1. An apparatus, comprising: a processor; a communication interface; an indicator mechanism, which, when activated, outputs binary information perceivable to a user of at least one of the apparatus and another device; a computer readable storage medium coupled to the processor comprising instructions which when executed by the processor cause the processor to: initiate a security transaction between the apparatus and the other device, the security transaction using one or more public keys according to a protocol to establish a shared secret state between the apparatus and the other device; wherein both the apparatus and the other device contribute to the shared secret state; and cause the indicator mechanism to output a binary sequence such that the binary sequence is perceivable to the user, wherein the binary sequence represents at least some noncontiguous bits of a representation of the shared secret state.