Cryptography
: Hidden Secret
Definition:
is the practice and study of techniques for secure communication in the presence of third
parties (called adversaries).
Before the Computer:

encryption: the conversion of information from a readable state to
apparent nonsense.

the originator of the encrypted message shared the decoding technique with only the recipient.
After the Computer:

heavily based on mathematical theory and computer science.

the originator tells the computer how to code a m
essage the computer does the work.

the recipient receives a coded message and the computer decodes it.
ATM cards, Computer passwords and electronic commerce.
Vocabulary
Encipher:
“to encipher a message” to change from readable to apparent nonsense.
Pl
aintext:
when information is readable, prior to encryption
Cipher text:
the apparent nonsense after encryption
Decipher
:
to transform the cipher text back to the original message.
Ciphers:
different methods of encrypting information
Caesar Cipher:
ea
ch letter in the plaintext is replace by a letter some fixed number of positions down
the alphabet.
Named after Julius Caesar who used it with a shift of three to protect messaged of military significance.
Example
1
:
Ciphertext: WKH TXLFN EURZQ IRA
MXPSV RYHU WKH ODCB GRJ
Plaintext: the quick brown fox jumps over the lazy dog
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC
Everything is shifted 3 to the left. Key = 3
Example2:
Ciphertext:
MAX YTNEM, WXTK UKNMNL, EBX
L GHM BG HNK LMTKL UNM BG HNKLXEOXL.
Plaintext: THE FAULT, DEAR BRUTUS, LIES NOT IN OUR STARS BUT IN OURSELVES.
Plain
: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher
: T U V W X Y Z A B C D E F G H I J K L M N O P Q R S
Everything is shifte
d 19 to the left. Key = 19
.
Breaking a Ceasar Cipher: The unknown shift is one of 26 possible shifts. One technique might be
to try each of the 26 possible shifts and check which of these resulted in readable English text.
Better Approach: Statis
tical data about English frequencies. Frequency Analysis:
Here is the alphabet in order of the frequency that each letter is used.
E, T, A, O, I, N, S, R, H, L, D, C, U, M, F, P, G, W, Y, B, V, K, X, J, Q, Z
Example 3:
Ciphertext:
Bj mfaj tsqd f kjb bj
jpx qjky zsynq xzrrjw gwjfp!
Example 4:
Ciphertext:
Wfi vmvip rtkzfe kyviv zj re fggfjzkv reu vhlrc ivrtkzfe.
Example 5:
Create your own! Pick a key and write out a plain and cipher alphabet. Encrypt your
message. And give to a friend to decode!
Si
mple Substitution Cipher
:
It basically consists of substituting every plaintext character for a different ciphertext character. It differs
from Caesar cipher in that the cipher alphabet is not simply the alphabet shifted, it is completely jumbled.
Exampl
e 6:
Cip
h
ertext:
giuifg cei iprc tpnn du cei qprcni
Example 7:
Ciphertext: FAX FWQ QYC WGVWYP EG QKJKH TKRWIQK QKJKH KYDAN HYHK!
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