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PORSA decryption using modulus and exponent
 

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  1. PORSA decryption using modulus and exponent
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    <p><strong>My task:</strong> I have encrypted (RSA) data and public key as modulus and exponent. I have to write decryption code.<br/> <strong>My problem with it:</strong> My implementation doesn't work ;) As far as I know philosophy is simple <strike>"open text" == rsa(public_key, rsa(private_key, "open text"))</strike> <strong>Edit: Exactly my assumption was wrong (Assumption is mother of all fu..ups ;) ). It should be <code>"open text" == rsa(private_key, rsa(public_key, "open text"))</code> because in RSA, public key is used for encryption and private for decryption.</strong> <br/><br/>I assumed that I can have public key which doesn't correspond to private key using during encryption so for tests I created own keys in such way:</p> <pre><code>openssl genrsa -des3 -out server.key 1024 openssl req -new -key server.key -out server.csr openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt </code></pre> <p>I got public key modulus and exponent using command:</p> <pre><code>openssl x509 -in server.crt -text </code></pre> <p>For encryption testing I'm using code</p> <pre><code>//Reads private key from file //StringPasswordFinder is my tmp implementation of PasswordFinder PEMReader pemReader = new PEMReader(new FileReader("/path/to/server.key"), new StringPasswordFinder()); KeyPair keyPair = (KeyPair) pemReader.readObject(); PrivateKey pk = keyPair.getPrivate(); //text for encryption String openText = "openText"; //encryption Cipher rsaCipher = Cipher.getInstance("RSA", "BC"); rsaCipher.init(Cipher.ENCRYPT_MODE, pk); byte[] encrypted = rsaCipher.doFinal(openText.getBytes("utf-8")); </code></pre> <p>And for decryption of encrypted text I use code</p> <pre><code>//modulus hex got using openssl byte[] modulus = Hex.decodeHex("very long hex".toCharArray()); //exponent hex got using openssl byte[] exponent = Hex.decodeHex("010001".toCharArray()); //initialization of rsa decryption engine RSAEngine rsaEngine = new RSAEngine(); rsaEngine.init(false, new RSAKeyParameters(false, new BigInteger(modulus), new BigInteger(exponent))); //input - encrypted stream ByteArrayInputStream bais = new ByteArrayInputStream(encrypted); ByteArrayOutputStream baos = new ByteArrayOutputStream(); //reading blocks from the input stream and decrypting them int bytesRead = 0; byte[] block = new byte[rsaEngine.getInputBlockSize()]; while ((bytesRead = bais.read(block)) &gt; -1) { baos.write(rsaEngine.processBlock(block, 0, bytesRead)); } //dispalying decrypted text System.out.println(new String(baos.toByteArray(), "utf-8")); </code></pre> <p>And after all displayed text is not. Can anybody show me where I'm wrong?</p> <p><strong>Edit: Summing up this problem has no solution. Because it's not possible encrypt message using private key and later decrypt it using public one. At general I mixed up encryption with signing message and decryption with verification. Because during making signature private key is used and public is used during verification. Btw, MByD thx for important clue.</strong></p>
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