# Definition:Trapdoor Function

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## Definition

A **trapdoor function** is a mapping $f$ which is:

- for a given $x$ in the domain of $f$ is easy to calculate
- for a given $y$ in the image of $f$ is hard to calculate unless a specific piece of information $t$ is supplied, in which case it is easy.

That piece of information is known as the **trapdoor**.

## Historical Note

The concept of a trapdoor function was invented by Whitfield Diffie and Martin Hellman in $1975$.

## Sources

- 1986: David Wells:
*Curious and Interesting Numbers*... (previous) ... (next): '$..........$' - 1997: David Wells:
*Curious and Interesting Numbers*(2nd ed.) ... (previous) ... (next): '$..........$'