Getting 1 byte in the RSA Key module, and sometimes for exhibitors

Question

Here is my code snippet:

int eValue = 79, t; int bitLength = 1024; // KeySize BigInteger e = new BigInteger(Integer.toString(eValue)); KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA"); kpg.initialize(bitLength); KeyPair kp = kpg.generateKeyPair(); KeyFactory kfactory = KeyFactory.getInstance("RSA"); RSAPublicKeySpec kspec = (RSAPublicKeySpec) kfactory.getKeySpec(kp.getPublic(), RSAPublicKeySpec.class); System.out.println("Byte Length is : " + kspec.getModulus().toByteArray().length); String testString; try { testString = new String (kspec.getModulus().toByteArray() , "ISO-8859-1"); StringBuilder tt = new StringBuilder(); for(t =0 ; t< testString.length() ; t++) { tt.append((int) testString.charAt(t)+","); } String encryptedBytes = tt.toString(); System.out.println("Mod is : " + encryptedBytes); }catch (Exception ex) { // TODO: handle exception } 

And here is the conclusion:

 Byte Length is : 129 Mod is : 0,190,193,141,230,128,124,6,201,254,135,66,162,65,147,160,76,160,181,7,141,113,8,57,193,185,206,42,125,9,169,209,124,74,233,151,10,128,180,35,24,206,213,32,48,4,39,178,60,10,249,151,50,218,220,11,124,72,64,148,135,251,133,23,54,171,25,202,157,28,21,39,239,234,48,56,79,36,127,59,203,108,189,232,216,231,237,237,90,253,19,118,29,18,142,126,254,193,189,82,15,126,139,136,45,31,133,242,187,81,62,52,5,23,11,217,171,233,7,137,115,30,93,206,236,31,196,111,153 

For a module with a bit length of 1024 bits, there should be 128 bytes, and for 2048 it should be 256, but I get one additional byte (adding 0 is always in the very first byte), I need help to redo this.

Thanks Pavan

Answer 1

The reason for the byte with byte 00h at the beginning is because BigInteger.toByteArray () returns a signed view. As long as the key length in bits is N * 8 (or the key length is% 8 = 0), then the signed representation of the RSA module will always contain the 00 byte at the beginning.

Just delete the start byte if it is zero by copying it to an array of key length in bytes. Please note: if you have a private exponent, it can also be shorter than the key length in bytes, so copy it to the end of the new byte array. Typically, this method is known as I2OS or I2O (an integer with an octet string), where the octet structure (byte array in java) has the specified length.

 /** * Encodes the given value as a unsigned Big Endian within an octet string * of octetStringSize bytes. * * @param i * the integer to encode * @param octetStringSize * the number of octets in the octetString returned * @return the encoding of i * @throws IllegalArgumentException * if the given integer i is negative * @throws IllegalArgumentException * if the octetStringSize is zero or lower * @throws IllegalArgumentException * if the given BigInteger does not fit into octetStringSize * bytes */ public static byte[] integerToOctetString(final BigInteger i, final int octetStringSize) { // throws NullPointerException if i = null if (i.signum() < 0) { throw new IllegalArgumentException( "argument i should not be negative"); } if (octetStringSize <= 0) { throw new IllegalArgumentException("octetStringSize argument (" + octetStringSize + ") should be higher than 0 to store any integer"); } if (i.bitLength() > octetStringSize * Byte.SIZE) { throw new IllegalArgumentException("argument i (" + i + ") does not fit into " + octetStringSize + " octets"); } final byte[] signedEncoding = i.toByteArray(); final int signedEncodingLength = signedEncoding.length; if (signedEncodingLength == octetStringSize) { return signedEncoding; } final byte[] unsignedEncoding = new byte[octetStringSize]; if (signedEncoding[0] == (byte) 0x00) { // skip first padding byte to create a (possitive) unsigned encoding for this number System.arraycopy(signedEncoding, 1, unsignedEncoding, octetStringSize - signedEncodingLength + 1, signedEncodingLength - 1); } else { System.arraycopy(signedEncoding, 0, unsignedEncoding, octetStringSize - signedEncodingLength, signedEncodingLength); } return unsignedEncoding; } /** * Returns a BigInteger that is the value represented by the unsigned, Big * Endian encoding within the given octetString. * * @param octetString * the octetString containing (only) the encoding * @return the value represented by the octetString */ public static BigInteger octetStringToInteger(final byte[] octetString) { // arguments are signum, magnitude as unsigned, Big Endian encoding return new BigInteger(1, octetString); } /** * Returns the minimum number of bytes required to directly store the given * number of bits. * * @param bitSize * the bitSize * @return the size as a number of bytes * @throws IllegalArgumentException * if the given bitSize argument is negative */ public static int bitSizeToByteSize(final int bitSize) { if (bitSize < 0) { throw new IllegalArgumentException("bitSize (" + bitSize + " should not be negative"); } return (bitSize + Byte.SIZE - 1) / Byte.SIZE; } 

Answer 2

You can use Arrays.deepToString () to directly print an array of bytes:

 String encryptedBytes = Arrays.deepToString(new Object[] { kspec.getModulus().toByteArray() }) 

I suspect you have problems with signed and unsigned numbers. A 128-bit unsigned module, but sometimes it can take 129 bits to store it in BigInteger, hence an extra byte.

Answer 3

As Maarten Bodewes replied , the extra byte is the space for the BigInteger character.

If the expected size is known and Hex is valid, I would use something like this:

 System.out.printf("Mod is : %0256x%n" , kspec.getModulus());