In an example there is provided a method for certifying transactions of a workflow for a workflow comprising a set of transactions and a permitted order of execution of transactions. The method comprises generating verification data in response to a request from a client to certify a transaction of the workflow on the basis of a secure ledger where the verification data certifies the validity of the secure ledger. The method further comprises communicating the verification data to the client. The secure ledger is determined on the basis of workflow transactions.