package contentenc import ( "log" "github.com/rfjakob/gocryptfs/internal/tlog" ) // Contentenc methods that translate offsets between ciphertext and plaintext // PlainOffToBlockNo converts a plaintext offset to the ciphertext block number. func (be *ContentEnc) PlainOffToBlockNo(plainOffset uint64) uint64 { return plainOffset / be.plainBS } // CipherOffToBlockNo converts the ciphertext offset to the plaintext block number. func (be *ContentEnc) CipherOffToBlockNo(cipherOffset uint64) uint64 { if cipherOffset < HeaderLen { log.Panicf("BUG: offset %d is inside the file header", cipherOffset) } return (cipherOffset - HeaderLen) / be.cipherBS } // BlockNoToCipherOff gets the ciphertext offset of block "blockNo" func (be *ContentEnc) BlockNoToCipherOff(blockNo uint64) uint64 { return HeaderLen + blockNo*be.cipherBS } // BlockNoToPlainOff gets the plaintext offset of block "blockNo" func (be *ContentEnc) BlockNoToPlainOff(blockNo uint64) uint64 { return blockNo * be.plainBS } // CipherSizeToPlainSize calculates the plaintext size `plainSize` from a // ciphertext size `cipherSize` (in bytes). // // Not all ciphertext sizes are legal due to the per-block overheads. // For an illegal cipherSize, we return a best guess plainSize. func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 { // Zero-sized files stay zero-sized if cipherSize == 0 { return 0 } if cipherSize == HeaderLen { // This can happen between createHeader() and Write() and is harmless. tlog.Debug.Printf("cipherSize %d == header size: interrupted write?\n", cipherSize) return 0 } if cipherSize < HeaderLen { tlog.Warn.Printf("cipherSize %d < header size %d: corrupt file\n", cipherSize, HeaderLen) return 0 } // If the last block is incomplete, pad it to 1 byte of plaintext // (= 33 bytes of ciphertext). lastBlockSize := (cipherSize - HeaderLen) % be.cipherBS if lastBlockSize > 0 && lastBlockSize <= be.BlockOverhead() { tmp := cipherSize - lastBlockSize + be.BlockOverhead() + 1 tlog.Warn.Printf("cipherSize %d: incomplete last block (%d bytes), padding to %d bytes", cipherSize, lastBlockSize, tmp) cipherSize = tmp } // Block number at last byte blockNo := be.CipherOffToBlockNo(cipherSize - 1) blockCount := blockNo + 1 overhead := be.BlockOverhead()*blockCount + HeaderLen if overhead > cipherSize { tlog.Warn.Printf("cipherSize %d < overhead %d: corrupt file\n", cipherSize, overhead) return 0 } return cipherSize - overhead } // PlainSizeToCipherSize calculates the ciphertext size from a plaintext size func (be *ContentEnc) PlainSizeToCipherSize(plainSize uint64) uint64 { // Zero-sized files stay zero-sized if plainSize == 0 { return 0 } // Block number at last byte blockNo := be.PlainOffToBlockNo(plainSize - 1) blockCount := blockNo + 1 overhead := be.BlockOverhead()*blockCount + HeaderLen return plainSize + overhead } // ExplodePlainRange splits a plaintext byte range into (possibly partial) blocks // Returns an empty slice if length == 0. 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 plaintext 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 } // ExplodeCipherRange splits a ciphertext byte range into (possibly partial) // blocks This is used in reverse mode when reading files func (be *ContentEnc) ExplodeCipherRange(offset uint64, length uint64) []IntraBlock { var blocks []IntraBlock var nextBlock IntraBlock nextBlock.fs = be for length > 0 { nextBlock.BlockNo = be.CipherOffToBlockNo(offset) nextBlock.Skip = offset - be.BlockNoToCipherOff(nextBlock.BlockNo) // This block can carry up to "maxLen" payload bytes maxLen := be.cipherBS - nextBlock.Skip nextBlock.Length = maxLen // But if the user requested less, we truncate the block to "length". if length < maxLen { nextBlock.Length = length } blocks = append(blocks, nextBlock) offset += nextBlock.Length length -= nextBlock.Length } return blocks } // BlockOverhead returns the per-block overhead. func (be *ContentEnc) BlockOverhead() uint64 { return be.cipherBS - be.plainBS } // MinUint64 returns the minimum of two uint64 values. func MinUint64(x uint64, y uint64) uint64 { if x < y { return x } return y }