Calcolo di checksum

L'esempio seguente mostra come calcolare la struttura hash SHA256 di un file utilizzando Java. Puoi eseguire questo esempio specificando una percorso di file come argomento oppure utilizzare il metodo TreeHashExample.computeSHA256TreeHash direttamente nel codice.

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class TreeHashExample {

static final int ONE_MB = 1024 * 1024;

    /**
     * Compute the Hex representation of the SHA-256 tree hash for the specified
     * File
     * 
     * @param args
     *            args[0]: a file to compute a SHA-256 tree hash for
     */
    public static void main(String[] args) {

        if (args.length < 1) {
            System.err.println("Missing required filename argument");
            System.exit(-1);
        }

        File inputFile = new File(args[0]);
        try {

            byte[] treeHash = computeSHA256TreeHash(inputFile);
            System.out.printf("SHA-256 Tree Hash = %s\n", toHex(treeHash));

        } catch (IOException ioe) {
            System.err.format("Exception when reading from file %s: %s", inputFile,
                    ioe.getMessage());
            System.exit(-1);

        } catch (NoSuchAlgorithmException nsae) {
            System.err.format("Cannot locate MessageDigest algorithm for SHA-256: %s",
                    nsae.getMessage());
            System.exit(-1);
        }
    }

    /**
     * Computes the SHA-256 tree hash for the given file
     * 
     * @param inputFile
     *            a File to compute the SHA-256 tree hash for
     * @return a byte[] containing the SHA-256 tree hash
     * @throws IOException
     *             Thrown if there's an issue reading the input file
     * @throws NoSuchAlgorithmException
     */
    public static byte[] computeSHA256TreeHash(File inputFile) throws IOException,
            NoSuchAlgorithmException {

        byte[][] chunkSHA256Hashes = getChunkSHA256Hashes(inputFile);
        return computeSHA256TreeHash(chunkSHA256Hashes);
    }

    /**
     * Computes a SHA256 checksum for each 1 MB chunk of the input file. This
     * includes the checksum for the last chunk even if it is smaller than 1 MB.
     * 
     * @param file
     *            A file to compute checksums on
     * @return a byte[][] containing the checksums of each 1 MB chunk
     * @throws IOException
     *             Thrown if there's an IOException when reading the file
     * @throws NoSuchAlgorithmException
     *             Thrown if SHA-256 MessageDigest can't be found
     */
    public static byte[][] getChunkSHA256Hashes(File file) throws IOException,
            NoSuchAlgorithmException {

        MessageDigest md = MessageDigest.getInstance("SHA-256");

        long numChunks = file.length() / ONE_MB;
        if (file.length() % ONE_MB > 0) {
            numChunks++;
        }

        if (numChunks == 0) {
            return new byte[][] { md.digest() };
        }

        byte[][] chunkSHA256Hashes = new byte[(int) numChunks][];
        FileInputStream fileStream = null;

        try {
            fileStream = new FileInputStream(file);
            byte[] buff = new byte[ONE_MB];

            int bytesRead;
            int idx = 0;
            int offset = 0;

            while ((bytesRead = fileStream.read(buff, offset, ONE_MB)) > 0) {
                md.reset();
                md.update(buff, 0, bytesRead);
                chunkSHA256Hashes[idx++] = md.digest();
                offset += bytesRead;
            }

            return chunkSHA256Hashes;

        } finally {
            if (fileStream != null) {
                try {
                    fileStream.close();
                } catch (IOException ioe) {
                    System.err.printf("Exception while closing %s.\n %s", file.getName(),
                            ioe.getMessage());
                }
            }
        }
    }

    /**
     * Computes the SHA-256 tree hash for the passed array of 1 MB chunk
     * checksums.
     * 
     * This method uses a pair of arrays to iteratively compute the tree hash
     * level by level. Each iteration takes two adjacent elements from the
     * previous level source array, computes the SHA-256 hash on their
     * concatenated value and places the result in the next level's destination
     * array. At the end of an iteration, the destination array becomes the
     * source array for the next level.
     * 
     * @param chunkSHA256Hashes
     *            An array of SHA-256 checksums
     * @return A byte[] containing the SHA-256 tree hash for the input chunks
     * @throws NoSuchAlgorithmException
     *             Thrown if SHA-256 MessageDigest can't be found
     */
    public static byte[] computeSHA256TreeHash(byte[][] chunkSHA256Hashes)
            throws NoSuchAlgorithmException {

        MessageDigest md = MessageDigest.getInstance("SHA-256");

        byte[][] prevLvlHashes = chunkSHA256Hashes;

        while (prevLvlHashes.length > 1) {

            int len = prevLvlHashes.length / 2;
            if (prevLvlHashes.length % 2 != 0) {
                len++;
            }

            byte[][] currLvlHashes = new byte[len][];

            int j = 0;
            for (int i = 0; i < prevLvlHashes.length; i = i + 2, j++) {

                // If there are at least two elements remaining
                if (prevLvlHashes.length - i > 1) {

                    // Calculate a digest of the concatenated nodes
                    md.reset();
                    md.update(prevLvlHashes[i]);
                    md.update(prevLvlHashes[i + 1]);
                    currLvlHashes[j] = md.digest();

                } else { // Take care of remaining odd chunk
                    currLvlHashes[j] = prevLvlHashes[i];
                }
            }

            prevLvlHashes = currLvlHashes;
        }

        return prevLvlHashes[0];
    }

    /**
     * Returns the hexadecimal representation of the input byte array
     * 
     * @param data
     *            a byte[] to convert to Hex characters
     * @return A String containing Hex characters
     */
    public static String toHex(byte[] data) {
        StringBuilder sb = new StringBuilder(data.length * 2);

        for (int i = 0; i < data.length; i++) {
            String hex = Integer.toHexString(data[i] & 0xFF);

            if (hex.length() == 1) {
                // Append leading zero.
                sb.append("0");
            }
            sb.append(hex);
        }
        return sb.toString().toLowerCase();
    }
}

L'esempio seguente mostra come calcolare la struttura hash SHA256 di un file. Puoi eseguire questo esempio specificando un percorso di file come argomento.

using System;
using System.IO;

using System.Security.Cryptography;

namespace ExampleTreeHash
{
    class Program
    {
        static int ONE_MB = 1024 * 1024;

        /**
        * Compute the Hex representation of the SHA-256 tree hash for the
        * specified file
        * 
        * @param args
        *            args[0]: a file to compute a SHA-256 tree hash for
        */
        public static void Main(string[] args)
        {
            if (args.Length < 1)
            {
                Console.WriteLine("Missing required filename argument");
                Environment.Exit(-1);
            }
            FileStream inputFile = File.Open(args[0], FileMode.Open, FileAccess.Read);
            try
            {
                byte[] treeHash = ComputeSHA256TreeHash(inputFile);
                Console.WriteLine("SHA-256 Tree Hash = {0}", BitConverter.ToString(treeHash).Replace("-", "").ToLower());
                Console.ReadLine();
                Environment.Exit(-1);
            }
            catch (IOException ioe)
            {
                Console.WriteLine("Exception when reading from file {0}: {1}",
                    inputFile, ioe.Message);
                Console.ReadLine();
                Environment.Exit(-1);
            }
            catch (Exception e)
            {
                Console.WriteLine("Cannot locate MessageDigest algorithm for SHA-256: {0}",
                    e.Message);
                Console.WriteLine(e.GetType());
                Console.ReadLine();
                Environment.Exit(-1);
            }
            Console.ReadLine();
        }


        /**
         * Computes the SHA-256 tree hash for the given file
         * 
         * @param inputFile
         *            A file to compute the SHA-256 tree hash for
         * @return a byte[] containing the SHA-256 tree hash
         */
        public static byte[] ComputeSHA256TreeHash(FileStream inputFile)
        {
            byte[][] chunkSHA256Hashes = GetChunkSHA256Hashes(inputFile);
            return ComputeSHA256TreeHash(chunkSHA256Hashes);
        }


        /**
         * Computes a SHA256 checksum for each 1 MB chunk of the input file. This
         * includes the checksum for the last chunk even if it is smaller than 1 MB.
         * 
         * @param file
         *            A file to compute checksums on
         * @return a byte[][] containing the checksums of each 1MB chunk
         */
        public static byte[][] GetChunkSHA256Hashes(FileStream file)
        {
            long numChunks = file.Length / ONE_MB;
            if (file.Length % ONE_MB > 0)
            {
                numChunks++;
            }

            if (numChunks == 0)
            {
                return new byte[][] { CalculateSHA256Hash(null, 0) };
            }
            byte[][] chunkSHA256Hashes = new byte[(int)numChunks][];

            try
            {
                byte[] buff = new byte[ONE_MB];

                int bytesRead;
                int idx = 0;

                while ((bytesRead = file.Read(buff, 0, ONE_MB)) > 0)
                {
                    chunkSHA256Hashes[idx++] = CalculateSHA256Hash(buff, bytesRead);
                }
                return chunkSHA256Hashes;
            }
            finally
            {
                if (file != null)
                {
                    try
                    {
                        file.Close();
                    }
                    catch (IOException ioe)
                    {
                        throw ioe;
                    }
                }
            }

        }

        /**
         * Computes the SHA-256 tree hash for the passed array of 1MB chunk
         * checksums.
         * 
         * This method uses a pair of arrays to iteratively compute the tree hash
         * level by level. Each iteration takes two adjacent elements from the
         * previous level source array, computes the SHA-256 hash on their
         * concatenated value and places the result in the next level's destination
         * array. At the end of an iteration, the destination array becomes the
         * source array for the next level.
         * 
         * @param chunkSHA256Hashes
         *            An array of SHA-256 checksums
         * @return A byte[] containing the SHA-256 tree hash for the input chunks
         */
        public static byte[] ComputeSHA256TreeHash(byte[][] chunkSHA256Hashes)
        {
            byte[][] prevLvlHashes = chunkSHA256Hashes;
            while (prevLvlHashes.GetLength(0) > 1)
            {

                int len = prevLvlHashes.GetLength(0) / 2;
                if (prevLvlHashes.GetLength(0) % 2 != 0)
                {
                    len++;
                }

                byte[][] currLvlHashes = new byte[len][];

                int j = 0;
                for (int i = 0; i < prevLvlHashes.GetLength(0); i = i + 2, j++)
                {

                    // If there are at least two elements remaining
                    if (prevLvlHashes.GetLength(0) - i > 1)
                    {

                        // Calculate a digest of the concatenated nodes
                        byte[] firstPart = prevLvlHashes[i];
                        byte[] secondPart = prevLvlHashes[i + 1];
                        byte[] concatenation = new byte[firstPart.Length + secondPart.Length];
                        System.Buffer.BlockCopy(firstPart, 0, concatenation, 0, firstPart.Length);
                        System.Buffer.BlockCopy(secondPart, 0, concatenation, firstPart.Length, secondPart.Length);

                        currLvlHashes[j] = CalculateSHA256Hash(concatenation, concatenation.Length);

                    }
                    else
                    { // Take care of remaining odd chunk
                        currLvlHashes[j] = prevLvlHashes[i];
                    }
                }

                prevLvlHashes = currLvlHashes;
            }

            return prevLvlHashes[0];
        }

        public static byte[] CalculateSHA256Hash(byte[] inputBytes, int count)
        {
            SHA256 sha256 = System.Security.Cryptography.SHA256.Create();
            byte[] hash = sha256.ComputeHash(inputBytes, 0, count);
            return hash;
        }
    }
}