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POExtracting RSA public key modulus and exponent stored in unsigned char arrays
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  1. POExtracting RSA public key modulus and exponent stored in unsigned char arrays
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    copied!<p>So, I'm using a proprietary library that has its own implementation for the creation of RSA key pairs. The public key struct looks like this:</p> <pre><code>typedef struct { unsigned int bits; //Length of modulus in bits unsigned char modulus[MAX_RSA_MOD_LEN]; //Modulus unsigned char exponent[MAX_RSA_MOD_LEN]; //Exponent } RSA_PUB_KEY </code></pre> <p>I need to figure out a way to extract both the exponent and the module so I can send them to a server as part of a validation scheme. I guess that this is a pretty standard procedure (or so I hope). I've already read these two similar questions:</p> <ul> <li><p><a href="https://stackoverflow.com/questions/5661101/how-to-convert-an-unsigned-character-array-into-a-hexadecimal-string-in-c">How to convert an Unsigned Character array into a hexadecimal string in C</a></p></li> <li><p><a href="https://stackoverflow.com/questions/10375932/printing-the-hexadecimal-representation-of-a-char-array">Printing the hexadecimal representation of a char array[]</a></p></li> </ul> <p>But so far I've had no luck. I'm also not sure of how to use if at all necessary the "bits" field to extract the modulus. In short what I have to do is be able to recreate this public key in Java:</p> <pre><code>BigInteger m = new BigInteger(MODULUS); BigInteger e = new BigInteger(EXPONENT); RSAPublicKeySpec keySpec = new RSAPublicKeySpec(m, e); KeyFactory fact = KeyFactory.getInstance("RSA"); PublicKey pubKey = fact.generatePublic(keySpec); return pubKey; </code></pre> <p><strong>Edit:</strong></p> <p>This is what I'm doing right now: (RSAPublic is a RSA_PUB_KEY struct as described above).</p> <pre><code>//RSAPublic.bits = length of modulus in bits log("Modulus length: "+std::to_string(RSAPublic.bits)); log("Key length: "+std::to_string(keyLengthInBits)); //Calculating buffer size for converted hexadec. representations int modulusLengthInBytes = (RSAPublic.bits+7)/8 ; int exponentLengthInBytes = (keyLengthInBits+7)/8; char convertedMod[modulusLengthInBytes*2+1]; char convertedExp[exponentLengthInBytes*2+1]; //Conversion int i; for(i=0; i&lt;modulusLengthInBytes ; i++){ sprintf(&amp;convertedMod[i*2], "%02X", RSAPublic.modulus[i]); } for(i=0; i&lt;exponentLengthInBytes ; i++){ sprintf(&amp;convertedExp[i*2], "%02X", RSAPublic.exponent[i]); } //Print results printf("Modulus: %s\n", convertedMod); printf("Exponent: %s\n", convertedExp); </code></pre> <p>And this is the output:</p> <pre><code>Modulus length: 16 Key length: 512 Modulus: 0000 Exponent: 0A000200FFFFFFFFFFFF0000600007004DDA0100B01D0000AEC642017A4513000000000000000000000000000000000000000000000000000000000000000000 </code></pre>
 

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