|
|
| (11 intermediate revisions by 2 users not shown) |
| Line 1: |
Line 1: |
| − | Cryptography is the process of hiding messages; either by concealing them (eg. hiding them in an image), or by obfuscating them outright (eg. substitution cipher).
| + | To learn about cryptography, check out the [http://gamedetectives.net/academy Game Detectives Academy] - a free, interactive series of tutorials! |
| − | | |
| − | == Basic Terminology ==
| |
| − | | |
| − | * '''Cipher''': a method of encryption
| |
| − | * '''Plaintext''': the legible text of a hidden message
| |
| − | * '''Ciphertext''': the text after a message is concealed in it
| |
| − | * '''Encryption''': The process of turning plaintext into ciphertext
| |
| − | * '''Decryption''': The process of converting ciphertext back into plaintext
| |
| − | * '''Key''': a string used in the encryption and decryption processes of some ciphers, akin to a password
| |
| − | | |
| − | == Basic Ciphers ==
| |
| − | | |
| − | === Caesar cipher ===
| |
| − | | |
| − | ''Click [http://www.xarg.org/tools/caesar-cipher/ here] to experiment with the Caesar cipher.''
| |
| − | | |
| − | The simplest example of a cipher is the Caesar cipher. The rules of the cipher are as follows:
| |
| − | | |
| − | Let '''n''' equal a value from 1 to 25
| |
| − | Shift each letter in the plaintext forward by '''n''' positions in the alphabet
| |
| − | The resultant string is the ciphertext
| |
| − | | |
| − | For example, to encrypt the string <code>Game Detectives</code> using the Caesar cipher, using an arbitrary '''n''' value of 2, then:
| |
| − | | |
| − | G -> H -> I
| |
| − | a -> b -> c
| |
| − | m -> n -> o
| |
| − | e -> f -> g
| |
| − | ...
| |
| − | | |
| − | and the resultant ciphertext would be <code>Icog Fgvgevkxgu</code>. To decrypt this string back into <code>Game Detectives</code>, the process can simply be reversed by shifting each letter of the ciphertext 2 places backwards. (''Note: another common name for the Caesar cipher is ROT<n> - ROT13 indicates that each letter is shifted halfway through the alphabet)''
| |
| − | | |
| − | === Binary ciphers ===
| |
| − | | |
| − | '''What is binary?'''
| |
| − | | |
| − | Binary is a system of counting, used by computers, that is different than the typical system of counting. You're used to counting by using 10 different digits: 0 to 9. This is known as '''base 10''', or decimal. Binary only uses 2 digits: 0 and 1, so it is known as '''base 2'''. Let me give you an example.
| |
| − | | |
| − | This is how you write the number ''one hundred and nine'' normally, in base-10. The top row represents the values of each digit place; you can see that, starting from the right-hand side and moving left, each consecutive decimal place is worth '''10 times more''' than the previous one in '''base 10'''. The bottom row can use digits from 0 to 9.
| |
| − | +-----+-----+-----+
| |
| − | | 100 | 10 | 1 |
| |
| − | +-----+-----+-----+
| |
| − | | 1 | 0 | 9 |
| |
| − | +-----+-----+-----+
| |
| − | 1*100 + 0*10 + 9*1
| |
| − | = 100 + 0 + 9
| |
| − | = 109
| |
| − | | |
| − | Okay, now, here's how you write the same number, ''one hundred and nine,'' in base-2. Again, the top row represents the values of each digit place; but this time, each decimal place is only worth '''2 times more''' than the previous one in '''base 2''. Now, the bottom row can only use the digits 0 and 1.
| |
| − | | |
| − | +-----+-----+-----+-----+-----+-----+-----+-----+
| |
| − | | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
| |
| − | +-----+-----+-----+-----+-----+-----+-----+-----+
| |
| − | | 0 | 1 | 1 | 0 | 1 | 1 | 0 | 1 |
| |
| − | +-----+-----+-----+-----+-----+-----+-----+-----+
| |
| − | 0*128 + 1*64 + 1*32 + 0*16 + 1*8 + 1*4 + 0*2 + 1*1
| |
| − | = 0 + 64 + 32 + 0 + 8 + 4 + 0 + 1
| |
| − | = 109
| |
| − | | |
| − | So, the base-10 (decimal) number of 109 is equal to the base-2 (binary) number of 01101101.
| |
| − | | |
| − | '''How is binary used in a cipher?'''
| |
| − | | |
| − | The binary cipher relies on the fact that each ASCII character that you can type on your keyboard has a unique identifying number, in binary. For example,
| |
| − | | |
| − | * Uppercase <code>A</code> has a binary code of <code>01000001</code> (converting to <code>65</code> in decimal)
| |
| − | * Lowercase <code>a</code> has a binary code of <code>01100001</code> (converting to <code>97</code> in decimal)
| |
| − | * Ampersand <code>&</code> has a binary code of <code>00100110</code> (converting to <code>38</code> in decimal)
| |
| − | * Plus sign <code>+</code> has a binary code of <code>00101011</code> (converting to <code>43</code> in decimal)
| |
