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[cipher-training.git] / slides / affine-encipher.html
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47
48 # Affine ciphers
49
50 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
51 --|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|--
52 b | e | h | k | n | q | t | w | z | c | f | i | l | o | r | u | x | a | d | g | j | m | p | s | v | y
53
54 An extension of Caesar ciphers
55
56 * Count the gaps in the letters.
57
58 ---
59 # How affine ciphers work
60
61 _ciphertext_letter_ =_plaintext_letter_ × a + b
62
63 * Convert letters to numbers
64 * Take the total modulus 26
65
66 # Enciphering is easy
67
68 * Build the `affine_encipher()` function
69
70 ---
71
72 # Deciphering affine ciphers is harder
73
74 `$$p = \frac{c - b}{a} \mod 26$$`
75
76 But modular division is hard!
77
78 Define division as mutiplication by the inverse: `\(\frac{x}{y} = x \times \frac{1}{y} = x \times y^{-1}\)`
79
80 A number _x_, when multiplied by its inverse _x_<sup>-1</sup>, gives result of 1.
81
82 This is not always defined in modular arithmetic. For instance, 7 × 4 = 28 = 2 mod 26, but 20 × 4 = 80 = 2 mod 26. Therefore, 4 doesn't have a multiplicative inverse (and therefore makes a bad key for affine ciphers).
83
84 Result from number theory: only numbers coprime with _n_ have multiplicative inverses in arithmetic mod _n_.
85
86 Another result from number theory: for non-negative integers _a_ and _n_, and there exist unique integers _x_ and _y_ such that _ax_ + _ny_ = gcd(_a_, _b_)
87
88 Coprime numbers have gcd of 1.
89
90 _ax_ + _ny_ = 1 mod _n_. But _ny_ mod _n_ = 0, so _ax_ = 1 mod _n_, so _a_ = _x_<sup>-1</sup>.
91
92 Perhaps the algorithm for finding gcds could be useful?
93
94 ---
95
96 # Euclid's algorithm
97
98 .float-right[![right-aligned GCD](gcd.svg)]
99
100 World's oldest algorithm.
101
102 _a_ = _qb_ + _r_ ; gcd(_a_, _b_) = gcd(_qb_ + _r_, _b_) = gcd(_r_, _b_) = gcd(_b_, _r_)
103
104 Repeatedly apply these steps until _r_ = 0, when the other number = gcd(_a_, _b_). For instance, _a_ = 81, _b_ = 57
105
106 * 81 = 1 × 57 + 24
107 * 57 = 2 × 24 + 9
108 * 24 = 2 × 9 + 6
109 * 9 = 1 × 6 + 3
110 * 6 = 2 × 3 + 0
111
112 Therefore, gcd(81, 57) = 3 and 81_x_ + 57_y_ = 3
113
114 Now unfold the derivation to find _x_ and _y_
115
116 * 3 = 9 × 1 + 6 × -1
117 * 3 = 9 × 1 + (24 - 2 × 9) × -1 = 9 × 3 + 24 × -1
118 * 3 = (57 - 2 × 24) × 3 + 24 × -1 = 57 × 3 + 24 × -7
119 * 3 = 57 × 3 + (81 - 57 × 1) × -7 = 57 × 10 + 81 × -7
120
121 Can we do this in one pass?
122
123 ---
124
125 # Hands up if you're lost
126
127 ## (Be honest)
128
129 ---
130
131 # Triple constraints
132
133 .float-right[![right-aligned GCD](fast-good-cheap.gif)]
134
135 ## Fast, cheap, good: pick two
136
137 ## Programmer time, execution time, space: pick one, get some of another.
138
139 (Scripting languages like Python are popular because they reduce programmer time. Contrast with Java and Pascal.)
140
141 Extended Euclid's algorithm has lots of programmer time (and risk of bugs), but will take virtually no space (6 numbers).
142
143 Can we trade space for ease?
144
145 A standard technique is memoisation: store the results somewhere, then just look them up.
146
147 ---
148
149 # Modular multiplication table for 7
150
151 (7) | 0 | 1 | 2 | 3 | 4 | 5 | 6
152 ----|---|---|---|---|---|---|---
153 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0
154 1 | 0 | 1 | 2 | 3 | 4 | 5 | 6
155 2 | 0 | 2 | 4 | 6 | 1 | 3 | 5
156 3 | 0 | 3 | 6 | 2 | 5 | 1 | 4
157 4 | 0 | 4 | 1 | 5 | 2 | 6 | 3
158 5 | 0 | 5 | 3 | 1 | 6 | 4 | 2
159 6 | 0 | 6 | 5 | 4 | 3 | 2 | 1
160
161 Can use this to find the multiplicative inverses.
162
163 (7) | 1 | 2 | 3 | 4 | 5 | 6
164 ----|---|---|---|---|---|---
165 | 1 | 4 | 5 | 2 | 3 | 6
166
167 How much to store?
168
169 ---
170
171 # How much to store?
172
173 1. The inverses for this modular base.
174 2. The inverses for all bases (12 of them)
175 3. All the _x_ ÷ _y_ = _z_ mod _n_ triples...
176 4. ...for all _n_
177 5. The decipherment table for this key
178 6. The decipherment table for all keys
179
180 The choice is a design decision, taking into account space needed, time to create and use, expected use patterns, etc.
181
182 ## Thoughts?
183
184 ---
185
186 # How much to store?
187
188 Keeping the decipherment close to encipherment seems aesthetically better to me.
189
190 Giving the ability to do division is the most obvious (to me).
191
192 As there are only a few possible modular bases, might as well calculate the whole table at startup.
193
194 ## Now implement affine decipherment.
195
196 Check both enciphering and deciphering work. Round-trip some text.
197 ---
198
199 # Counting from 0 or 1
200
201 When converting letters to numbers, we're using the range 0-25.
202
203 Another convention is to use numbers in range 1-26.
204
205 Implement this.
206
207 * You'll need another parameter:
208 ```python
209 affine_encipher_letter(letter, multiplier, adder, one_based=False)
210 ```
211
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