Spike Clancy (hclancy@dimacs.rutgers.edu)

**General Information**

This lesson is based on Adi Shamir's talk on "Cryptography Without Computers" and the UCSMP Geometry book definition of discrete points or pixels. By using a TI-83 calculator we can hide information.

**Category**

This lesson can be used in geometry, Algebra I or II, and courses in discrete mathematics or cryptography.

**Grade Levels**

Grades 9 - 12 (also 7 & 8 with revisions)

**Activity 1**

Hiding Informations with the TI-83

**Suggested Materials**

TI-83 calculators

TI-83 view screen

**Procedure**

1. Draw face or message on the TI-83 using <Graph> <2nd> <Draw> Circle.

2. Draw circle for face and eyes.

3. <2nd> <Draw> <Pen> to draw mouth.

4. <2nd> <Draw> >> <Sto> <1> Store pic in 1.

5. To recall the picture <2nd> <Draw> >> <2>.

6. Use <2nd> and hold down <^> to darken screen and hide picture.

7. Use <2nd> and hold down <v> to regain hidden picture.

8. To hide a text use <2nd> <Draw> ??? text.

9. Write out text and follow directions for hiding pictures.

*Hints:*

1.If mistakes are made hit <2nd> <Draw> <1> "ClrDraw" and start over again.

2. Use <2nd> <Format> and turn axes off.

**References**

UCSMP __Geometry__, Scott Foresman (c)1997

Shamir, Adi, "Cryptography Without Computers"