運算檢查總和 - Amazon S3 Glacier
樹雜湊範例 1：在單一請求上傳封存樹雜湊範例 2：使用分段上傳來上傳封存運算檔案的樹雜湊

如果您不熟悉 Amazon Simple Storage Service (Amazon S3) 中的封存儲存，建議您從深入了解 Amazon S3 中的 S3 Glacier 儲存類別、S3 Glacier Instant Retrieval、S3 Glacier Flexible Retrieval 和 S3 Glacier Deep Archive 開始。如需詳細資訊，請參閱 Amazon S3 使用者指南中的 S3 Glacier 儲存類別和存檔物件的儲存類別。

本文為英文版的機器翻譯版本，如內容有任何歧義或不一致之處，概以英文版為準。

運算檢查總和

當上傳封存時，您必須同時包含 x-amz-sha256-tree-hash 和 x-amz-content-sha256 標頭。x-amz-sha256-tree-hash 標題是您的請求內文中承載的檢查總和。此主題說明如何計算 x-amz-sha256-tree-hash 標頭。x-amz-content-sha256 標頭是整個承載的雜湊，並且是授權所需。如需詳細資訊，請參閱 串流 API 的簽章計算範例。

請求的承載內容可以是：

  • 整個封存：在單一請求中使用上傳封存 API 上傳封存時，您在請求內文傳送整個封存。在這種情況下，您必須包含整個封存的檢查總和。

  • 封存部分：使用分段上傳 API 以部分形式上傳封存時，您僅在請求內文傳送部分封存。在這種情況下，會包含封存部分的檢查總和。而且在上傳所有的部分後，您會完成分段上傳請求，其必須包含整個封存的檢查總和。

承載的檢查總和是 SHA-256 樹雜湊。其稱為樹雜湊的原因是，在運算檢查總和的過程中，您會計算 SHA-256 樹雜湊值。根的雜湊值是整個封存的檢查總和。

注意

本節說明運算 SHA-256 樹雜湊的方式。不過，您可以使用任何會產生相同結果的程序。

您運算 SHA-256 樹雜湊，如下所示：

  1. 對於每個 1 MB 區塊的承載資料，運算 SHA-256 雜湊。最後區塊的資料可能小於 1 MB。例如，如果上傳 3.2 MB 的封存，您為前三個 1 MB 的資料區塊的每一個運算 SHA-256 雜湊值，然後運算剩餘 0.2 MB 資料的 SHA-256 雜湊。這些雜湊值形成樹狀結構的分葉節點。

  2. 建置下一個層級的樹狀結構。

    1. 串連兩個連續的子節點雜湊值，並且運算已串連雜湊值的 SHA-256 雜湊。這個串連及 SHA-256 雜湊的產生會產生兩個子節點的父節點。

    2. 如果只剩一個子節點，就會將該雜湊值提升到樹狀結構的下一個層級。

  3. 重複步驟 2，直到產生的樹狀結構有根。樹狀結構的根提供整個封存的雜湊，而適當子樹狀結構則提供分段上傳部分的雜湊。

主題

樹雜湊範例 1：在單一請求上傳封存

在單一請求中使用上傳封存 API 上傳封存時 (請參閱 上傳封存 (POST 封存)) 請求承載包含整個封存。因此，您必須在 x-amz-sha256-tree-hash 請求標頭中包含整個封存的樹雜湊。假設您想要上傳 6.5 MB 的封存。下圖說明建立封存的 SHA-256 雜湊的程序。您讀取封存並運算每 1 MB 區塊的 SHA-256 雜湊。您也運算剩餘 0.5 MB 資料的雜湊，然後依所述的樹狀結構建置程序。

顯示在單個請求中上傳歸檔的樹哈希示例的圖表。

樹雜湊範例 2：使用分段上傳來上傳封存

在使用分段上傳來上傳封存時，其運算樹雜湊的程序和在單一請求上傳封存的程序是相同的。唯一的差別是，在分段上傳只上傳每個請求的一部分封存 (使用 分段上傳 (PUT uploadID) API)，因此，您只提供 x-amz-sha256-tree-hash 請求標頭部分的檢查總和。不過，在上傳所有部分後，您必須隨著 完成分段上傳 (POST uploadID) 請求標頭中整個封存的樹雜湊，傳送「完成分段上傳」(請參閱 x-amz-sha256-tree-hash) 請求。

圖顯示使用多部分上傳上傳歸檔的樹哈希示例。

運算檔案的樹雜湊

這裡所顯示的演算法僅示範之目定而選定。您可以視您實作情況的需要最佳化程式碼。如果您使用 Amazon 開發套件，針對 Amazon S3 Glacier (S3 Glacier) 進行程式設計，系統便會為您完成樹雜湊計算，您只需提供檔案參考即可。

範例 1: Java 範例

以下範例說明如何計算使用 Java 的檔案的 SHA256 樹雜湊。您可以執行這個範例，做法是提供檔案位置做為引數，或可直接從您的程式碼使用 TreeHashExample.computeSHA256TreeHash 方法。

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();
    }
}
範例 2: C# .NET 範例

以下範例說明如何計算使檔案的 SHA256 樹雜湊。您可以執行此範例，做法是提供檔案位置做為引數。

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;
        }
    }
}