identity

A sentry would yell "Flash!", and only a fellow American soldier would know to respond with "Thunder!", confirming they were friendly. 

In the book of Judges describes Gileadite soldiers using the word "shibboleth" to detect their Ephraimite enemies, who pronounced it differently as "sibboleth". 

Passwords!

Let x represent the pronunciation:
x = 1 if the person pronounces it correctly as "shibboleth"
x = 0 if the person pronounces it as "sibboleth"

Where:
x = 1 means the person is a Gileadite (ally)
x = 0 means the person is an Ephraimite (enemy)

Identity reduced to the indivisible for a fleeting moment. 
The function - Rapidly discern the following: Ally v. Enemy 
x ∈ {0, 1}

Identifiers are irrelevant in the way they are represented, but require a web of context to become meaningful.