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// Package contentenc encrypts and decrypts file blocks.
package contentenc
import (
"bytes"
"encoding/binary"
"encoding/hex"
"errors"
"github.com/rfjakob/gocryptfs/internal/cryptocore"
"github.com/rfjakob/gocryptfs/internal/tlog"
)
type NonceMode int
const (
// Default plaintext block size
DefaultBS = 4096
// We always use 128-bit IVs for file content encryption
IVBitLen = 128
_ = iota // skip zero
RandomNonce NonceMode = iota
ReverseDeterministicNonce NonceMode = iota
ExternalNonce NonceMode = iota
)
type ContentEnc struct {
// Cryptographic primitives
cryptoCore *cryptocore.CryptoCore
// Plaintext block size
plainBS uint64
// Ciphertext block size
cipherBS uint64
// All-zero block of size cipherBS, for fast compares
allZeroBlock []byte
// All-zero block of size IVBitLen/8, for fast compares
allZeroNonce []byte
}
func New(cc *cryptocore.CryptoCore, plainBS uint64) *ContentEnc {
cipherBS := plainBS + uint64(cc.IVLen) + cryptocore.AuthTagLen
return &ContentEnc{
cryptoCore: cc,
plainBS: plainBS,
cipherBS: cipherBS,
allZeroBlock: make([]byte, cipherBS),
allZeroNonce: make([]byte, cc.IVLen),
}
}
// PlainBS returns the plaintext block size
func (be *ContentEnc) PlainBS() uint64 {
return be.plainBS
}
// CipherBS returns the ciphertext block size
func (be *ContentEnc) CipherBS() uint64 {
return be.cipherBS
}
// DecryptBlocks - Decrypt a number of blocks
// TODO refactor to three-param for
func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, fileId []byte) ([]byte, error) {
cBuf := bytes.NewBuffer(ciphertext)
var err error
var pBuf bytes.Buffer
for cBuf.Len() > 0 {
cBlock := cBuf.Next(int(be.cipherBS))
var pBlock []byte
pBlock, err = be.DecryptBlock(cBlock, firstBlockNo, fileId)
if err != nil {
break
}
pBuf.Write(pBlock)
firstBlockNo++
}
return pBuf.Bytes(), err
}
// DecryptBlock - Verify and decrypt GCM block
//
// Corner case: A full-sized block of all-zero ciphertext bytes is translated
// to an all-zero plaintext block, i.e. file hole passtrough.
func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileId []byte) ([]byte, error) {
// Empty block?
if len(ciphertext) == 0 {
return ciphertext, nil
}
// All-zero block?
if bytes.Equal(ciphertext, be.allZeroBlock) {
tlog.Debug.Printf("DecryptBlock: file hole encountered")
return make([]byte, be.plainBS), nil
}
if len(ciphertext) < be.cryptoCore.IVLen {
tlog.Warn.Printf("DecryptBlock: Block is too short: %d bytes", len(ciphertext))
return nil, errors.New("Block is too short")
}
// Extract nonce
nonce := ciphertext[:be.cryptoCore.IVLen]
if bytes.Equal(nonce, be.allZeroNonce) && be.cryptoCore.AEADBackend != cryptocore.BackendGCMSIV {
panic("Hit an all-zero nonce with GCMSIV off. This MUST NOT happen!")
}
ciphertextOrig := ciphertext
ciphertext = ciphertext[be.cryptoCore.IVLen:]
// Decrypt
var plaintext []byte
aData := make([]byte, 8)
aData = append(aData, fileId...)
binary.BigEndian.PutUint64(aData, blockNo)
plaintext, err := be.cryptoCore.AEADCipher.Open(plaintext, nonce, ciphertext, aData)
if err != nil {
tlog.Warn.Printf("DecryptBlock: %s, len=%d", err.Error(), len(ciphertextOrig))
tlog.Debug.Println(hex.Dump(ciphertextOrig))
return nil, err
}
return plaintext, nil
}
// EncryptBlocks - Encrypt a number of blocks
// Used for reverse mode
func (be *ContentEnc) EncryptBlocks(plaintext []byte, firstBlockNo uint64, fileId []byte, nMode NonceMode) []byte {
inBuf := bytes.NewBuffer(plaintext)
var outBuf bytes.Buffer
for blockNo := firstBlockNo; inBuf.Len() > 0; blockNo++ {
inBlock := inBuf.Next(int(be.plainBS))
outBlock := be.EncryptBlock(inBlock, blockNo, fileId, nMode, nil)
outBuf.Write(outBlock)
}
return outBuf.Bytes()
}
// encryptBlock - Encrypt and add IV and MAC
func (be *ContentEnc) EncryptBlock(plaintext []byte, blockNo uint64, fileID []byte, nMode NonceMode, externalNonce []byte) []byte {
// Empty block?
if len(plaintext) == 0 {
return plaintext
}
var nonce []byte
switch nMode {
case ExternalNonce:
if be.cryptoCore.AEADBackend != cryptocore.BackendGCMSIV {
panic("MUST NOT use deterministic nonces unless in GCMSIV mode!")
}
nonce = externalNonce
case ReverseDeterministicNonce:
if be.cryptoCore.AEADBackend != cryptocore.BackendGCMSIV {
panic("MUST NOT use deterministic nonces unless in GCMSIV mode!")
}
l := be.cryptoCore.IVLen
nonce = make([]byte, l)
copy(nonce, fileID)
// Add the block number to the last 8 byte. Plus one so the block-zero
// IV is distinct from the fileID.
counter := binary.BigEndian.Uint64(nonce[l-8 : l])
binary.BigEndian.PutUint64(nonce[l-8:l], counter+blockNo+1)
case RandomNonce:
// Get a fresh random nonce
nonce = be.cryptoCore.IVGenerator.Get()
default:
panic("invalid nonce mode")
}
if len(nonce) != be.cryptoCore.IVLen {
panic("wrong nonce length")
}
// Authenticate block with block number and file ID
aData := make([]byte, 8)
binary.BigEndian.PutUint64(aData, blockNo)
aData = append(aData, fileID...)
// Encrypt plaintext and append to nonce
ciphertext := be.cryptoCore.AEADCipher.Seal(nonce, nonce, plaintext, aData)
return ciphertext
}
// MergeBlocks - Merge newData into oldData at offset
// New block may be bigger than both newData and oldData
func (be *ContentEnc) MergeBlocks(oldData []byte, newData []byte, offset int) []byte {
// Make block of maximum size
out := make([]byte, be.plainBS)
// Copy old and new data into it
copy(out, oldData)
l := len(newData)
copy(out[offset:offset+l], newData)
// Crop to length
outLen := len(oldData)
newLen := offset + len(newData)
if outLen < newLen {
outLen = newLen
}
return out[0:outLen]
}
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