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 }