classified, it is necessary to determine the intended use of the key. The mere existence of a key does not indicate which operations may be performed with it or the consequences of its compromise. Key material can serve purposes such as encryption, digital signatures, authentication, electronic seals, or timestamping. From a technical standpoint, some of these purposes may utilize the same cryptographic algorithms; however, this does not mean it is appropriate to freely reuse the same key across different purposes.

Therefore, determining the purpose is not merely a matter of record-keeping. It is a rule that restricts the key's usage and establishes a link between the key, its owner, access rights, and a specific cryptographic operation.

The Purpose of Key Material

Key material is used for cryptographic operations, such as digital signatures, authentication, or encryption. From a cryptographic perspective, there are symmetric keys and asymmetric pairs consisting of a private key and a public key. The public and private keys are mathematically linked; the public key may be made public, whereas the private key must be protected.

PKI (Public Key Infrastructure) is an infrastructure based primarily on asymmetric cryptography and the binding of public keys to identities or other entities via certificates. However, practical systems commonly combine asymmetric and symmetric mechanisms. In a PKI environment, keys and certificates are used for various purposes, such as:

• signing,
• authentication,
• encryption,
• electronic seals,
• TSA services,
• Certification Authority signatures,
• OCSP signatures,
• CRL signatures,
• code signing,
• device identity.

The purpose is usually also expressed through certificate parameters and the use of appropriate extensions. From a technical standpoint, however, the private key itself might still be usable for other operations. This is precisely where an important distinction arises between what the cryptographic mechanism permits and what is permitted under the rules.
For instance, the same signing key could technically be used for different signature formats, such as S/MIME or CMS/CAdES. However, it does not automatically follow that the same key should be used for all these purposes. The decision depends on the key classification, its management method, authorizations, the requirements of the specific service, and any potential legal or evidentiary implications. Separation of purposes is therefore an important aspect of key material management. Without it, a key intended for a specific security or legal purpose might come to be used in a manner not envisaged at the time of its creation or classification.

Electronic signature

For a personal signing key, the required signature level dictates the need to ensure an appropriate link to the signer and protection against unauthorized use. For an eIDAS-compliant advanced electronic signature, requirements include a unique link to the signer, the ability to identify the signer, and signature creation under the signer's sole control. For a qualified electronic signature, these requirements are supplemented by the use of a qualified signature creation device and a qualified certificate.
In practice, protection of the private key can be ensured by, for example, a QSCD, a smart card, a cryptographic token, or a suitably designed remote signing service. The key should not be shared or transferred between individuals in a way that would compromise the link between the key and the signer. Therefore, the following relationship is crucial for an electronic signature:
signer – private key – signed data – signing operation

Each signing operation must be performed in a manner that meets the required signature level and maintains the authorized holder's control over the private key. Once there is a reasonable possibility that the same personal signing key could have been used by another person, the evidentiary value of the link between the signer and the specific signing operation is weakened. Therefore, the concept of non-repudiation is best understood primarily as a security or evidentiary attribute. A digital signature in itself does not automatically guarantee a specific legal effect; that also depends on the identification of the holder, the protection of the key, the service used, and the applicable legal regime.

Digital signature and biometric signature

It is necessary to distinguish between a cryptographic digital signature and a biometric method of signing.
With a digital signature, the link between the signed data and the private key is cryptographically verified. Consequently, the question of who had control over the private key and whether someone else could have used it is crucial.
In contrast, a biometric signature records characteristics of the signer's actual signing action. These may include, for example, pen tilt, movement speed, or pressure. While such data can assist in assessing whether a specific signature was created by a particular person, they do not in themselves establish the same cryptographic link Cryptography therefore addresses a different problem than biometric analysis. One method cannot simply be substituted for the other.

Electronic Seal

In the case of an electronic seal, the trust model differs. The key and certificate are linked to a legal entity. The purpose of the seal is not to express the intent of a specific natural person, but primarily to demonstrate the origin and integrity of the data. However, the organization must be able to demonstrate how the sealing key is managed and who is authorized to initiate its use.
The sealing key may be managed centrally and used by automated systems or by multiple authorized persons acting on behalf of the organization. However, the key should not be freely exportable or copyable. In practice, it may be stored, for example, in an HSM (Hardware Security Module), which enables the management of access rights, the separation of administrative roles, and the auditing of cryptographic operations. The specific protection method must align with the required service level and operational model. For a seal, the following link is particularly important:
organization – private key – usage authorization – seal – data

The evidentiary value of the seal therefore does not rely solely on the mathematical verifiability of the signature. Equally significant is the organization's ability to demonstrate how the key was protected, who could use it, and under what conditions.

Electronic Time Stamp

An electronic time stamp constitutes a distinct cryptographic service. Its purpose is neither to confirm the intent of the signatory nor to establish the origin of the document itself. It serves to prove that specific data existed in a given form no later than the moment indicated in the timestamp. The private key used to generate timestamps is managed by a Time Stamping Authority (TSA). The key is designated for this specific purpose, and its use should be kept separate from other cryptographic functions.

In addition to protecting the private key, other crucial elements include a trusted time source, its synchronization with Coordinated Universal Time (UTC), the auditing of operations, and protection against retroactive manipulation. The trustworthiness of a timestamp therefore depends on a chain of interlinked components:
TSA – private key – time source – timestamp – data

A timestamp does not, in itself, prove when a document was created, who created it, or who signed it. It proves the existence of specific data at a particular time, in the form in which it was submitted to the service.

Legal Significance

The legal significance of electronic signatures, seals, and other trust services within the European Union is governed primarily by the eIDAS Regulation (electronic Identification, Authentication and Trust Services) and related regulations. eIDAS is a legal framework, not a technical standard. It establishes legal effects and requirements for electronic identification and trust services; Technical requirements and interoperability are subsequently elaborated through European and international standards and implementing acts.
However, the use of cryptographic key material within an organization entails broader security and procedural requirements that cannot be reduced solely to eIDAS. The following overview is therefore not an eIDAS taxonomy; rather, it represents a practical distinction between certain functions of cryptographic mechanisms. The individual items have different legal significance, and some do not constitute separate legal categories under eIDAS.

In this context, non-repudiation is best understood as an evidentiary or security attribute rather than a separate legal category. Specific legal effects depend on the service used, the level of assurance, and the legal regime.
eIDAS is a European Union legal framework. Technical requirements and interoperability are supported in a number of areas by ETSI standards and other specifications and implementing acts of the European Commission. Other jurisdictions have their own legal and technical frameworks. International standards, such as ISO/IEC 27001 or ISO/IEC 27002, represent security frameworks and should not be confused with the legal framework for electronic signatures.

Where authentication fits in

At first glance, authentication might appear to be missing from the overview of purposes between the document and the signature as is the case with a cryptographic digital signature. Authentication using a digital signature. Technically, the same or similar cryptographic algorithms may be used as for an electronic signature; however, the legal and operational significance differs.
The primary goal of authentication is to prove that the counterparty controls the corresponding cryptographic key. A typical example is TLS, where a client or server can prove possession of a private key by signing a challenge. However, this does not automatically constitute an electronic signature of a legally significant document.

Thus, the same cryptographic algorithm can be used for two operations that serve entirely different purposes. That is precisely why, when managing key material, it is important to record not only the algorithm and the key itself but also its intended purpose.

Defining Purpose as a Security Rule

The definition of a key's purpose should be practical and actionable. Simply noting that it is a "signing key" is insufficient. It is advisable to specify at least the following:

• who owns the key,
• which entity uses the key,
• which service the key is assigned to,
• which cryptographic operations are permitted,
• which operations are prohibited,
• what data or protocols the key may protect,
• what privileges are associated with the key's use,
• the intended duration of the key's use,
• how compromise and replacement are handled.

This establishes a link between the key's purpose and its lifecycle. For instance, a key created for authentication should not subsequently be used as a signing key without further authorization, simply because the algorithm technically supports both operations. Likewise, it is unwise to assume that a single, universal key will simplify management. While it may reduce the number of keys, it can simultaneously increase the scope of consequences should a key be compromised, and complicate both auditing and the ability to definitively determine which operations were authorized.

Conclusion

Defining the purpose of key material establishes a boundary between what a cryptographic key is technically capable of doing and what an organization actually permits it to be used for.
Electronic signatures, electronic seals, time-stamping, code signing, and authentication may rely on similar cryptographic principles, yet their security, operational, and—potentially—legal implications differ. Therefore, the algorithm should not be conflated with the purpose.
For a personal signing key, the link to a specific signer is crucial. For a sealing key, the link to the organization and its control over the key's usage is paramount. For a TSA key, the connection to a trusted service and a time source is important. For an authentication key, the key factor is the scope of authorization associated with the identity the key represents.
A correctly defined purpose provides the foundation for the subsequent stages of key material management: determining who is authorized to create the key, who may access it, and who may use it.

Autor článku:

Jan Dušátko

Jan Dušátko has been working with computers and computer security for almost a quarter of a century. In the field of cryptography, he has cooperated with leading experts such as Vlastimil Klíma or Tomáš Rosa. Currently he works as a security consultant, his main focus is on topics related to cryptography, security, e-mail communication and Linux systems.