1. 問題

今天在寫一段RSA加解密的代碼挎春，代碼本身不復(fù)雜。

  private static final String RSA = "RSA";
  private static final String UTF8 = "UTF-8";
  private static final String KEY_STORE = "PKCS12";
  private static final String X509 = "X.509";


  private static final int MAX_ENCRYPT_BLOCK = 117;
  private static final int MAX_DECRYPT_BLOCK = 128;

  public static byte[] encryptToBase64(byte[] originalContent, Key key) throws NoSuchPaddingException,
      NoSuchAlgorithmException, InvalidKeyException, IOException, BadPaddingException, IllegalBlockSizeException {
    Cipher cipher = Cipher.getInstance(key.getAlgorithm());
    cipher.init(Cipher.ENCRYPT_MODE, key);
    //cipher一次能加密的長度有限制豆拨，因此需要分段加密
    byte[] originalBytes = originalContent;
    int totalLength = originalBytes.length;
    int offset = 0;
    ByteArrayOutputStream resultStream = new ByteArrayOutputStream();
    while (totalLength > offset) {
      byte[] encryptedBlock = cipher.doFinal(
          originalBytes, offset, Math.min(totalLength - offset, RSAUtils.MAX_ENCRYPT_BLOCK));
      resultStream.write(encryptedBlock);

      offset = offset + RSAUtils.MAX_ENCRYPT_BLOCK;
    }
    
    byte[] encryptedContent = resultStream.toByteArray();
    return encryptedContent;
  }

  public static byte[] decryptFromBase64(byte[] encryptedContent, Key key) throws NoSuchPaddingException,
      NoSuchAlgorithmException, InvalidKeyException, IOException, BadPaddingException, IllegalBlockSizeException {
    Cipher cipher = Cipher.getInstance(key.getAlgorithm());
    cipher.init(Cipher.DECRYPT_MODE, key);
    //cipher一次能解密的長度有限制直奋，因此需要分段解密
    byte[] encryptedBytes = encryptedContent;
    int totalLength = encryptedBytes.length;
    int offset = 0;
    ByteArrayOutputStream resultStream = new ByteArrayOutputStream();
    while (totalLength > offset) {
      int blockLength = Math.min(totalLength - offset, RSAUtils.MAX_DECRYPT_BLOCK);
      byte[] decryptedBlock = cipher.doFinal(encryptedBytes, offset, blockLength);
      resultStream.write(decryptedBlock);

      offset = offset + RSAUtils.MAX_DECRYPT_BLOCK;
    }
    resultStream.close();
    byte[] originalContent = resultStream.toByteArray();
    return originalContent;
  }

但是在執(zhí)行單元測試的時候，總是報(bào)如下的錯誤

javax.crypto.BadPaddingException: Decryption error

    at sun.security.rsa.RSAPadding.unpadV15(RSAPadding.java:379)
    at sun.security.rsa.RSAPadding.unpad(RSAPadding.java:290)
    at com.sun.crypto.provider.RSACipher.doFinal(RSACipher.java:365)
    at com.sun.crypto.provider.RSACipher.engineDoFinal(RSACipher.java:391)
    at javax.crypto.Cipher.doFinal(Cipher.java:2226)
    at wang.afrag.base.util.RSAUtils.decryptFromBase64(RSAUtils.java:72)
    at wang.afrag.base.util.RSAUtilsTest.testEncrypt(RSAUtilsTest.java:86)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:686)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:212)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:208)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:137)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:71)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:135)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.util.ArrayList.forEach(ArrayList.java:1259)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.util.ArrayList.forEach(ArrayList.java:1259)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:248)
    at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$5(DefaultLauncher.java:211)
    at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:226)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:199)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:132)
    at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:220)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:53)

2. 解決的方法

經(jīng)過一番折騰施禾，總算是找到了原因脚线，在這里記錄下來。

主要的原因在于加密和解密是的block size弥搞，也就是 MAX_ENCRYPT_BLOCK 和 MAX_DECRYPT_BLOCK 這兩個常量的值邮绿。這兩個值必須和使用的密鑰的長度匹配渠旁。 我使用的密鑰是2048 bit的密鑰，因此這兩個值設(shè)置的長度分別應(yīng)該分別設(shè)置為245和256船逮。

3. 根本的原因

在進(jìn)行RSA加密的時候顾腊，必須考慮密鑰長度，明文長度和密文長度挖胃。

3.1 密鑰及密鑰長度

在RSA中杂靶，我們常說的密鑰，其實(shí)指的是 （公鑰 + 模值）冠骄、或（私鑰 + 模值）中的一組伪煤。 這一組中的數(shù)據(jù)是需要配合起來使用的。

我們所說的密鑰長度凛辣，指的是模值的位長度抱既。目前主流的密鑰長度是在1024bit以上，上不封頂扁誓。

3.2 加解密數(shù)據(jù)塊的長度

由于密鑰的長度是有限的防泵，因此一次能夠加/解密的長度也是有限的，和密鑰的長度是相同的蝗敢。但是要注意到捷泞，我們所說的密鑰長度，單位是bit寿谴。但是我們在計(jì)算明文或者密文的長度的時候锁右，通常單位是byte，因此要除以8讶泰。比如我用的密鑰長度是2048咏瑟，除以8之后為256， 因此我一次能夠加密或者解密的數(shù)據(jù)的最大長度是256個byte痪署。如果需要加密或者解密的數(shù)據(jù)多于這個值怎么辦码泞？答案是分塊處理，前面代碼中就是這樣處理的狼犯。

那為什么加密的數(shù)據(jù)塊大小和解密的數(shù)據(jù)塊大小不一致呢余寥？因?yàn)樵诩用艿臅r候，我們需要在明文中填充一些隨機(jī)數(shù)悯森，這樣每次產(chǎn)生的密文都會發(fā)生變化宋舷，這個過程稱為padding。在加密的過程中瓢姻，有各種各樣的padding標(biāo)準(zhǔn)肥缔，在前面的代碼中，采用的是默認(rèn)的PKCS1Padding標(biāo)準(zhǔn)汹来，這個標(biāo)準(zhǔn)中生成的隨機(jī)數(shù)要占用11個字節(jié)续膳，因此明文最長只能取245個字節(jié)，合起來是256個字節(jié)收班。

4 其他

在RSA中坟岔，還有很多其他的Padding類型，例如使用OAEP算法的OAEPWITHSHA-256ANDMGF1PADDING等摔桦，這些Padding對應(yīng)的密文長度社付，就需要各位自己去發(fā)現(xiàn)了……