How do Machines Keep Secrets I
Ryan: FYS Computational Reasoning Fall 2025
Title Text
How machines keep secrets I
Ryan: FYS Computational Reasoning Fall 2025
Vocabulary
"plaintext"
cyphertext
key
encryption
means taking a message or
transforming it with an algorithm called a
cypher
into a form
called a
that can only be read if you have a
which is a text or number that can b e used to "reverse" the
cypher to
decrypt
the message
Caesar or Shift Cypher
GHIJKLMNOPQRSTUVWXYZABCDEF
Encrypt letter with a letter N locations away
Weakness: cryptanalyzable with statistics
ABCDEFGHIJKLMNOPQRSTUVWXYZ
GHIJKLMNOPQRSTUVWXYZABCDEF
ABCDEFGHIJKLMNOPQRSTUVWXYZ
For N=6
my secret > seykixkz
Better: Use a "KEY"
Shift Cypher with Key
Shift each letter in plaintext by different amount
9, 14, 6, 15
Select a key phrase.
Note alphabet position of each letter.
info
ABCDEFGHIJKLMNOPQRSTUVWXYZ JKLMNOPQRSTUVWXYZABCDEFGHI
ABCDEFGHIJKLMNOPQRSTUVWXYZ
OPQRSTUVWXYZABCDEFGHIJKLMN
ABCDEFGHIJKLMNOPQRSTUVWXYZ
GHIJKLMNOPQRSTUVWXYZABCDEF
ABCDEFGHIJKLMNOPQRSTUVWXYZ
PQRSTUVWXYZABCDEFGHIJKLMNO
i
n
f
o
in foinfo
NOTE: Lots of ways to do this. Here the rule is "rotate alphabet N positions to the left"
my secret >
i
k
f
l
t
y
m
v
STOP+THINK
If the key is
starry starry night
what is the cyphertext of
seven pm
s e v e n p m s t a r r y s
S ABCDEFGHIJKLMNOPQRSTUVWXYZ
TUVWXYZABCDEFGHIJKLMNOPQRS
T ABCDEFGHIJKLMNOPQRSTUVWXYZ
UVWXYZABCDEFGHIJKLMNOPQRST
A ABCDEFGHIJKLMNOPQRSTUVWXYZ
BCDEFGHIJKLMNOPQRSTUVWXYZA
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
Y ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZABCDEFGHIJKLMNOPQRSTUVWXY
S ABCDEFGHIJKLMNOPQRSTUVWXYZ
TUVWXYZABCDEFGHIJKLMNOPQRS
T ABCDEFGHIJKLMNOPQRSTUVWXYZ
UVWXYZABCDEFGHIJKLMNOPQRST
A ABCDEFGHIJKLMNOPQRSTUVWXYZ
BCDEFGHIJKLMNOPQRSTUVWXYZA
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
Y ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZABCDEFGHIJKLMNOPQRSTUVWXY
N ABCDEFGHIJKLMNOPQRSTUVWXYZ
OPQRSTUVWXYZABCDEFGHIJKLMN
I ABCDEFGHIJKLMNOPQRSTUVWXYZ
JKLMNOPQRSTUVWXYZABCDEFGHI
L
Y
W
W
F
O
F
STOP+THINK
If the key is
starry starry night
what is the cyphertext of
seven pm
Decrypt the message
vfbkkhliwwj
STOP+THINK
S ABCDEFGHIJKLMNOPQRSTUVWXYZ
TUVWXYZABCDEFGHIJKLMNOPQRS
T ABCDEFGHIJKLMNOPQRSTUVWXYZ
UVWXYZABCDEFGHIJKLMNOPQRST
A ABCDEFGHIJKLMNOPQRSTUVWXYZ
BCDEFGHIJKLMNOPQRSTUVWXYZA
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
Y ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZABCDEFGHIJKLMNOPQRSTUVWXY
S ABCDEFGHIJKLMNOPQRSTUVWXYZ
TUVWXYZABCDEFGHIJKLMNOPQRS
T ABCDEFGHIJKLMNOPQRSTUVWXYZ
UVWXYZABCDEFGHIJKLMNOPQRST
A ABCDEFGHIJKLMNOPQRSTUVWXYZ
BCDEFGHIJKLMNOPQRSTUVWXYZA
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
R ABCDEFGHIJKLMNOPQRSTUVWXYZ
STUVWXYZABCDEFGHIJKLMNOPQR
Y ABCDEFGHIJKLMNOPQRSTUVWXYZ
ZABCDEFGHIJKLMNOPQRSTUVWXY
N ABCDEFGHIJKLMNOPQRSTUVWXYZ
OPQRSTUVWXYZABCDEFGHIJKLMN
I ABCDEFGHIJKLMNOPQRSTUVWXYZ
JKLMNOPQRSTUVWXYZABCDEFGHI
L
A
S
S
I
S
O
V
C
E
R
Instead of a finite key, how about an infinite supply of random numbers?
From Better to Best
The One-Time Pad
Cryptography 101: One-Time Pads
infinite supply of random "numbers"
plaintext here
cyphertext here
You encode your message using the OTP and then you burn the OTP. I can decode because I have a copy of the OTP.
STOP+THINK: Decrypt CWJHJDVOKVXHIATBEGFPSSGK
D
1
2
3
4
STOP+THINK: Encrypt "meet me at eight"
key:YHMTMUBGHGBYE
STOP+THINK: Decrypt CWJHJDVOKVXHIATBEGFPSSGK
How Machines Keep Secrets I
By Dan Ryan
How Machines Keep Secrets I
Cryptography 101 and the one time pad
