Document `SecretKey` extraction from Java keystore

Closes #629.

content/posts/2019-08-02-extract-secret-key-java-keystore.md

+---
+title: "Extract Secret Key Java Keystore"
+description: "How to extract a symmetric `SecretKey` entry from a Java keystore."
+tags:
+- blogumentation
+- java
+- keystore
+license_code: Apache-2.0
+license_prose: CC-BY-NC-SA-4.0
+date: 2019-08-02T23:43:03+0100
+slug: "extract-secret-key-java-keystore"
+---
+Yesterday, I was trying to pull a shared secret (a `SecretKeyEntry` not a `PrivateKeyEntry`) out of a Java keystore.
+
+I'd created it quite some time ago and annoyingly didn't have a copy of the secret stored anywhere. What I did have, however, was the `keystorepass` and the `keypass`, so wanted to pull the key out.
+
+This was achievable using the below Java class:
+
+```java
+import java.io.FileInputStream;
+import java.math.BigInteger;
+import java.security.KeyStore;
+import java.security.KeyStoreException;
+import javax.crypto.SecretKey;
+
+public class OutputSecretKey {
+  public static void main(String[] args) throws Exception {
+    final String fileName = args[0];
+    final String alias = args[1];
+    final char[] storepass = args[2].toCharArray();
+    final char[] keypass = args[3].toCharArray();
+
+    KeyStore ks = KeyStore.getInstance("JCEKS");
+
+    try (FileInputStream fis = new FileInputStream(fileName)) {
+      ks.load(fis, storepass);
+      SecretKey secretKey = (SecretKey) ks.getKey(alias, keypass);
+      String secretAsHex = new BigInteger(1, secretKey.getEncoded()).toString(16);
+      System.out.println(hexToAscii(secretAsHex));
+    }
+  }
+
+  /* https://www.baeldung.com/java-convert-hex-to-ascii */
+  private static String hexToAscii(String hexStr) {
+    StringBuilder output = new StringBuilder("");
+
+    for (int i = 0; i < hexStr.length(); i += 2) {
+      String str = hexStr.substring(i, i + 2);
+      output.append((char) Integer.parseInt(str, 16));
+    }
+
+    return output.toString();
+  }
+}
+```
+
+This can then be run as follows:
+
+```
+$ java OutputSecretKey.java
+$ java OutputSecretKey keystore.jceks alias thisisthekeystorepass thekeyhasthispassword
+supersecretpassword
+```
+
+Note that this code has been adapted from [_How to display Java keystore SecretKeyEntry from command line_](https://stackoverflow.com/a/37491400/2257038) and [_Convert Hex to ASCII in Java_](https://www.baeldung.com/java-convert-hex-to-ascii).

permalinks.yml

 ---
 posts:
+- https://www.jvt.me/posts/2019/08/02/extract-secret-key-java-keystore/
 - https://www.jvt.me/posts/2019/08/01/node-parse-url-fragment/
 - https://www.jvt.me/posts/2019/07/27/rsvp-calendar/
 - https://www.jvt.me/posts/2019/07/27/meetupcom-calendar/