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package cryptfs
// Filename encryption / decryption functions
import (
"crypto/aes"
"crypto/cipher"
"encoding/base64"
"errors"
"fmt"
"github.com/rfjakob/eme"
)
// DecryptName - decrypt base64-encoded encrypted filename "cipherName"
// The used encryption is either CBC or EME, depending on the "EMENames" argument.
//
// This function is exported because it allows for a very efficient readdir
// implementation (read IV once, decrypt all names using this function).
func (be *CryptFS) DecryptName(cipherName string, iv []byte, EMENames bool) (string, error) {
return be.decryptName(cipherName, iv, EMENames)
}
// decryptName - decrypt base64-encoded encrypted filename "cipherName".
// The used encryption is either CBC or EME, depending on the "EMENames" argument.
func (be *CryptFS) decryptName(cipherName string, iv []byte, EMENames bool) (string, error) {
// Make sure relative symlinks still work after encryption
// by passing these through unchanged
if cipherName == "." || cipherName == ".." {
return cipherName, nil
}
bin, err := base64.URLEncoding.DecodeString(cipherName)
if err != nil {
return "", err
}
if len(bin)%aes.BlockSize != 0 {
return "", fmt.Errorf("Decoded length %d is not a multiple of the AES block size", len(bin))
}
if EMENames {
bin = eme.Transform(be.blockCipher, iv, bin, eme.DirectionDecrypt)
} else {
cbc := cipher.NewCBCDecrypter(be.blockCipher, iv)
cbc.CryptBlocks(bin, bin)
}
bin, err = be.unPad16(bin)
if err != nil {
return "", err
}
plain := string(bin)
return plain, err
}
// encryptName - encrypt "plainName", return base64-encoded "cipherName64"
// The used encryption is either CBC or EME, depending on the "EMENames" argument.
func (be *CryptFS) encryptName(plainName string, iv []byte, EMENames bool) (cipherName64 string) {
// Make sure relative symlinks still work after encryption
// by passing these trough unchanged
if plainName == "." || plainName == ".." {
return plainName
}
bin := []byte(plainName)
bin = be.pad16(bin)
if EMENames {
bin = eme.Transform(be.blockCipher, iv, bin, eme.DirectionEncrypt)
} else {
cbc := cipher.NewCBCEncrypter(be.blockCipher, iv)
cbc.CryptBlocks(bin, bin)
}
cipherName64 = base64.URLEncoding.EncodeToString(bin)
return cipherName64
}
// pad16 - pad filename to 16 byte blocks using standard PKCS#7 padding
// https://tools.ietf.org/html/rfc5652#section-6.3
func (be *CryptFS) pad16(orig []byte) (padded []byte) {
oldLen := len(orig)
if oldLen == 0 {
panic("Padding zero-length string makes no sense")
}
padLen := aes.BlockSize - oldLen%aes.BlockSize
if padLen == 0 {
padLen = aes.BlockSize
}
newLen := oldLen + padLen
padded = make([]byte, newLen)
copy(padded, orig)
padByte := byte(padLen)
for i := oldLen; i < newLen; i++ {
padded[i] = padByte
}
return padded
}
// unPad16 - remove padding
func (be *CryptFS) unPad16(orig []byte) ([]byte, error) {
oldLen := len(orig)
if oldLen%aes.BlockSize != 0 {
return nil, errors.New("Unaligned size")
}
// The last byte is always a padding byte
padByte := orig[oldLen-1]
// The padding byte's value is the padding length
padLen := int(padByte)
// Padding must be at least 1 byte
if padLen <= 0 {
return nil, errors.New("Padding cannot be zero-length")
}
// Larger paddings make no sense
if padLen > aes.BlockSize {
return nil, errors.New("Padding cannot be larger than 16")
}
// All padding bytes must be identical
for i := oldLen - padLen; i < oldLen; i++ {
if orig[i] != padByte {
return nil, errors.New(fmt.Sprintf("Padding byte at i=%d is invalid", i))
}
}
newLen := oldLen - padLen
// Padding an empty string makes no sense
if newLen == 0 {
return nil, errors.New("Unpadded length is zero")
}
return orig[0:newLen], nil
}
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