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package pathiv
import (
"crypto/sha256"
"encoding/binary"
"github.com/rfjakob/gocryptfs/internal/nametransform"
)
// Purpose identifies for which purpose the IV will be used. This is mixed into the
// derivation.
type Purpose string
const (
// PurposeDirIV means the value will be used as a directory IV
PurposeDirIV Purpose = "DIRIV"
// PurposeFileID means the value will be used as the file ID in the file header
PurposeFileID Purpose = "FILEID"
// PurposeSymlinkIV means the value will be used as the IV for symlink encryption
PurposeSymlinkIV Purpose = "SYMLINKIV"
// PurposeBlock0IV means the value will be used as the IV of ciphertext block #0.
PurposeBlock0IV Purpose = "BLOCK0IV"
)
// Derive derives an IV from an encrypted path by hashing it with sha256
func Derive(path string, purpose Purpose) []byte {
// Use null byte as separator as it cannot occur in the path
extended := []byte(path + "\000" + string(purpose))
hash := sha256.Sum256(extended)
return hash[:nametransform.DirIVLen]
}
// FileIVs contains both IVs that are needed to create a file.
type FileIVs struct {
ID []byte
Block0IV []byte
}
// DeriveFile derives both IVs that are needed to create a file and returns them
// in a container struct.
func DeriveFile(path string) (fileIVs FileIVs) {
fileIVs.ID = Derive(path, PurposeFileID)
fileIVs.Block0IV = Derive(path, PurposeBlock0IV)
return fileIVs
}
// BlockIV returns the block IV for block number "blockNo". "block0iv" is the block
// IV of block #0.
func BlockIV(block0iv []byte, blockNo uint64) []byte {
iv := make([]byte, len(block0iv))
copy(iv, block0iv)
// Add blockNo to one half of the iv
lowBytes := iv[8:]
lowInt := binary.BigEndian.Uint64(lowBytes)
binary.BigEndian.PutUint64(lowBytes, lowInt+blockNo)
return iv
}
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