Add -forcedecode

Force decode of encrypted files even if the integrity check fails, instead of
failing with an IO error. Warning messages are still printed to syslog if corrupted
files are encountered.
It can be useful to recover files from disks with bad sectors or other corrupted
media.

Closes https://github.com/rfjakob/gocryptfs/pull/102 .

2 files changed, 19 insertions, 9 deletions

diff --git a/internal/contentenc/content.go b/internal/contentenc/content.go
index a2a263c..9998c06 100644
--- a/internal/contentenc/content.go
+++ b/internal/contentenc/content.go
@@ -9,6 +9,7 @@ import (
 	"log"
 
 	"github.com/rfjakob/gocryptfs/internal/cryptocore"
+	"github.com/rfjakob/gocryptfs/internal/stupidgcm"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -46,10 +47,12 @@ type ContentEnc struct {
 	allZeroBlock []byte
 	// All-zero block of size IVBitLen/8, for fast compares
 	allZeroNonce []byte
+	// Force decode even if integrity check fails (openSSL only)
+	forceDecode bool
 }
 
 // New returns an initialized ContentEnc instance.
-func New(cc *cryptocore.CryptoCore, plainBS uint64) *ContentEnc {
+func New(cc *cryptocore.CryptoCore, plainBS uint64, forceDecode bool) *ContentEnc {
 	cipherBS := plainBS + uint64(cc.IVLen) + cryptocore.AuthTagLen
 
 	return &ContentEnc{
@@ -58,6 +61,7 @@ func New(cc *cryptocore.CryptoCore, plainBS uint64) *ContentEnc {
 		cipherBS:     cipherBS,
 		allZeroBlock: make([]byte, cipherBS),
 		allZeroNonce: make([]byte, cc.IVLen),
+		forceDecode:  forceDecode,
 	}
 }
 
@@ -82,7 +86,9 @@ func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, file
 		var pBlock []byte
 		pBlock, err = be.DecryptBlock(cBlock, firstBlockNo, fileID)
 		if err != nil {
-			break
+			if be.forceDecode == false || (be.forceDecode == true && stupidgcm.AuthError != err) {
+				break
+			}
 		}
 		pBuf.Write(pBlock)
 		firstBlockNo++
@@ -133,7 +139,11 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileID []b
 	if err != nil {
 		tlog.Warn.Printf("DecryptBlock: %s, len=%d", err.Error(), len(ciphertextOrig))
 		tlog.Debug.Println(hex.Dump(ciphertextOrig))
-		return nil, err
+		if be.forceDecode == true {
+			return plaintext, err
+		} else {
+			return nil, err
+		}
 	}
 
 	return plaintext, nil
diff --git a/internal/contentenc/content_test.go b/internal/contentenc/content_test.go
index 8ce496d..e4d4a3e 100644
--- a/internal/contentenc/content_test.go
+++ b/internal/contentenc/content_test.go
@@ -23,8 +23,8 @@ func TestSplitRange(t *testing.T) {
 		testRange{6654, 8945})
 
 	key := make([]byte, cryptocore.KeyLen)
-	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true)
-	f := New(cc, DefaultBS)
+	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true, false)
+	f := New(cc, DefaultBS, false)
 
 	for _, r := range ranges {
 		parts := f.ExplodePlainRange(r.offset, r.length)
@@ -51,8 +51,8 @@ func TestCiphertextRange(t *testing.T) {
 		testRange{6654, 8945})
 
 	key := make([]byte, cryptocore.KeyLen)
-	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true)
-	f := New(cc, DefaultBS)
+	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true, false)
+	f := New(cc, DefaultBS, false)
 
 	for _, r := range ranges {
 
@@ -74,8 +74,8 @@ func TestCiphertextRange(t *testing.T) {
 
 func TestBlockNo(t *testing.T) {
 	key := make([]byte, cryptocore.KeyLen)
-	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true)
-	f := New(cc, DefaultBS)
+	cc := cryptocore.New(key, cryptocore.BackendOpenSSL, DefaultIVBits, true, false)
+	f := New(cc, DefaultBS, false)
 
 	b := f.CipherOffToBlockNo(788)
 	if b != 0 {