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|
package s3crypto
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"io"
)
// AESCBC is a symmetric crypto algorithm. This algorithm
// requires a padder due to CBC needing to be of the same block
// size. AES CBC is vulnerable to Padding Oracle attacks and
// so should be avoided when possible.
type aesCBC struct {
encrypter cipher.BlockMode
decrypter cipher.BlockMode
padder Padder
}
// newAESCBC creates a new AES CBC cipher. Expects keys to be of
// the correct size.
func newAESCBC(cd CipherData, padder Padder) (Cipher, error) {
block, err := aes.NewCipher(cd.Key)
if err != nil {
return nil, err
}
encrypter := cipher.NewCBCEncrypter(block, cd.IV)
decrypter := cipher.NewCBCDecrypter(block, cd.IV)
return &aesCBC{encrypter, decrypter, padder}, nil
}
// Encrypt will encrypt the data using AES CBC by returning
// an io.Reader. The io.Reader will encrypt the data as Read
// is called.
func (c *aesCBC) Encrypt(src io.Reader) io.Reader {
reader := &cbcEncryptReader{
encrypter: c.encrypter,
src: src,
padder: c.padder,
}
return reader
}
type cbcEncryptReader struct {
encrypter cipher.BlockMode
src io.Reader
padder Padder
size int
buf bytes.Buffer
}
// Read will read from our io.Reader and encrypt the data as necessary.
// Due to padding, we have to do some logic that when we encounter an
// end of file to pad properly.
func (reader *cbcEncryptReader) Read(data []byte) (int, error) {
n, err := reader.src.Read(data)
reader.size += n
blockSize := reader.encrypter.BlockSize()
reader.buf.Write(data[:n])
if err == io.EOF {
b := make([]byte, getSliceSize(blockSize, reader.buf.Len(), len(data)))
n, err = reader.buf.Read(b)
if err != nil && err != io.EOF {
return n, err
}
// The buffer is now empty, we can now pad the data
if reader.buf.Len() == 0 {
b, err = reader.padder.Pad(b[:n], reader.size)
if err != nil {
return n, err
}
n = len(b)
err = io.EOF
}
// We only want to encrypt if we have read anything
if n > 0 {
reader.encrypter.CryptBlocks(data, b)
}
return n, err
}
if err != nil {
return n, err
}
if size := reader.buf.Len(); size >= blockSize {
nBlocks := size / blockSize
if size > len(data) {
nBlocks = len(data) / blockSize
}
if nBlocks > 0 {
b := make([]byte, nBlocks*blockSize)
n, _ = reader.buf.Read(b)
reader.encrypter.CryptBlocks(data, b[:n])
}
} else {
n = 0
}
return n, nil
}
// Decrypt will decrypt the data using AES CBC
func (c *aesCBC) Decrypt(src io.Reader) io.Reader {
return &cbcDecryptReader{
decrypter: c.decrypter,
src: src,
padder: c.padder,
}
}
type cbcDecryptReader struct {
decrypter cipher.BlockMode
src io.Reader
padder Padder
buf bytes.Buffer
}
// Read will read from our io.Reader and decrypt the data as necessary.
// Due to padding, we have to do some logic that when we encounter an
// end of file to pad properly.
func (reader *cbcDecryptReader) Read(data []byte) (int, error) {
n, err := reader.src.Read(data)
blockSize := reader.decrypter.BlockSize()
reader.buf.Write(data[:n])
if err == io.EOF {
b := make([]byte, getSliceSize(blockSize, reader.buf.Len(), len(data)))
n, err = reader.buf.Read(b)
if err != nil && err != io.EOF {
return n, err
}
// We only want to decrypt if we have read anything
if n > 0 {
reader.decrypter.CryptBlocks(data, b)
}
if reader.buf.Len() == 0 {
b, err = reader.padder.Unpad(data[:n])
n = len(b)
if err != nil {
return n, err
}
err = io.EOF
}
return n, err
}
if err != nil {
return n, err
}
if size := reader.buf.Len(); size >= blockSize {
nBlocks := size / blockSize
if size > len(data) {
nBlocks = len(data) / blockSize
}
// The last block is always padded. This will allow us to unpad
// when we receive an io.EOF error
nBlocks -= blockSize
if nBlocks > 0 {
b := make([]byte, nBlocks*blockSize)
n, _ = reader.buf.Read(b)
reader.decrypter.CryptBlocks(data, b[:n])
} else {
n = 0
}
}
return n, nil
}
// getSliceSize will return the correct amount of bytes we need to
// read with regards to padding.
func getSliceSize(blockSize, bufSize, dataSize int) int {
size := bufSize
if bufSize > dataSize {
size = dataSize
}
size = size - (size % blockSize) - blockSize
if size <= 0 {
size = blockSize
}
return size
}
|
