From 6d4f1a6888cafd218cb97bd11de6a7553d9bc8f1 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Sun, 9 Aug 2020 22:11:46 +0200
Subject: v2api/reverse: implement Read

---
 internal/fusefrontend_reverse/file_helpers.go | 62 +++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 internal/fusefrontend_reverse/file_helpers.go

(limited to 'internal/fusefrontend_reverse/file_helpers.go')

diff --git a/internal/fusefrontend_reverse/file_helpers.go b/internal/fusefrontend_reverse/file_helpers.go
new file mode 100644
index 0000000..f024e69
--- /dev/null
+++ b/internal/fusefrontend_reverse/file_helpers.go
@@ -0,0 +1,62 @@
+package fusefrontend_reverse
+
+import (
+	"bytes"
+	"io"
+	"sync"
+
+	"github.com/rfjakob/gocryptfs/internal/contentenc"
+	"github.com/rfjakob/gocryptfs/internal/pathiv"
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+var inodeTable sync.Map
+
+// encryptBlocks - encrypt "plaintext" into a number of ciphertext blocks.
+// "plaintext" must already be block-aligned.
+func (rf *File) encryptBlocks(plaintext []byte, firstBlockNo uint64, fileID []byte, block0IV []byte) []byte {
+	inBuf := bytes.NewBuffer(plaintext)
+	var outBuf bytes.Buffer
+	bs := int(rf.contentEnc.PlainBS())
+	for blockNo := firstBlockNo; inBuf.Len() > 0; blockNo++ {
+		inBlock := inBuf.Next(bs)
+		iv := pathiv.BlockIV(block0IV, blockNo)
+		outBlock := rf.contentEnc.EncryptBlockNonce(inBlock, blockNo, fileID, iv)
+		outBuf.Write(outBlock)
+	}
+	return outBuf.Bytes()
+}
+
+// readBackingFile: read from the backing plaintext file, encrypt it, return the
+// ciphertext.
+// "off" ... ciphertext offset (must be >= HEADER_LEN)
+// "length" ... ciphertext length
+func (f *File) readBackingFile(off uint64, length uint64) (out []byte, err error) {
+	blocks := f.contentEnc.ExplodeCipherRange(off, length)
+
+	// Read the backing plaintext in one go
+	alignedOffset, alignedLength := contentenc.JointPlaintextRange(blocks)
+	plaintext := make([]byte, int(alignedLength))
+	n, err := f.fd.ReadAt(plaintext, int64(alignedOffset))
+	if err != nil && err != io.EOF {
+		tlog.Warn.Printf("readBackingFile: ReadAt: %s", err.Error())
+		return nil, err
+	}
+	// Truncate buffer down to actually read bytes
+	plaintext = plaintext[0:n]
+
+	// Encrypt blocks
+	ciphertext := f.encryptBlocks(plaintext, blocks[0].BlockNo, f.header.ID, f.block0IV)
+
+	// Crop down to the relevant part
+	lenHave := len(ciphertext)
+	skip := blocks[0].Skip
+	endWant := int(skip + length)
+	if lenHave > endWant {
+		out = ciphertext[skip:endWant]
+	} else if lenHave > int(skip) {
+		out = ciphertext[skip:lenHave]
+	} // else: out stays empty, file was smaller than the requested offset
+
+	return out, nil
+}
-- 
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