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package cryptfs
// File content encryption / decryption
import (
"bytes"
"os"
"errors"
"crypto/cipher"
"crypto/md5"
"encoding/hex"
)
const (
// A block of 4124 zero bytes has this md5
ZeroBlockMd5 = "64331af89bd15a987b39855338336237"
)
// md5sum - debug helper, return md5 hex string
func md5sum(buf []byte) string {
rawHash := md5.Sum(buf)
hash := hex.EncodeToString(rawHash[:])
return hash
}
type CryptFile struct {
file *os.File
gcm cipher.AEAD
}
// DecryptBlocks - Decrypt a number of blocks
func (be *CryptFS) DecryptBlocks(ciphertext []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)
if err != nil {
break
}
pBuf.Write(pBlock)
}
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 *CryptFS) DecryptBlock(ciphertext []byte) ([]byte, error) {
// Empty block?
if len(ciphertext) == 0 {
return ciphertext, nil
}
// All-zero block?
if bytes.Equal(ciphertext, be.allZeroBlock) {
Debug.Printf("DecryptBlock: file hole encountered\n")
return make([]byte, be.plainBS), nil
}
if len(ciphertext) < NONCE_LEN {
Warn.Printf("decryptBlock: Block is too short: %d bytes\n", len(ciphertext))
return nil, errors.New("Block is too short")
}
// Extract nonce
nonce := ciphertext[:NONCE_LEN]
ciphertextOrig := ciphertext
ciphertext = ciphertext[NONCE_LEN:]
// Decrypt
var plaintext []byte
plaintext, err := be.gcm.Open(plaintext, nonce, ciphertext, nil)
if err != nil {
Warn.Printf("DecryptBlock: %s, len=%d, md5=%s\n", err.Error(), len(ciphertextOrig), Warn.Md5sum(ciphertextOrig))
return nil, err
}
return plaintext, nil
}
// encryptBlock - Encrypt and add MAC using GCM
func (be *CryptFS) EncryptBlock(plaintext []byte) []byte {
// Empty block?
if len(plaintext) == 0 {
return plaintext
}
// Get fresh nonce
nonce := gcmNonce.Get()
// Encrypt plaintext and append to nonce
ciphertext := be.gcm.Seal(nonce, nonce, plaintext, nil)
return ciphertext
}
// Split a plaintext byte range into (possibly partial) blocks
func (be *CryptFS) SplitRange(offset uint64, length uint64) []intraBlock {
var b intraBlock
var parts []intraBlock
b.fs = be
for length > 0 {
b.BlockNo = offset / be.plainBS
b.Offset = offset % be.plainBS
b.Length = be.minu64(length, be.plainBS - b.Offset)
parts = append(parts, b)
offset += b.Length
length -= b.Length
}
return parts
}
// PlainSize - calculate plaintext size from ciphertext size
func (be *CryptFS) PlainSize(size uint64) uint64 {
// Zero sized files stay zero-sized
if size == 0 {
return 0
}
overhead := be.cipherBS - be.plainBS
nBlocks := (size + be.cipherBS - 1) / be.cipherBS
if nBlocks * overhead > size {
Warn.Printf("PlainSize: Negative size, returning 0 instead\n")
return 0
}
size -= nBlocks * overhead
return size
}
// CipherSize - calculate ciphertext size from plaintext size
func (be *CryptFS) CipherSize(size uint64) uint64 {
overhead := be.cipherBS - be.plainBS
nBlocks := (size + be.plainBS - 1) / be.plainBS
size += nBlocks * overhead
return size
}
func (be *CryptFS) minu64(x uint64, y uint64) uint64 {
if x < y {
return x
}
return y
}
// CiphertextRange - Get byte range in backing ciphertext corresponding
// to plaintext range. Returns a range aligned to ciphertext blocks.
func (be *CryptFS) CiphertextRange(offset uint64, length uint64) (alignedOffset uint64, alignedLength uint64, skipBytes int) {
// Decrypting the ciphertext will yield too many plaintext bytes. Skip this number
// of bytes from the front.
skip := offset % be.plainBS
firstBlockNo := offset / be.plainBS
lastBlockNo := ( offset + length - 1 ) / be.plainBS
alignedOffset = firstBlockNo * be.cipherBS
alignedLength = (lastBlockNo - firstBlockNo + 1) * be.cipherBS
skipBytes = int(skip)
return alignedOffset, alignedLength, skipBytes
}
// Get the byte range in the ciphertext corresponding to blocks
// (full blocks!)
func (be *CryptFS) JoinCiphertextRange(blocks []intraBlock) (uint64, uint64) {
offset, _ := blocks[0].CiphertextRange()
last := blocks[len(blocks)-1]
length := (last.BlockNo - blocks[0].BlockNo + 1) * be.cipherBS
return offset, length
}
// Crop plaintext that correspons to complete cipher blocks down to what is
// requested according to "iblocks"
func (be *CryptFS) CropPlaintext(plaintext []byte, blocks []intraBlock) []byte {
offset := blocks[0].Offset
last := blocks[len(blocks)-1]
length := (last.BlockNo - blocks[0].BlockNo + 1) * be.plainBS
var cropped []byte
if offset + length > uint64(len(plaintext)) {
cropped = plaintext[offset:len(plaintext)]
} else {
cropped = plaintext[offset:offset+length]
}
return cropped
}
// MergeBlocks - Merge newData into oldData at offset
// New block may be bigger than both newData and oldData
func (be *CryptFS) 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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