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CS Unplugged activity

Secret Messages in ASCII

CS Unplugged activity slides Lesson plan (and sub notes)

About 20 minutesSoloPencil

Computers do not store letters. They store numbers. ASCII is the code that turns each letter into a number, and you are about to crack it.

Your code table

A	B	C	D	E	F	G	H	I
65	66	67	68	69	70	71	72	73
J	K	L	M	N	O	P	Q	R
74	75	76	77	78	79	80	81	82
S	T	U	V	W	X	Y	Z	space
83	84	85	86	87	88	89	90	32

What to do

1. Find each number in the table above.
2. Write the matching letter in the box under it.
3. A hatched box is the code 32. That means “space.” Leave it blank.
4. Read your finished answer to check it makes sense.

Now decode these

1. A computer stores every letter as a ___.

78

85

77

66

69

82

2. Binary only uses two digits: zero and ___.

79

78

69

3. A group of eight bits is called a ___.

66

89

84

69

4. Fun fact: the first computer bug was a real ___.

84

73

78

89

32

77

79

84

72

Lowercase, punctuation, and digits

Real messages need more than capital letters. Every lowercase letter’s code is 32 more than its capital: A is 65, so a is 97.

a	b	c	d	e	f	g	h	i
97	98	99	100	101	102	103	104	105
j	k	l	m	n	o	p	q	r
106	107	108	109	110	111	112	113	114
s	t	u	v	w	x	y	z
115	116	117	118	119	120	121	122

space	!	,	.	?
32	33	44	46	63

0	1	2	3	4	5	6	7	8	9
48	49	50	51	52	53	54	55	56	57

Notice that the digit 7 is stored as 55, not 7. To a computer, the character “7” and the number 7 are different things. (That’s why a program that reads typed text has to convert “12” into the number 12 before it can do math with it.)

For example, 72 105 33 is Hi!

Decode this friendly question.

72

105

44

32

104

111

119

32

97

114

101

32

121

111

117

63

Now with digits. Decode this one.

73

32

97

109

32

49

50

32

121

101

97

114

115

32

111

108

100

46

Write your name in ASCII

Now go the other way. Write one letter of your name in each top box, then use the table to write its code underneath.

Short name? Leave the extra boxes blank. Longer than eight letters? Keep going on the back of the page.

Secret messages for a friend

Think of two short messages (up to 16 letters, spaces, and marks each) and write them on the lines below the fold line. Then write each message’s codes in its boxes, one character per box. Fold the bottom strip under so only the codes show, and trade with a friend. Can they read your messages?

Message 1 codes:

Message 2 codes:

fold here

Message 1:

Message 2:

Your friend’s messages

Decode your friend’s codes, one character in each box.

Friend's message 1:

Friend's message 2:

ASCII table: 32 to 122

Every character from code 32 to 122. Use it for your secret messages.

Code	Char	Code	Char	Code	Char	Code	Char	Code	Char
32	space	51	3	70	F	89	Y	108	l
33	!	52	4	71	G	90	Z	109	m
34	"	53	5	72	H	91	[	110	n
35	#	54	6	73	I	92	\	111	o
36	$	55	7	74	J	93	]	112	p
37	%	56	8	75	K	94	^	113	q
38	&	57	9	76	L	95	_	114	r
39	'	58	:	77	M	96	`	115	s
40	(	59	;	78	N	97	a	116	t
41	)	60	<	79	O	98	b	117	u
42	*	61	=	80	P	99	c	118	v
43	+	62	>	81	Q	100	d	119	w
44	,	63	?	82	R	101	e	120	x
45	-	64	@	83	S	102	f	121	y
46	.	65	A	84	T	103	g	122	z
47	/	66	B	85	U	104	h
48	0	67	C	86	V	105	i
49	1	68	D	87	W	106	j
50	2	69	E	88	X	107	k

Want even more (codes 0 to 127, in hex too)? See the full ASCII / Hex table.

Answer key

1. NUMBER
2. ONE
3. BYTE
4. TINY MOTH

Lowercase, punctuation, and digits: Hi, how are you? / I am 12 years old.