hardcoresushi/libgocryptfs
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contentenc: fix CipherSizeToPlainSize non-monoticity

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For an illegal cipherSize, pretend we have an additional
1-byte block.

See code comment for details.
This commit is contained in:
Jakob Unterwurzacher 2021-05-26 09:08:52 +02:00
parent bebd7ed81f
commit b4794bedec
1 changed files with 14 additions and 1 deletions
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15
internal/contentenc/offsets.go
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@ -31,7 +31,11 @@ func (be *ContentEnc) BlockNoToPlainOff(blockNo uint64) uint64 {
return blockNo * be.plainBS
}
// CipherSizeToPlainSize calculates the plaintext size from a ciphertext size
// 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 {
@ -49,6 +53,15 @@ func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 {
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