standardize on lowercase SHA-256 hashes

fdroidserver produces lowercase hashes, so its easiest to just mimic
that.  This makes hash comparisons easy.

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

@@ -386,7 +386,7 @@ public final class Utils {
             }
 
             byte[] mdbytes = md.digest();
-            return toHexString(mdbytes);
+            return toHexString(mdbytes).toLowerCase(Locale.ENGLISH);
         } catch (IOException e) {
             Log.e(TAG, "Error reading \"" + apk.getAbsolutePath()
                     + "\" to compute " + algo + " hash.", e);

app/src/main/java/org/fdroid/fdroid/data/Apk.java

@@ -25,7 +25,7 @@ public class Apk extends ValueObject implements Comparable<Apk>, Parcelable {
     public int versionCode;
     public int size; // Size in bytes - 0 means we don't know!
     public long repo; // ID of the repo it comes from
-    public String hash;
+    public String hash; // checksum of the APK, in lowercase hex
     public String hashType;
     public int minSdkVersion = SDK_VERSION_MIN_VALUE; // 0 if unknown
     public int targetSdkVersion = SDK_VERSION_MIN_VALUE; // 0 if unknown

app/src/main/java/org/fdroid/fdroid/data/DBHelper.java

@@ -120,7 +120,7 @@ class DBHelper extends SQLiteOpenHelper {
             + " );";
     private static final String DROP_TABLE_INSTALLED_APP = "DROP TABLE " + InstalledAppTable.NAME + ";";
 
-    private static final int DB_VERSION = 60;
+    private static final int DB_VERSION = 61;
 
     private final Context context;
 
@@ -327,6 +327,16 @@ class DBHelper extends SQLiteOpenHelper {
         migrateAppPrimaryKeyToRowId(db, oldVersion);
         removeApkPackageNameColumn(db, oldVersion);
         addAppPrefsTable(db, oldVersion);
+        lowerCaseApkHashes(db, oldVersion);
+    }
+
+    private void lowerCaseApkHashes(SQLiteDatabase db, int oldVersion) {
+        if (oldVersion >= 61) {
+            return;
+        }
+        Utils.debugLog(TAG, "Lowercasing all APK hashes");
+        db.execSQL("UPDATE " + InstalledAppTable.NAME + " SET " + InstalledAppTable.Cols.HASH
+                + " = lower(" + InstalledAppTable.Cols.HASH + ")");
     }
 
     private void addAppPrefsTable(SQLiteDatabase db, int oldVersion) {

app/src/main/java/org/fdroid/fdroid/localrepo/LocalRepoManager.java

@@ -470,7 +470,7 @@ public final class LocalRepoManager {
         private void tagHash(App app) throws IOException {
             serializer.startTag("", "hash");
             serializer.attribute("", "type", app.installedApk.hashType);
-            serializer.text(app.installedApk.hash.toLowerCase(Locale.US));
+            serializer.text(app.installedApk.hash);
             serializer.endTag("", "hash");
         }
     }