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|
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 from a ciphertext size
func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 {
// Zero-sized files stay zero-sized
if cipherSize == 0 {
return 0
}
if cipherSize == HeaderLen {
tlog.Warn.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
}
// 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
}
|
