contentenc: add helpers for reverse mode

Add the reverse variant of DecryptBlocks etc:

* EncryptBlocks
* JointPlaintextRange
* ExplodeCipherRange

internal/contentenc/content.go

@@ -50,6 +50,7 @@ func (be *ContentEnc) CipherBS() uint64 {
 }
 
 // DecryptBlocks - Decrypt a number of blocks
+// TODO refactor to three-param for
 func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, fileId []byte) ([]byte, error) {
 	cBuf := bytes.NewBuffer(ciphertext)
 	var err error
@@ -110,6 +111,19 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileId []b
 	return plaintext, nil
 }
 
+// EncryptBlocks - Encrypt a number of blocks
+// Used for reverse mode
+func (be *ContentEnc) EncryptBlocks(plaintext []byte, firstBlockNo uint64, fileId []byte) []byte {
+	inBuf := bytes.NewBuffer(plaintext)
+	var outBuf bytes.Buffer
+	for blockNo := firstBlockNo; inBuf.Len() > 0; blockNo++ {
+		inBlock := inBuf.Next(int(be.plainBS))
+		outBlock := be.EncryptBlock(inBlock, blockNo, fileId)
+		outBuf.Write(outBlock)
+	}
+	return outBuf.Bytes()
+}
+
 // encryptBlock - Encrypt and add IV and MAC
 func (be *ContentEnc) EncryptBlock(plaintext []byte, blockNo uint64, fileID []byte) []byte {
 

internal/contentenc/intrablock.go

@@ -2,9 +2,18 @@ package contentenc
 
 // intraBlock identifies a part of a file block
 type intraBlock struct {
-	BlockNo uint64 // Block number in file
+	// Block number in the file
-	Skip    uint64 // Offset into block plaintext
+	BlockNo uint64
-	Length  uint64 // Length of plaintext data in this block
+	// 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
 }
 
@@ -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
+}

internal/contentenc/offsets.go

@@ -11,8 +11,11 @@ func (be *ContentEnc) PlainOffToBlockNo(plainOffset uint64) uint64 {
 	return plainOffset / be.plainBS
 }
 
-// get the block number at ciphter-text offset
+// get the block number at cipher-text offset
 func (be *ContentEnc) CipherOffToBlockNo(cipherOffset uint64) uint64 {
+	if cipherOffset < HEADER_LEN {
+		panic("BUG: offset is inside the file header")
+	}
 	return (cipherOffset - HEADER_LEN) / be.cipherBS
 }
 
@@ -89,6 +92,32 @@ func (be *ContentEnc) ExplodePlainRange(offset uint64, length uint64) []intraBlo
 	return blocks
 }
 
+// Split a ciphertext byte range into (possibly partial) blocks
+// This is used in reverse mode when reading files
+func (be *ContentEnc) ExplodeCipherRange(offset uint64, length uint64) []intraBlock {
+	var blocks []intraBlock
+	var nextBlock intraBlock
+	nextBlock.fs = be
+
+	for length > 0 {
+		nextBlock.BlockNo = be.CipherOffToBlockNo(offset)
+		nextBlock.Skip = offset - be.BlockNoToCipherOff(nextBlock.BlockNo)
+
+		// This block can carry up to "maxLen" payload bytes
+		maxLen := be.cipherBS - nextBlock.Skip
+		nextBlock.Length = maxLen
+		// But if the user requested less, we truncate the block to "length".
+		if length < maxLen {
+			nextBlock.Length = length
+		}
+
+		blocks = append(blocks, nextBlock)
+		offset += nextBlock.Length
+		length -= nextBlock.Length
+	}
+	return blocks
+}
+
 func (be *ContentEnc) BlockOverhead() uint64 {
 	return be.cipherBS - be.plainBS
 }