According to an example aspect of the present invention, there is provided an apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to obtain a document, segmented into a first number of segments (510), obtain the first number of outputs of a cryptographic operation, such that for each output, a random value and a document segment are employed to generate an input to the cryptographic operation (520), build a Merkle tree based on the outputs of the cryptographic operation (530), and store a top hash of the Merkle tree in a block chain (540).