A cryptographic method for carrying out selective access control to attributes of a plurality of documents assigned to a first user, the attributes being stored in a tamper-proof manner in one or more memories (266, 286, 306), the method comprising: providing a non-linear personal document blockchain structure, which is assigned to the first user and comprises a plurality of linear blockchain substructures (104, 108, 109, 110, 600), each of the linear blockchain substructures comprising a plurality of blocks that are bidirectionally linked to one another, a first linear blockchain substructure of the document blockchain structure forming a primary structure of the document blockchain structure, blocks of the primary structure in each case being assigned to an individual document of the plurality of documents, forming a starting block of a second linear blockchain substructure, which is comprised by a document-specific secondary structure of the document blockchain structure and assigned to the corresponding document, and comprising an identifier of the corresponding document, each of the documents comprising a plurality of attributes, which are stored distributed across a plurality of blocks of the document-specific secondary structure assigned to the corresponding document; providing an authorization blockchain structure for monitoring the access rights to the blocks of the document blockchain structure, the authorization blockchain structure comprising a starting block and one or more further blocks, the further blocks in each case granting an access right to a further user to one of the blocks of the document blockchain structure; in response to an access authorization request for an access right of a second user to one of the blocks of the personal document blockchain structure of the first user comprising one or more attributes of one of the documents, verifying access rights of the second user for access to the corresponding block, access to the corresponding block only being granted when the second user is comprised by the authorization blockchain structure as a user who has been granted access rights; in response to a successful verification, confirming the successful verification for the second user; and in response to an access request by the second user while providing the confirmation of the successful verification, granting access to the corresponding block to the second user. providing a non-linear personal document blockchain structure, which is assigned to the first user and comprises a plurality of linear blockchain substructures (104, 108, 109, 110, 600), each of the linear blockchain substructures comprising a plurality of blocks that are bidirectionally linked to one another, a first linear blockchain substructure of the document blockchain structure forming a primary structure of the document blockchain structure, blocks of the primary structure in each case being assigned to an individual document of the plurality of documents, forming a starting block of a second linear blockchain substructure, which is comprised by a document-specific secondary structure of the document blockchain structure and assigned to the corresponding document, and comprising an identifier of the corresponding document, each of the documents comprising a plurality of attributes, which are stored distributed across a plurality of blocks of the document-specific secondary structure assigned to the corresponding document; providing an authorization blockchain structure for monitoring the access rights to the blocks of the document blockchain structure, the authorization blockchain structure comprising a starting block and one or more further blocks, the further blocks in each case granting an access right to a further user to one of the blocks of the document blockchain structure; in response to an access authorization request for an access right of a second user to one of the blocks of the personal document blockchain structure of the first user comprising one or more attributes of one of the documents, verifying access rights of the second user for access to the corresponding block, access to the corresponding block only being granted when the second user is comprised by the authorization blockchain structure as a user who has been granted access rights; in response to a successful verification, confirming the successful verification for the second user; and in response to an access request by the second user while providing the confirmation of the successful verification, granting access to the corresponding block to the second user.