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Keystream
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<h2 id="example">Example</h2> <p>In this simple example we use the English alphabet of 26 characters from a-z. Thus we can not encrypt numbers, commas, spaces and other symbols. The random numbers in the keystream then have to be at least between 0 and 25. </p><p>To encrypt we add the keystream numbers to the plaintext. And to decrypt we subtract the same keystream numbers from the ciphertext to get the plaintext. </p><p>If a ciphertext number becomes larger than 25 we wrap it to a value between 0-25. Thus 26 becomes 0 and 27 becomes 1 and so on. (Such wrapping is called <a href="/facts/Modular_arithmetic/0wtRa5dU">modular arithmetic</a>.) </p><p>Here the plaintext message "attack at dawn" is combined by addition with the keystream "kjcngmlhylyu" and produces the ciphertext "kcvniwlabluh". </p> <table><tbody><tr><th>Plaintext</th><td>a</td><td>t</td><td>t</td><td>a</td><td>c</td><td>k</td><td>a</td><td>t</td><td>d</td><td>a</td><td>w</td><td>n</td></tr><tr><th>Plaintext as numbers</th><td>0</td><td>19</td><td>19</td><td>0</td><td>2</td><td>10</td><td>0</td><td>19</td><td>3</td><td>0</td><td>22</td><td>13</td></tr><tr><th>Keystream</th><td>k</td><td>j</td><td>c</td><td>n</td><td>g</td><td>m</td><td>l</td><td>h</td><td>y</td><td>l</td><td>y</td><td>u</td></tr><tr><th>Keystream as numbers</th><td>10</td><td>9</td><td>2</td><td>13</td><td>6</td><td>12</td><td>11</td><td>7</td><td>24</td><td>11</td><td>24</td><td>20</td></tr><tr><th>Ciphertext as numbers</th><td>10</td><td>28</td><td>21</td><td>13</td><td>8</td><td>22</td><td>11</td><td>26</td><td>27</td><td>11</td><td>46</td><td>33</td></tr><tr><th>Ciphertext as numbers wrapped to 0-25</th><td>10</td><td>2</td><td>21</td><td>13</td><td>8</td><td>22</td><td>11</td><td>0</td><td>1</td><td>11</td><td>20</td><td>7</td></tr><tr><th>Ciphertext as text</th><td>k</td><td>c</td><td>v</td><td>n</td><td>i</td><td>w</td><td>l</td><td>a</td><td>b</td><td>l</td><td>u</td><td>h</td></tr></tbody></table> <ul><li><i><a href="http://www.cacr.math.uwaterloo.ca/hac/">Handbook of Applied Cryptography</a></i> by Menezes, van Oorschot and Vanstone (2001), chapter 1, 6 and 7.</li></ul>