1 files changed, 97 insertions, 0 deletions
diff --git a/internal/contentenc/offsets.go b/internal/contentenc/offsets.go
new file mode 100644
index 0000000..1b5952f
--- /dev/null
+++ b/internal/contentenc/offsets.go
@@ -0,0 +1,97 @@
+package contentenc
+
+import (
+	"github.com/rfjakob/gocryptfs/internal/toggledlog"
+)
+
+// Contentenc methods that translate offsets between ciphertext and plaintext
+
+// get the block number at plain-text offset
+func (be *ContentEnc) PlainOffToBlockNo(plainOffset uint64) uint64 {
+	return plainOffset / be.plainBS
+}
+
+// get the block number at ciphter-text offset
+func (be *ContentEnc) CipherOffToBlockNo(cipherOffset uint64) uint64 {
+	return (cipherOffset - HEADER_LEN) / be.cipherBS
+}
+
+// get ciphertext offset of block "blockNo"
+func (be *ContentEnc) BlockNoToCipherOff(blockNo uint64) uint64 {
+	return HEADER_LEN + blockNo*be.cipherBS
+}
+
+// get plaintext offset of block "blockNo"
+func (be *ContentEnc) BlockNoToPlainOff(blockNo uint64) uint64 {
+	return blockNo * be.plainBS
+}
+
+// PlainSize - calculate plaintext size from ciphertext size
+func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 {
+
+	// Zero sized files stay zero-sized
+	if cipherSize == 0 {
+		return 0
+	}
+
+	if cipherSize == HEADER_LEN {
+		toggledlog.Warn.Printf("cipherSize %d == header size: interrupted write?\n", cipherSize)
+		return 0
+	}
+
+	if cipherSize < HEADER_LEN {
+		toggledlog.Warn.Printf("cipherSize %d < header size: corrupt file\n", cipherSize)
+		return 0
+	}
+
+	// Block number at last byte
+	blockNo := be.CipherOffToBlockNo(cipherSize - 1)
+	blockCount := blockNo + 1
+
+	overhead := be.BlockOverhead()*blockCount + HEADER_LEN
+
+	return cipherSize - overhead
+}
+
+// CipherSize - calculate ciphertext size from plaintext size
+func (be *ContentEnc) PlainSizeToCipherSize(plainSize uint64) uint64 {
+
+	// Block number at last byte
+	blockNo := be.PlainOffToBlockNo(plainSize - 1)
+	blockCount := blockNo + 1
+
+	overhead := be.BlockOverhead()*blockCount + HEADER_LEN
+
+	return plainSize + overhead
+}
+
+// Split a plaintext byte range into (possibly partial) blocks
+func (be *ContentEnc) ExplodePlainRange(offset uint64, length uint64) []intraBlock {
+	var blocks []intraBlock
+	var nextBlock intraBlock
+	nextBlock.fs = be
+
+	for length > 0 {
+		nextBlock.BlockNo = be.PlainOffToBlockNo(offset)
+		nextBlock.Skip = offset - be.BlockNoToPlainOff(nextBlock.BlockNo)
+
+		// Minimum of remaining data and remaining space in the block
+		nextBlock.Length = MinUint64(length, be.plainBS-nextBlock.Skip)
+
+		blocks = append(blocks, nextBlock)
+		offset += nextBlock.Length
+		length -= nextBlock.Length
+	}
+	return blocks
+}
+
+func (be *ContentEnc) BlockOverhead() uint64 {
+	return be.cipherBS - be.plainBS
+}
+
+func MinUint64(x uint64, y uint64) uint64 {
+	if x < y {
+		return x
+	}
+	return y
+}