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The Java Developers Almanac 1.4Order this book from Amazon. |
e465. Converting a 56-bit Value to a DES KeyThe DES encrypter/decrypter requires a 64-bit key where only 56-bit are significant. The other 8-bit are parity bits used to ensure that the key has not been corrupted. To make the 64-bit key, the 56-bit value is broken up into 7-bit chunks. Each 7-bit chunk is moved into an 8-bit slot taking up the most significant bit positions. The least significant bit (the parity bit) is set to either 1 or 0 in order to make the quantity of 1 bits in the byte an odd number.This example implements a method to convert a 56-bit value into a valid DES key. Such a method could be used to convert a 7-character string password to a valid DES key. See also e462 Encrypting a String with DES. // Takes a 7-byte quantity and returns a valid 8-byte DES key. // The input and output bytes are big-endian, where the most significant // byte is in element 0. public static byte[] addParity(byte[] in) { byte[] result = new byte[8]; // Keeps track of the bit position in the result int resultIx = 1; // Used to keep track of the number of 1 bits in each 7-bit chunk int bitCount = 0; // Process each of the 56 bits for (int i=0; i<56; i++) { // Get the bit at bit position i boolean bit = (in[6-i/8]&(1<<(i%8))) > 0; // If set, set the corresponding bit in the result if (bit) { result[7-resultIx/8] |= (1<<(resultIx%8))&0xFF; bitCount++; } // Set the parity bit after every 7 bits if ((i+1) % 7 == 0) { if (bitCount % 2 == 0) { // Set low-order bit (parity bit) if bit count is even result[7-resultIx/8] |= 1; } resultIx++; bitCount = 0; } resultIx++; } return result; } // Get the 56-bit value byte[] raw = new byte[]{0x01, 0x72, 0x43, 0x3E, 0x1C, 0x7A, 0x55}; byte[] keyBytes = addParity(raw); // You can check that the parity has been set properly try { boolean b = DESKeySpec.isParityAdjusted(keyBytes, 0); } catch (java.security.InvalidKeyException e) { // The DES is invalid } // Convert the bytes into a SecretKey suitable for use by Cipher SecretKey key = new SecretKeySpec(keyBytes, "DES");
e462. Encrypting a String with DES e463. Encrypting a File or Stream with DES e464. Encrypting with DES Using a Pass Phrase e466. Encrypting an Object with DES
© 2002 Addison-Wesley. |