crypt: add --crypt-pass-bad-blocks to allow corrupted file output

backend/crypt/cipher.go

@@ -179,6 +179,7 @@ type Cipher struct {
 	buffers        sync.Pool // encrypt/decrypt buffers
 	cryptoRand     io.Reader // read crypto random numbers from here
 	dirNameEncrypt bool
+	passBadBlocks  bool // if set passed bad blocks as zeroed blocks
 }
 
 // newCipher initialises the cipher.  If salt is "" then it uses a built in salt val
@@ -199,6 +200,11 @@ func newCipher(mode NameEncryptionMode, password, salt string, dirNameEncrypt bo
 	return c, nil
 }
 
+// Call to set bad block pass through
+func (c *Cipher) setPassBadBlocks(passBadBlocks bool) {
+	c.passBadBlocks = passBadBlocks
+}
+
 // Key creates all the internal keys from the password passed in using
 // scrypt.
 //
@@ -864,7 +870,14 @@ func (fh *decrypter) fillBuffer() (err error) {
 		if err != nil && err != io.EOF {
 			return err // return pending error as it is likely more accurate
 		}
-		return ErrorEncryptedBadBlock
+		if !fh.c.passBadBlocks {
+			return ErrorEncryptedBadBlock
+		}
+		fs.Errorf(nil, "crypt: ignoring: %v", ErrorEncryptedBadBlock)
+		// Zero out the bad block and continue
+		for i := range fh.buf[:n] {
+			fh.buf[i] = 0
+		}
 	}
 	fh.bufIndex = 0
 	fh.bufSize = n - blockHeaderSize

backend/crypt/cipher_test.go

@@ -1536,6 +1536,17 @@ func TestDecrypterRead(t *testing.T) {
 		}
 		file16copy[i] ^= 0xFF
 	}
+
+	// Test that we can corrupt a byte and read zeroes if
+	// passBadBlocks is set
+	copy(file16copy, file16)
+	file16copy[len(file16copy)-1] ^= 0xFF
+	c.passBadBlocks = true
+	fh, err = c.newDecrypter(io.NopCloser(bytes.NewBuffer(file16copy)))
+	assert.NoError(t, err)
+	buf, err := io.ReadAll(fh)
+	assert.NoError(t, err)
+	assert.Equal(t, make([]byte, 16), buf)
 }
 
 func TestDecrypterClose(t *testing.T) {

backend/crypt/crypt.go

@@ -119,6 +119,15 @@ names, or for debugging purposes.`,
 					Help:  "Encrypt file data.",
 				},
 			},
+		}, {
+			Name: "pass_bad_blocks",
+			Help: `If set this will pass bad blocks through as all 0.
+
+This should not be set in normal operation, it should only be set if
+trying to recover a crypted file with errors and it is desired to
+recover as much of the file as possible.`,
+			Default:  false,
+			Advanced: true,
 		}, {
 			Name: "filename_encoding",
 			Help: `How to encode the encrypted filename to text string.
@@ -174,6 +183,7 @@ func newCipherForConfig(opt *Options) (*Cipher, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to make cipher: %w", err)
 	}
+	cipher.setPassBadBlocks(opt.PassBadBlocks)
 	return cipher, nil
 }
 
@@ -262,6 +272,7 @@ type Options struct {
 	Password2               string `config:"password2"`
 	ServerSideAcrossConfigs bool   `config:"server_side_across_configs"`
 	ShowMapping             bool   `config:"show_mapping"`
+	PassBadBlocks           bool   `config:"pass_bad_blocks"`
 	FilenameEncoding        string `config:"filename_encoding"`
 }