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-rw-r--r--backend_content.go140
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
+}