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package cryptfs
// File content encryption / decryption
import (
"bytes"
"crypto/cipher"
"crypto/md5"
"encoding/binary"
"encoding/hex"
"errors"
"os"
)
// 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, 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 *CryptFS) 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) {
Debug.Printf("DecryptBlock: file hole encountered")
return make([]byte, be.plainBS), nil
}
if len(ciphertext) < be.gcmIVLen {
Warn.Printf("DecryptBlock: Block is too short: %d bytes", len(ciphertext))
return nil, errors.New("Block is too short")
}
// Extract nonce
nonce := ciphertext[:be.gcmIVLen]
ciphertextOrig := ciphertext
ciphertext = ciphertext[be.gcmIVLen:]
// Decrypt
var plaintext []byte
aData := make([]byte, 8)
aData = append(aData, fileId...)
binary.BigEndian.PutUint64(aData, blockNo)
plaintext, err := be.gcm.Open(plaintext, nonce, ciphertext, aData)
if err != nil {
Warn.Printf("DecryptBlock: %s, len=%d, md5=%s", err.Error(), len(ciphertextOrig), md5sum(ciphertextOrig))
Debug.Println(hex.Dump(ciphertextOrig))
return nil, err
}
return plaintext, nil
}
// encryptBlock - Encrypt and add IV and MAC
func (be *CryptFS) EncryptBlock(plaintext []byte, blockNo uint64, fileID []byte) []byte {
// Empty block?
if len(plaintext) == 0 {
return plaintext
}
// Get fresh nonce
nonce := be.gcmIVGen.Get()
// 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.gcm.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 *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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