Encryption is the conversion of data into a form, called a cipher, that cannot be easily understood by unauthorized people. Decryption is the process of converting encrypted data back into its original form, so it can be understood.

Index:
Basic Concepts:
A Brief History of Cryptography:

Ancient Ciphers
Machine Ciphers
Classical Cryptographic Techniques
Caesar Cipher - a monoalphabetic cipher
Cryptanalysis of the Caesar Cipher
Character Frequencies
Cryptanalysis:
Mixed Alphabets
Cryptanalysis
Cryptanalytic Attacks:

ciphertext only
known plaintext
chosen plaintext
chosen plaintext-ciphertext
Unconditional and Computational Security
Strategies for Deciphering:

     Compile a list of known codes and cipher methodologies that you can use on secret messages. Once you have tried all the methods you know of, use a hit and miss method, trial and error. The structure of the English language enables cryptologists to see patterns that emerge from normal messages. Just by looking at this paragraph of text, or playing Wheel of Fortune, you can figure out which letters are most common.

     Vowels and spaces are the most frequent in the English language. A few constants, such as "S, T, N, etc." are more common than others. Finally, a few known bigrams (2 letters), such as "ll, ee, ss, etc," are more common than others. By recognizing patterns in ciphers you can often guess what the letters are decipher the message. The computer application "Cryptogrammer" teaches you how to decipher this way.


8 Frequency Tables for Deciphering 
Most Common Letters. In order of most common to least:
1. E
2. T
3. A, O, N, R, I, S
4. H
5. D, L, F, C, M, U
6. G, Y, P, W, B
7. V, K, X, J, Q, Z

Bigram Frequency. In order from most common to least:
TH, HE, AN, RE, ER, IN, ON, AT, ND, ST, ES, EN, OF, TE

Bigram Same Letter Frequency. In order from most common to least:
LL, EE, SS, OO, TT, FF, RR, NN, PP, CC, MM, GG

Trigram Frequency:
THE, ING, CON, ENT, ERE, ERS, EVE, FOR, HER, TED, TER, TIO, VER

Intial Letters:
T, A, O, M, H, W, C, I, P, B, E, S

Second Letters:
H, O, E, I, A, U, N, R, T

Third Letters:
E, S, A, R, N, I

Final Letters:
E, T, S, D, N, R, Y, G

*More than 50% of English words end with "E."
*More than 50% of English words start with T, A, O, S, or W.
     It's a good idea to create a table of the counts for letters of the alphabet, bigrams, trigrams, and initial and ending letters to see how many times a certain letter(s), number(s), or symbol(s) from an encrypted message occurs. The highest occurance of a single letter/number/symbol is most likely to be "E" as seen in the frequency table above. The most common 2 letter/number/symbol bigram is likely to be "TH." If you can place a few likely letters you will often see a few short words, like "THE" appear.

     Once words start to appear you're on to something. See if there is an easy pattern to get the rest of the letters. If not, continue analyzing frequency tables to decode the rest of the letters.

     Use trial and error and replace letters/numbers/symbols with different letters until more words begin to appear. Try this method on the simple cipher below. Can you figure out what the encrypted message says?

Wpxfmt bsf uif nptu dpnnpo mfuufs