refactor into reusable static method for checking file hashes

This takes the APK file hash checker and turns it into a generic static
utility method for checking that a given file matches a given hash.  This
will be needed as F-Droid handles other file types, like OBB and media.

app/src/main/java/org/fdroid/fdroid/Hasher.java

@@ -97,6 +97,21 @@ public class Hasher {
         return hashCache.equals(otherHash.toLowerCase(Locale.ENGLISH));
     }
 
+    /**
+     * Checks the file against the provided hash, returning whether it is a match.
+     */
+    public static boolean isFileMatchingHash(File file, String hash, String hashType) {
+        if (!file.exists()) {
+            return false;
+        }
+        try {
+            Hasher hasher = new Hasher(hashType, file);
+            return hasher.match(hash);
+        } catch (NoSuchAlgorithmException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
     public static String hex(Certificate cert) {
         byte[] encoded;
         try {

app/src/main/java/org/fdroid/fdroid/installer/ApkCache.java

@@ -31,7 +31,6 @@ import org.fdroid.fdroid.data.SanitizedFile;
 
 import java.io.File;
 import java.io.IOException;
-import java.security.NoSuchAlgorithmException;
 
 public class ApkCache {
 
@@ -45,23 +44,16 @@ public class ApkCache {
      */
     public static SanitizedFile copyApkFromCacheToFiles(Context context, File apkFile, Apk expectedApk)
             throws IOException {
-        SanitizedFile sanitizedApkFile = null;
+        SanitizedFile sanitizedApkFile = SanitizedFile.knownSanitized(
-
-        try {
-            sanitizedApkFile = SanitizedFile.knownSanitized(
                 File.createTempFile("install-", ".apk", context.getFilesDir()));
         FileUtils.copyFile(apkFile, sanitizedApkFile);
 
         // verify copied file's hash with expected hash from Apk class
-            if (!verifyApkFile(sanitizedApkFile, expectedApk.hash, expectedApk.hashType)) {
+        if (!Hasher.isFileMatchingHash(sanitizedApkFile, expectedApk.hash, expectedApk.hashType)) {
             FileUtils.deleteQuietly(apkFile);
             throw new IOException(apkFile + " failed to verify!");
         }
 
-            return sanitizedApkFile;
-        } catch (NoSuchAlgorithmException e) {
-            throw new RuntimeException(e);
-        } finally {
         // 20 minutes the start of the install process, delete the file
         final File apkToDelete = sanitizedApkFile;
         new Thread() {
@@ -76,19 +68,8 @@ public class ApkCache {
                 }
             }
         }.start();
-        }
-    }
 
-    /**
+        return sanitizedApkFile;
-     * Checks the APK file against the provided hash, returning whether it is a match.
-     */
-    private static boolean verifyApkFile(File apkFile, String hash, String hashType)
-            throws NoSuchAlgorithmException {
-        if (!apkFile.exists()) {
-            return false;
-        }
-        Hasher hasher = new Hasher(hashType, apkFile);
-        return hasher.match(hash);
     }
 
     /**
@@ -108,19 +89,15 @@ public class ApkCache {
      * Bails out if the file sizes don't match to prevent having to do the work of hashing the file.
      */
     public static boolean apkIsCached(File apkFile, Apk apkToCheck) {
-        try {
         return apkFile.length() == apkToCheck.size &&
-                    verifyApkFile(apkFile, apkToCheck.hash, apkToCheck.hashType);
+                Hasher.isFileMatchingHash(apkFile, apkToCheck.hash, apkToCheck.hashType);
-        } catch (NoSuchAlgorithmException e) {
-            throw new RuntimeException(e);
-        }
     }
 
     /**
      * This location is only for caching, do not install directly from this location
      * because if the file is on the External Storage, any other app could swap out
      * the APK while the install was in process, allowing malware to install things.
-     * Using {@link Installer#installPackage(Uri, Uri, Apk)}
+     * Using {@link Installer#installPackage(Uri, Uri)}
      * is fine since that does the right thing.
      */
     public static File getApkCacheDir(Context context) {