139 lines
4.3 KiB
Go
139 lines
4.3 KiB
Go
// Package nametransform encrypts and decrypts filenames.
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package nametransform
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import (
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"bytes"
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"crypto/aes"
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"encoding/base64"
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"path/filepath"
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"syscall"
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"github.com/rfjakob/eme"
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"github.com/rfjakob/gocryptfs/internal/tlog"
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)
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const (
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// Like ext4, we allow at most 255 bytes for a file name.
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NameMax = 255
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)
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// NameTransformer is an interface used to transform filenames.
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type NameTransformer interface {
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DecryptName(cipherName string, iv []byte) (string, error)
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EncryptName(plainName string, iv []byte) string
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EncryptAndHashName(name string, iv []byte) (string, error)
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HashLongName(name string) string
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WriteLongNameAt(dirfd int, hashName string, plainName string) error
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B64EncodeToString(src []byte) string
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B64DecodeString(s string) ([]byte, error)
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}
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// NameTransform is used to transform filenames.
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type NameTransform struct {
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emeCipher *eme.EMECipher
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longNames bool
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// B64 = either base64.URLEncoding or base64.RawURLEncoding, depending
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// on the Raw64 feature flag
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B64 *base64.Encoding
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// Patterns to bypass decryption
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BadnamePatterns []string
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}
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// New returns a new NameTransform instance.
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func New(e *eme.EMECipher, longNames bool, raw64 bool) *NameTransform {
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b64 := base64.URLEncoding
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if raw64 {
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b64 = base64.RawURLEncoding
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}
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return &NameTransform{
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emeCipher: e,
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longNames: longNames,
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B64: b64,
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}
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}
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// DecryptName calls decryptName to try and decrypt a base64-encoded encrypted
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// filename "cipherName", and failing that checks if it can be bypassed
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func (n *NameTransform) DecryptName(cipherName string, iv []byte) (string, error) {
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res, err := n.decryptName(cipherName, iv)
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if err != nil {
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for _, pattern := range n.BadnamePatterns {
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match, err := filepath.Match(pattern, cipherName)
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if err == nil && match { // Pattern should have been validated already
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// Find longest decryptable substring
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// At least 16 bytes due to AES --> at least 22 characters in base64
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nameMin := n.B64.EncodedLen(aes.BlockSize)
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for charpos := len(cipherName) - 1; charpos >= nameMin; charpos-- {
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res, err = n.decryptName(cipherName[:charpos], iv)
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if err == nil {
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return res + cipherName[charpos:] + " GOCRYPTFS_BAD_NAME", nil
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}
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}
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return cipherName + " GOCRYPTFS_BAD_NAME", nil
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}
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}
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}
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return res, err
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}
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// decryptName decrypts a base64-encoded encrypted filename "cipherName" using the
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// initialization vector "iv".
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func (n *NameTransform) decryptName(cipherName string, iv []byte) (string, error) {
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bin, err := n.B64.DecodeString(cipherName)
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if err != nil {
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return "", err
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}
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if len(bin) == 0 {
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tlog.Warn.Printf("DecryptName: empty input")
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return "", syscall.EBADMSG
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}
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if len(bin)%aes.BlockSize != 0 {
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tlog.Debug.Printf("DecryptName %q: decoded length %d is not a multiple of 16", cipherName, len(bin))
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return "", syscall.EBADMSG
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}
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bin = n.emeCipher.Decrypt(iv, bin)
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bin, err = unPad16(bin)
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if err != nil {
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tlog.Debug.Printf("DecryptName: unPad16 error detail: %v", err)
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// unPad16 returns detailed errors including the position of the
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// incorrect bytes. Kill the padding oracle by lumping everything into
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// a generic error.
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return "", syscall.EBADMSG
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}
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// A name can never contain a null byte or "/". Make sure we never return those
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// to the kernel, even when we read a corrupted (or fuzzed) filesystem.
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if bytes.Contains(bin, []byte{0}) || bytes.Contains(bin, []byte("/")) {
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return "", syscall.EBADMSG
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}
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// The name should never be "." or "..".
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if bytes.Equal(bin, []byte(".")) || bytes.Equal(bin, []byte("..")) {
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return "", syscall.EBADMSG
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}
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plain := string(bin)
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return plain, err
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}
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// EncryptName encrypts "plainName", returns a base64-encoded "cipherName64",
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// encrypted using EME (https://github.com/rfjakob/eme).
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//
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// This function is exported because in some cases, fusefrontend needs access
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// to the full (not hashed) name if longname is used.
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func (n *NameTransform) EncryptName(plainName string, iv []byte) (cipherName64 string) {
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bin := []byte(plainName)
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bin = pad16(bin)
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bin = n.emeCipher.Encrypt(iv, bin)
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cipherName64 = n.B64.EncodeToString(bin)
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return cipherName64
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}
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// B64EncodeToString returns a Base64-encoded string
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func (n *NameTransform) B64EncodeToString(src []byte) string {
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return n.B64.EncodeToString(src)
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}
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// B64DecodeString decodes a Base64-encoded string
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func (n *NameTransform) B64DecodeString(s string) ([]byte, error) {
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return n.B64.DecodeString(s)
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}
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