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Simple otp (One Time Pad)

NONE of the implementations are intended for production usage. They have not been extensively validated for it. These files are simply an educational tool for looking at AN implementation that works to help people understand it.

There are two accompanying videos to this repository, one walks through the python implementation in detail, and another is an animated explanation of the one-time pad protocol on the whole:

Table of contents

Usage

Each folder contains a language implementation with details on how to setup a runtime environment and run the corresponding file. The most polished implementation is python since that's my preferred language. When you run the file it will generate three files pad.txt, ciphertext.txt and plaintext.txt these each correspond to the variables generated based on the terms below.

Glossary

Throughout the files I use a set of standardized terms to refer to variables and functions to make the implementations understandable:

Variables

text: The text that you want to encrypt, in all files this is hardcoded and I am using Do not go gentle into that good night by Dylan Thomas.

pad: The one time pad that is used to encrypt the text

ciphertext: The text generated through the iterative XORing of the text and pad

plaintext: The decrypted value generated through the XORing of the pad and ciphertext (should match the text variable if the implementation is correct)

Functions

Note the names change to suit whatever is the preferred language style so I have opted to just use names with uppercase first letters and spaces in between instead of either snake_case or camelCase.

Generate pad: Used to generate the one-time pad. Will take in an integer for the length of pad it needs to generate and return whatever type is most appropriate for the language (either a string or char array). This is completed during step 3 of the theory section.

Encrypt: Generates the ciphertext using the pad and text, then return it as whatever type is most appropriate for the language (either a string or char array). This is completed during step 4 of the theory section.

Decrypt: Takes in the ciphertext and pad, generates the plaintext and return it as whatever type is most appropriate for the language (either a string or char array). This is completed during step 5 of the theory section.

Save: Takes in either a string or char array based on which language it is, and a file path then serializes the string/char array to the path provided.

Theory

This explanation is meant to be somewhat simplistic and skip over some of the detail about secrecy and proofs why the math works. This description is meant to be a laymen's terms, step-by-step walkthrough of how the one-time pad protocol works.

Again there is also an animated explanation of the protocol you can watch.

1. Background

The one-time pad relies on a number of principles that aren't immediately obvious to everyone, two of the most basic are:

  1. Any character can be represented as an integer

  2. XORing 2 values, and then XORing the result of the values with one of the original values produces the other. So for example:

    a XOR b = c
c XOR b = a
c XOR a = b

    or

    text XOR pad = ciphertext
ciphertext XOR pad = text
ciphertext XOR text = pad

Additionally to maintain secrecy there are a few things that need to be done:

  1. Each pad must be used one time, or else the ciphertexts are susceptible to cribdragging (a technique where you can use only resulting ciphertexts to determine the original pad).
  2. The numbers generated must be random as flaws in the pseudorandom generation can be exploited.
  3. The attacker must not have any intuition (be able to guess) as to what's inside the files.

2. ASCII Conversion

One way to do a conversion from string/chars to integers is to use an ASCII table, which allows you to have a 1-1 mapping of characters to an (at most) 2 byte integer. For example let's convert the string 'Hello' using the ASCII table below. The 'H' corresponds to 72 (0x48 in hex and 0b1001000 in binary) using the chart so that would be our first integer. The whole sequence would then be (72, 101, 108, 108, 111).

3. Pad Generation

From there we would use a random number generator to generate a pad of random 2 byte integers that is the same length as the input text (in this case 5 characters) lets say they are (11, 212, 8, 224, 122) which corresponds to ("VT", "Ô", "BS", "à", "z" ) . Notice that not all of these are printable characters (any with two letters) and so the resulting text file may end up looking weird.

4. Generating a ciphertext

The ciphertext can now be generated by iterating over the integer representations of the pad and original text ("Hello") and XORing them together. To XOR two numbers take the integers and convert them to binary, from there line up each binary number into columns and on columns where one of the bits is a 1 and the other is a 0 the result is 1, any other combination is a 0.

For example for the first character of our pad and original text "VT" (11) and "H"(72) would be:

0000 1011 ("VT" or 11)
0100 1000 ("H" or 72)
----------
0100 0011 ("C" or 67)

The whole sequence would be:

Characters: ("H", "e", "l", "l", "o") XOR ("VT", "Ô", "BS", "à", "z" ) = ("C", "±", "d", "Œ", "NAK")

Decimal: (72, 101, 108, 108, 111) XOR (11, 212, 8, 224, 122) = (67, 177, 100, 140, 21)

Now that we have the pad and ciphertext, you can give someone the ciphertext over a public channel and it can only be decrypted if they also have the pad (which should be sent through a secured channel).

5. Decrypting to plaintext

Now to decrypt we do the same thing we did to generate the ciphertext, but with the ciphertext and pad characters to reverse it to the plaintext (remember ciphertext XOR pad = text). So in this case:

Characters: ("VT", "Ô", "BS", "à", "z" ) XOR ("C", "±", "d", "Œ", "NAK") = ("H", "e", "l", "l", "o")

Decimal: (11, 212, 8, 224, 122) XOR (67, 177, 100, 140, 21) = (72, 101, 108, 108, 111)

ASCII table (provided by the built in one on bing)

DECIMAL HEX CHAR NAME (if not printable)
0 00 NUL Null
1 01 SOH Start of heading
2 02 STX Start of text
3 03 ETX End of text
4 04 EOT End of trans.
5 05 ENQ Enquiry
6 06 ACK Ack.
7 07 BEL Bell
8 08 BS Back space
9 09 HT Horizontal tab
10 0A LF Line feed
11 0B VT Vertical tab
12 0C FF Form feed
13 0D CR Carriage return
14 0E SO Shift out
15 0F SI Shift in
16 10 DLE Data line escape
17 11 DC1 Device control 1
18 12 DC2 Device control 2
19 13 DC3 Device control 3
20 14 DC4 Device control 4
21 15 NAK Negative ack.
22 16 SYN Synchronous idle
23 17 ETB End of block
24 18 CAN Cancel
25 19 EM End of medium
26 1A SUB Substitute
27 1B ESC Escape
28 1C FS File separator
29 1D GS Group separator
30 1E RS Record separator
31 1F US Unit separator
32 20 Space
33 21 !
34 22 " "
35 23 #
36 24 $
37 25 %
38 26 & &
39 27 '
40 28 (
41 29 )
42 2A *
43 2B +
44 2C ,
45 2D -
46 2E .
47 2F /
48 30 0
49 31 1
50 32 2
51 33 3
52 34 4
53 35 5
54 36 6
55 37 7
56 38 8
57 39 9
58 3A :
59 3B ;
60 3C < <
61 3D =
62 3E > >
63 3F ?
64 40 @
65 41 A
66 42 B
67 43 C
68 44 D
69 45 E
70 46 F
71 47 G
72 48 H
73 49 I
74 4A J
75 4B K
76 4C L
77 4D M
78 4E N
79 4F O
80 50 P
81 51 Q
82 52 R
83 53 S
84 54 T
85 55 U
86 56 V
87 57 W
88 58 X
89 59 Y
90 5A Z
91 5B [
92 5C \
93 5D ]
94 5E ^
95 5F _
96 60 `
97 61 a
98 62 b
99 63 c
100 64 d
101 65 e
102 66 f
103 67 g
104 68 h
105 69 i
106 6A j
107 6B k
108 6C l
109 6D m
110 6E n
111 6F o
112 70 p
113 71 q
114 72 r
115 73 s
116 74 t
117 75 u
118 76 v
119 77 w
120 78 x
121 79 y
122 7A z
123 7B {
124 7C |
125 7D }
126 7E ~
127 7F � Delete
128 80 € €
129 81 �
130 82 ‚ ‚
131 83 ƒ ƒ
132 84 „ „
133 85 … …
134 86 † †
135 87 ‡ ‡
136 88 ˆ ˆ
137 89 ‰ ‰
138 8A Š Š
139 8B ‹ ‹
140 8C Œ Œ
141 8D �
142 8E Ž
143 8F �
144 90 �
145 91 ‘ ‘
146 92 ’ ’
147 93 “ “
148 94 ” ”
149 95 • •
150 96 – –
151 97 — —
152 98 ˜ ˜
153 99 ™ ™
154 9A š š
155 9B › ›
156 9C œ œ
157 9D �
158 9E ž
159 9F ÿ ÿ
160 A0  
161 A1 ¡ ¡
162 A2 ¢ ¢
163 A3 £ £
164 A4 ¤ ¤
165 A5 ¥ ¥
166 A6 ¦ ¦
167 A7 § §
168 A8 ¨ ¨
169 A9 © ©
170 AA ª ª
171 AB « «
172 AC ¬ ¬
173 AD ­ ­
174 AE ® ®
175 AF ¯ ¯
176 B0 ° °
177 B1 ± ±
178 B2 ² ²
179 B3 ³ ³
180 B4 ´ ´
181 B5 µ µ
182 B6
183 B7 · ·
184 B8 ¸ ¸
185 B9 ¹ ¹
186 BA º º
187 BB » »
188 BC ¼ ¼
189 BD ½ ½
190 BE ¾ ¾
191 BF ¿ ¿
192 C0 À À
193 C1 Á Á
194 C2 Â Â
195 C3 Ã Ã
196 C4 Ä Ä
197 C5 Å Å
198 C6 Æ Æ
199 C7 Ç Ç
200 C8 È È
201 C9 É É
202 CA Ê Ê
203 CB Ë Ë
204 CC Ì Ì
205 CD Í Í
206 CE Î Î
207 CF Ï Ï
208 D0 Ð Ð
209 D1 Ñ Ñ
210 D2 Ò Ò
211 D3 Ó Ó
212 D4 Ô Ô
213 D5 Õ Õ
214 D6 Ö Ö
215 D7 × ×
216 D8 Ø Ø
217 D9 Ù Ù
218 DA Ú Ú
219 DB Û Û
220 DC Ü Ü
221 DD Ý Ý
222 DE Þ Þ
223 DF ß ß
224 E0 à à
225 E1 á á
226 E2 â â
227 E3 ã ã
228 E4 ä ä
229 E5 å å
230 E6 æ æ
231 E7 ç ç
232 E8 è è
233 E9 é é
234 EA ê ê
235 EB ë ë
236 EC ì ì
237 ED í í
238 EE î î
239 EF ï ï
240 F0 ð ð
241 F1 ñ ñ
242 F2 ò ò
243 F3 ó ó
244 F4 ô ô
245 F5 õ õ
246 F6 ö ö
247 F7 ÷ ÷
248 F8 ø ø
249 F9 ù ù
250 FA ú ú
251 FB û û
252 FC ü ü
253 FD ý ý
254 FE þ þ
255 FF ÿ ÿ