decryptverify: add trivial detection of pgp ascii-armored data

OpenKeychain/src/main/java/org/sufficientlysecure/keychain/pgp/PgpDecryptVerifyOperation.java

@@ -535,14 +535,14 @@ public class PgpDecryptVerifyOperation extends BaseOperation<PgpDecryptVerifyInp
         if (!"".equals(originalFilename)) {
             log.add(LogType.MSG_DC_CLEAR_META_FILE, indent + 1, originalFilename);
         }
-        log.add(LogType.MSG_DC_CLEAR_META_MIME, indent + 1,
-                mimeType);
         log.add(LogType.MSG_DC_CLEAR_META_TIME, indent + 1,
                 new Date(literalData.getModificationTime().getTime()).toString());
 
         // return here if we want to decrypt the metadata only
         if (input.isDecryptMetadataOnly()) {
 
+            log.add(LogType.MSG_DC_CLEAR_META_MIME, indent + 1, mimeType);
+
             // this operation skips the entire stream to find the data length!
             Long originalSize = literalData.findDataLength();
 
@@ -584,6 +584,7 @@ public class PgpDecryptVerifyOperation extends BaseOperation<PgpDecryptVerifyInp
         long wholeSize = 0; // TODO inputData.getSize() - inputData.getStreamPosition();
         int length;
         byte[] buffer = new byte[1 << 16];
+        byte[] firstBytes = new byte[48];
         while ((length = dataIn.read(buffer)) > 0) {
             // Log.d(Constants.TAG, "read bytes: " + length);
             if (out != null) {
@@ -595,7 +596,13 @@ public class PgpDecryptVerifyOperation extends BaseOperation<PgpDecryptVerifyInp
                 signature.update(buffer, 0, length);
             }
 
+            // note down first couple of bytes for "magic bytes" file type detection
+            if (alreadyWritten == 0) {
+                System.arraycopy(buffer, 0, firstBytes, 0, length > firstBytes.length ? firstBytes.length : length);
+            }
+
             alreadyWritten += length;
+
             if (wholeSize > 0) {
                 long progress = 100 * alreadyWritten / wholeSize;
                 // stop at 100% for wrong file sizes...
@@ -607,6 +614,17 @@ public class PgpDecryptVerifyOperation extends BaseOperation<PgpDecryptVerifyInp
             // TODO: slow annealing to fake a progress?
         }
 
+        // special treatment to detect pgp mime types
+        if (matchesPrefix(firstBytes, "-----BEGIN PGP PUBLIC KEY BLOCK-----")
+                || matchesPrefix(firstBytes, "-----BEGIN PGP PRIVATE KEY BLOCK-----")) {
+            mimeType = "application/pgp-keys";
+        } else if (matchesPrefix(firstBytes, "-----BEGIN PGP MESSAGE-----")) {
+            // this is NOT pgp/encrypted, see RFC 3156!
+            mimeType = "application/pgp-message";
+        }
+
+        log.add(LogType.MSG_DC_CLEAR_META_MIME, indent + 1, mimeType);
+
         metadata = new OpenPgpMetadata(
                 originalFilename, mimeType, literalData.getModificationTime().getTime(), alreadyWritten, charset);
 
@@ -1291,4 +1309,17 @@ public class PgpDecryptVerifyOperation extends BaseOperation<PgpDecryptVerifyInp
         String nl = System.getProperty("line.separator");
         return nl.getBytes();
     }
+
+    /// Convenience method - Trivially checks if a byte array matches the bytes of a plain text string
+    // Assumes data.length >= needle.length()
+    static boolean matchesPrefix(byte[] data, String needle) {
+        byte[] needleBytes = needle.getBytes();
+        for (int i = 0; i < needle.length(); i++) {
+            if (data[i] != needleBytes[i]) {
+                return false;
+            }
+        }
+        return true;
+    }
+
 }