contentenc: add helpers for reverse mode

Add the reverse variant of DecryptBlocks etc:

* EncryptBlocks
* JointPlaintextRange
* ExplodeCipherRange

1 files changed, 24 insertions, 4 deletions

diff --git a/internal/contentenc/intrablock.go b/internal/contentenc/intrablock.go
index 9a22ea2..632e76b 100644
--- a/internal/contentenc/intrablock.go
+++ b/internal/contentenc/intrablock.go
@@ -2,10 +2,19 @@ package contentenc
 
 // intraBlock identifies a part of a file block
 type intraBlock struct {
-	BlockNo uint64 // Block number in file
-	Skip    uint64 // Offset into block plaintext
-	Length  uint64 // Length of plaintext data in this block
-	fs      *ContentEnc
+	// Block number in the file
+	BlockNo uint64
+	// Offset into block payload
+	// In forwared mode: block plaintext
+	// In reverse mode: offset into block ciphertext. Takes the header into
+	// account.
+	Skip uint64
+	// Length of payload data in this block
+	// In forwared mode: length of the plaintext
+	// In reverse mode: length of the ciphertext. Takes header and trailer into
+	// account.
+	Length uint64
+	fs     *ContentEnc
 }
 
 // isPartial - is the block partial? This means we have to do read-modify-write.
@@ -47,3 +56,14 @@ func (ib *intraBlock) JointCiphertextRange(blocks []intraBlock) (offset uint64,
 
 	return offset, length
 }
+
+// Plaintext range corresponding to the sum of all "blocks" (complete blocks)
+func JointPlaintextRange(blocks []intraBlock) (offset uint64, length uint64) {
+	firstBlock := blocks[0]
+	lastBlock := blocks[len(blocks)-1]
+
+	offset = firstBlock.BlockPlainOff()
+	length = lastBlock.BlockPlainOff() + lastBlock.fs.PlainBS() - offset
+
+	return offset, length
+}