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:

• Python Code Walkthrough
• Animated Explanation

Table of contents

• Usage
• Glossary
  • Variables
  • Functions
• Theory
  • 1. Background
  • 2. ASCII Conversion
  • 3. Pad Generation
  • 4. Generating a ciphertext
  • 5. Decrypting to plaintext
• ASCII table

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