A digital message is signed and, if a request is approved, receives a time stamp. The request is computed as a first function of the message and a current one of a sequence of passwords computed such that each password corresponds to an index unit. Each of the passwords may be computed as a function, such as a hash function, pseudo-random function, or encryption function, of the subsequent password, whereby the sequence terminates with an initial password that forms a public key parameter for the password sequence. At least one hash tree uses at least a subset of the passwords as inputs to a hash tree used to verify the passwords.