Plug.Crypto.secure_compare

You're seeing just the function secure_compare, go back to Plug.Crypto module for more information.
Link to this function

secure_compare(left, right)

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Specs

secure_compare(binary(), binary()) :: boolean()

Compares the two binaries in constant-time to avoid timing attacks.

See: http://codahale.com/a-lesson-in-timing-attacks/