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age: mitigate multi-key attacks on ChaCha20Poly1305

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It's possible to craft ChaCha20Poly1305 ciphertexts that decrypt under
multiple keys. (I know, it's wild.)

The impact is different for different recipients, but in general only
applies to Chosen Ciphertext Attacks against online decryption oracles:

* With the scrypt recipient, it lets the attacker make a recipient
  stanza that decrypts with multiple passwords, speeding up a bruteforce
  in terms of oracle queries (but not scrypt work, which can be
  precomputed) to logN by binary search.

  Limiting the ciphertext size limits the keys to two, which makes this
  acceptable: it's a loss of only one bit of security in a scenario
  (online decryption oracles) that is not recommended.

* With the X25519 recipient, it lets the attacker search for accepted
  public keys without using multiple recipient stanzas in the message.
  That lets the attacker bypass the 20 recipients limit (which was not
  actually intended to defend against deanonymization attacks).

  This is not really in the threat model for age: we make no attempt to
  provide anonymity in an online CCA scenario.

  Anyway, limiting the keys to two by enforcing short ciphertexts
  mitigates the attack: it only lets the attacker test 40 keys per
  message instead of 20.

* With the ssh-ed25519 recipient, the attack should be irrelevant, since
  the recipient stanza includes a 32-bit hash of the public key, making
  it decidedly not anonymous.

  Also to avoid breaking the abstraction in the agessh package, we don't
  mitigate the attack for this recipient, but we document the lack of
  anonymity.

This was reported by Paul Grubbs in the context of the upcoming paper
"Partitioning Oracle Attacks", USENIX Security 2021 (to appear), by
Julia Len, Paul Grubbs, and Thomas Ristenpart.
This commit is contained in:
Filippo Valsorda
2020-09-19 18:18:59 +02:00
parent 07c72f3b69
commit 2194f6962c
10 changed files with 94 additions and 28 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
scrypt.go
View File

@@ -19,7 +19,8 @@ import (
const scryptLabel = "age-encryption.org/v1/scrypt"
// ScryptRecipient is a password-based recipient.
// ScryptRecipient is a password-based recipient. Anyone with the password can
// decrypt the message.
//
// If a ScryptRecipient is used, it must be the only recipient for the file: it
// can't be mixed with other recipient types and can't be used multiple times
@@ -60,8 +61,10 @@ func (r *ScryptRecipient) SetWorkFactor(logN int) {
r.workFactor = logN
}
const scryptSaltSize = 16
func (r *ScryptRecipient) Wrap(fileKey []byte) (*Stanza, error) {
salt := make([]byte, 16)
salt := make([]byte, scryptSaltSize)
if _, err := rand.Read(salt[:]); err != nil {
return nil, err
}
@@ -133,7 +136,7 @@ func (i *ScryptIdentity) Unwrap(block *Stanza) ([]byte, error) {
if err != nil {
return nil, fmt.Errorf("failed to parse scrypt salt: %v", err)
}
if len(salt) != 16 {
if len(salt) != scryptSaltSize {
return nil, errors.New("invalid scrypt recipient block")
}
logN, err := strconv.Atoi(block.Args[1])
@@ -153,7 +156,14 @@ func (i *ScryptIdentity) Unwrap(block *Stanza) ([]byte, error) {
return nil, fmt.Errorf("failed to generate scrypt hash: %v", err)
}
fileKey, err := aeadDecrypt(k, block.Body)
// This AEAD is not robust, so an attacker could craft a message that
// decrypts under two different keys (meaning two different passphrases) and
// then use an error side-channel in an online decryption oracle to learn if
// either key is correct. This is deemed acceptable because the usa case (an
// online decryption oracle) is not recommended, and the security loss is
// only one bit. This also does not bypass any scrypt work, but that work
// can be precomputed in an online oracle scenario.
fileKey, err := aeadDecrypt(k, fileKeySize, block.Body)
if err != nil {
return nil, ErrIncorrectIdentity
}