# Towards a Catalogue of Self-Sovereign Identity Design Patterns
## 3 Self-Sovereign Identity Design Patterns and Their Classification
### Patterns and Their Classification
On the other hand, keys can be secured and split into multiple pieces/shards, stored on multiple devices or by multiple trusted entities (i.e., social recovery), and restored by using enough pieces, utilising the _Key Shards_ pattern [14]. (4.8) The _DID Controller_ is related to DID document management, and allows differentiation between a DID subject and the DID controller(s), as, sometimes, the DID subject is not the same as the DID controller (e.g., parents control the identity of their underage children; pet owners control the identifier of their pets, etc. ), and some use cases require multiple entities to control a DID (e.g., multiple owners control the organisation identifier) which can be determined in the DID document. Thus, the controller(s) defined in the documents are allowed to make changes to DID document(s), supporting more complex use cases where DID subjects are not capable of controlling their identity (e.g., guardianship as a type of indirect identity control). (4.9) The _Master and Sub-Key Generation_ pattern can be used to minimise correlation and increase the privacy and security of entities' identity and their keys. Thus, each entity has multiple keys connected with a particular identity, used for different purposes, meaning that each entity has a master key for managing a set of sub-keys used for signing different types of transactions [14]. Patterns within the _Qualified Electronic Certificates (QEC)_ category (Table 5) can provide legal value, as they bind SSI objects (VCs and DIDs) with the QEC issued by a duly accredited trust service provider. The binding can, therefore, provide the highest legal guarantee, instill trust in the entity's identity, and assure the integrity of signed documents and their enforceability. This is crucial, because some transactions require a higher LoA to establish sufficient trust. Therefore, some credentials and presentations require a regulatory-supported legal value. VCs can be signed by DIDs' corresponding private keys. However, currently, there is no binding between the DID and real-world natural or legal persons, and no law confers binding between legal value to credentials. Thus, (5.1) _Qualified Verifiable Credentials_ enables entities to sign/seal VCs and VPs with QEC private keys that have a legally binding value/effect. Decentralised digital identity can be linked with a real-world identity by establishing binding with a QEC, either through VCs or DIDs. (5.2) _Binding VCs and QECs_ require issuing verifiable credentials that demand identification and authentication before \begin{table}
\begin{tabular}{l l} \hline \hline
**Design Pattern** & **Summary** \\ \hline Qualified & \begin{tabular}{l} Signing verifiable credentials (and presentations) with qualified digital signatures or seals (eSignature, eSeal) and adding legal value to credentials [59; 60]. \\ \end{tabular} \\ \hline Binding & VCs & \
\begin{tabular}{l} Linking verifiable credentials with qualified electronic certificates [59; 60]. \\ \end{tabular} \\ \hline Binding & DIDs &
\begin{tabular}{l} Linking decentralised identifiers with qualified electronic certificates, either by enriching the DID document or using trusted registries [61]. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: SSI design patterns, classified into the Qualified Electronic Certificates (QEC) category. VC issuance, and they contain information about an entity and can provide substantial proof and evidence. (5.3) _Binding DIDs and QECs_ allows entities, i.e., verifiers, to retrieve qualified certificates and verify them against a trusted list. Thus, verifiers can check reliably which entity is behind a particular DID. DIDs can be linked with QECs using different approaches, either by enriching DID documents or by utilising trusted registries. However, it should be noted that trusted registries should not link natural person identifiers with public certificates, to avoid revealing their identity and comply with GDPR.