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POGenerating cryptographc random number for modulus of powers of 10 with node.js
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  1. POGenerating cryptographc random number for modulus of powers of 10 with node.js
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    copied!<p>Using the crypto library I can require the library to give me a number of random bytes. The thing is that this bytes are numbers from 0 to 255 each (inclusive) and thus it is not that simple to create a uniformly distributed results.</p> <p>What I mean is the following:</p> <p>The function receives a number N, which is 10, 100, ... 10^b, where b is a number between 1 and 8 (could be bigger, but I don't need anything larger) and returns a number between 0 and the given number (not inclusive to the given number), so say if N is 100, the results of the function are from 0 to 99, and if N is 10 the results are from 0 to 9. </p> <p>You can create a random number by using Math.random and just multiplying by N and then using floor. However, Math.random is not cryptographically safe, so it must be done by using randomly generated number that are of 2^8m, where m is just whatever number of bytes is given to crypto.randomBytes.</p> <p>I created a simple function which is working, apparently. However, I am aware that it is fairly easy to induce some bias into random numbers and I would just like to have it validated, since it is somewhat important for the project.</p> <pre><code>genera_aleatorio_residuo_potencia10 : function (n, cb) { var digitos = Math.log(n) / Math.LN10; var extra_base2 = digitos &gt; 8 ? digitos - 8 : 0; if (Math.floor(digitos + .4) - digitos &gt; 0.00000001) { return cb("Numero no es potencia de 10 (10, 100, 1000...)", null); } digitos = Math.round(digitos); async.parallel({ r1 : crypto_helper.generador_random_bytes(1), r2 : crypto_helper.generador_random_bytes(1) }, function (err, res) { if (err) { return cb(err, null); } var r1 = res.r1[0] + 1; var r2 = res.r2[0] + 1; var aleatorio = (Math.pow(5, digitos) - 1) * Math.pow(2, extra_base2) * r1 + r2; cb(null, aleatorio % n); }); } </code></pre> <p>Needless to say: crypto_helper.generador_random_bytes is a wrapper of node.js' crypto.randomBytes that I frequently use to make it friendlier with the async library.</p> <p>My reasoning for using Math.pow(5, digitos) and the Math.pow(2, extra_base2) is for the least common multiple between N and 256. In practice, n will never be larger than 100000000, so the Math.pow(2, extra_base2) shouldn't be used in our product, but I would still like to be sure it makes sense to other people.</p>
 

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