1 files changed, 140 insertions, 0 deletions
diff --git a/backend_content.go b/backend_content.go
new file mode 100644
index 0000000..3cf2a87
--- /dev/null
+++ b/backend_content.go
@@ -0,0 +1,140 @@
+package gocryptfs
+
+import (
+	"fmt"
+	"os"
+	"io"
+	"errors"
+)
+
+// readCipherBlock - Read ciphertext block number "blockNo", decrypt,
+// return plaintext
+func (be *Backend) readCipherBlock(blockNo int64, f *os.File) ([]byte, error) {
+	off := blockNo * int64(be.cipherBS)
+	buf := make([]byte, be.cipherBS)
+
+	readN, err := f.ReadAt(buf, off)
+
+	if err != nil && err != io.EOF {
+		return nil, err
+	}
+
+	// Truncate buffer to actually read bytes
+	buf = buf[:readN]
+
+	// Empty block?file:///home/jakob/go/src/github.com/rfjakob/gocryptfs-bazil/backend/backend.go
+
+	if len(buf) == 0 {
+		return buf, nil
+	}
+
+	if len(buf) < NONCE_LEN {
+		warn.Printf("readCipherBlock: Block is too short: %d bytes\n", len(buf))
+		return nil, errors.New("Block is too short")
+	}
+
+	// Extract nonce
+	nonce := buf[:NONCE_LEN]
+	buf = buf[NONCE_LEN:]
+
+	// Decrypt
+	var plainBuf []byte
+	plainBuf, err = be.gcm.Open(plainBuf, nonce, buf, nil)
+	if err != nil {
+		fmt.Printf("gcm.Open() failed: %d\n", err)
+		return nil, err
+	}
+
+	return plainBuf, nil
+}
+
+// intraBlock identifies a part of a file block
+type intraBlock struct {
+	blockNo int64  // Block number in file
+	offset  int64  // Offset into block plaintext
+	length  int64  // Length of data from this block
+}
+
+// Split a plaintext byte range into (possible partial) blocks
+func (be *Backend) splitRange(offset int64, length int64, f *os.File) []intraBlock {
+	var b intraBlock
+	var parts []intraBlock
+
+	for length > 0 {
+		b.blockNo = offset / be.plainBS
+		b.offset = offset % be.plainBS
+		b.length = be.min64(length, be.plainBS - b.offset)
+		parts = append(parts, b)
+		offset += b.length
+		length -= b.length
+	}
+	return parts
+}
+
+func (be *Backend) min64(x int64, y int64) int64 {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+// writeCipherBlock - Encrypt plaintext and write it to file block "blockNo"
+func (be *Backend) writeCipherBlock(blockNo int64, plain []byte, f *os.File) error {
+
+	if int64(len(plain)) > be.plainBS {
+		panic("writeCipherBlock: Cannot write block that is larger than plainBS")
+	}
+
+	// Get fresh nonce
+	nonce := gcmNonce.Get()
+	// Encrypt data and append to nonce
+	cipherBuf := be.gcm.Seal(nonce, nonce, plain, nil)
+
+	// WriteAt retries short writes autmatically
+	written, err := f.WriteAt(cipherBuf, blockNo * be.cipherBS)
+
+	debug.Printf("writeCipherBlock: wrote %d ciphertext bytes to block %d\n",
+		written, blockNo)
+
+	return err
+}
+
+// Perform RMW cycle on block
+// Write "data" into file location specified in "b"
+func (be *Backend) rmwWrite(b intraBlock, data []byte, f *os.File) error {
+	if b.length != int64(len(data)) {
+		panic("Length mismatch")
+	}
+
+	oldBlock, err := be.readCipherBlock(b.blockNo, f)
+	if err != nil {
+		return err
+	}
+	newBlockLen := b.offset + b.length
+	debug.Printf("newBlockLen := %d + %d\n", b.offset, b.length)
+	var newBlock []byte
+
+	// Write goes beyond the old block and grows the file?
+	// Must create a bigger newBlock
+	if newBlockLen > int64(len(oldBlock)) {
+		newBlock = make([]byte, newBlockLen)
+	} else {
+		newBlock = make([]byte, len(oldBlock))
+	}
+
+	// Fill with old data
+	copy(newBlock, oldBlock)
+	// Then overwrite the relevant parts with new data
+	copy(newBlock[b.offset:b.offset + b.length], data)
+
+	// Actual write
+	err = be.writeCipherBlock(b.blockNo, newBlock, f)
+
+	if err != nil {
+		// An incomplete write to a ciphertext block means that the whole block
+		// is destroyed.
+		fmt.Printf("rmwWrite: Write error: %s\n", err)
+	}
+
+	return err
+}