libgocryptfs/cryptfs/address_translation.go
Jakob Unterwurzacher 296bdf3af2 CipherSizeToPlainSize: Handle illegal states
A file never gets a cipherSize <= HEADER_LEN in normal operation.
However, this can happen if header write it interrupted or the
underlying filesystem does not support fallocate.

Noticed while trying to store a CIPHERDIR in another gocryptfs mount
(gocryptfs does not support fallocate)
2015-11-15 14:15:21 +01:00

90 lines
2.3 KiB
Go

package cryptfs
// CryptFS methods that translate offsets between ciphertext and plaintext
// get the block number at plain-text offset
func (be *CryptFS) PlainOffToBlockNo(plainOffset uint64) uint64 {
return plainOffset / be.plainBS
}
// get the block number at ciphter-text offset
func (be *CryptFS) CipherOffToBlockNo(cipherOffset uint64) uint64 {
return (cipherOffset - HEADER_LEN) / be.cipherBS
}
// get ciphertext offset of block "blockNo"
func (be *CryptFS) BlockNoToCipherOff(blockNo uint64) uint64 {
return HEADER_LEN + blockNo*be.cipherBS
}
// get plaintext offset of block "blockNo"
func (be *CryptFS) BlockNoToPlainOff(blockNo uint64) uint64 {
return blockNo * be.plainBS
}
// PlainSize - calculate plaintext size from ciphertext size
func (be *CryptFS) CipherSizeToPlainSize(cipherSize uint64) uint64 {
// Zero sized files stay zero-sized
if cipherSize == 0 {
return 0
}
if cipherSize == HEADER_LEN {
Warn.Printf("cipherSize %d == header size: interrupted write?\n", cipherSize)
return 0
}
if cipherSize < HEADER_LEN {
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 := BLOCK_OVERHEAD*blockCount + HEADER_LEN
return cipherSize - overhead
}
// CipherSize - calculate ciphertext size from plaintext size
func (be *CryptFS) PlainSizeToCipherSize(plainSize uint64) uint64 {
// Block number at last byte
blockNo := be.PlainOffToBlockNo(plainSize - 1)
blockCount := blockNo + 1
overhead := BLOCK_OVERHEAD*blockCount + HEADER_LEN
return plainSize + overhead
}
// Split a plaintext byte range into (possibly partial) blocks
func (be *CryptFS) 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 MinUint64(x uint64, y uint64) uint64 {
if x < y {
return x
}
return y
}