Kestrel uses a simple combination of the Noise Protocol and a chunked file encryption scheme.
The noise protocol (Noise_X_25519_ChaChaPoly_SHA256) is used to encrypt a payload key. This payload key is then used for ChaCha20-Poly1305 file encryption. Files are split into encrypted and authenticated chunks, ensuring that unauthenticated data is never written to disk.
Users can also use a password instead of public keys. This password is used with scrypt to derive a symmetric key for file encryption.
Public Key Encryption
The X pattern of a noise protocol handshake is used to perform public key authenticated encryption between sender and recipient.
In order to send a message, senders are required to obtain a legitimate copy of the recipient's public key.
- When you successfully decrypt a file, you know that the file hasn't been modified and that it came from a specific, known public key.
- Deniability. If you send a message and your recipient reveals it and tries to claim that you sent it, you can deny it because the recipient has the ability to forge received messages.
- If your private key gets compromised later, the attacker can't read the messages that you've sent. The only way to decrypt the messages is to compromise your recipient's private key.
- If your private key gets compromised, the attacker can pretend to be you. You need to get a new key pair and be able to communicate the new public key to your contacts.
- Messages can be replayed. Replay prevention is out of scope for this application. However, replay is considered benign in this context. Imagine sending your encrypted tax files to an accountant. The attacker can resend your encrypted file to the accountant, but the accountant will end up with a benign, redundant copy.
- The encryption is meant to work as you would expect it to. If you get a file, you know who it came from, and that it hasn't been read or tampered with. When you send a file, only the person that you sent it to can read it.
Guarantees from the noise protocol
Each payload is assigned a "source" property regarding the degree of authentication of the sender provided to the recipient, and a "destination" property regarding the degree of confidentiality provided to the sender.
Sender authentication vulnerable to key-compromise impersonation (KCI). The sender authentication is based on a static-static DH ("ss") involving both parties' static key pairs. If the recipient's long-term private key has been compromised, this authentication can be forged.
Encryption to a known recipient, forward secrecy for sender compromise only, vulnerable to replay. This payload is encrypted based only on DHs involving the recipient's static key pair. If the recipient's static private key is compromised, even at a later date, this payload can be decrypted. This message can also be replayed, since there's no ephemeral contribution from the recipient.
- A fresh 256 bit symmetric key is generated from a CSPRNG. This is the payload key.
- A noise handshake is performed between the sender and recipient with the payload key included as the noise payload. The result is a noise handshake message that includes the encrypted payload key and the encrypted sender public key.
- The plaintext is encrypted using the payload key and the chunked encryption format.
- The recipient must choose to decrypt using the same key that the sender chose as the recipient key. Because the ciphertext contains no identifying information from either the sender or recipient, the recipient must choose the correct key pair from which to attempt decryption. If the recipient has, for example, a work key pair, and a personal key pair, the recipient must know to decrypt with either the work key or the personal key. Obviously decryption could be attempted with both keys if the recipient is unsure.
- The recipient decrypts the noise handshake message. If successful, this results in the decrypted payload key and sender's public key.
- The ciphertext is decrypted using the payload key and the chunked encryption format. The sender's public key is displayed upon successful decryption.
Scrypt is used to derive a symmetric key from a password which is then used with the chunked file encryption format.
Files are encrypted and authenticated. After a successful decryption, you can be certain that the file was not modified and that it came from the person in possession of the shared password.
Encryption and Decryption Steps
- A symmetric key is derived from a password using the scrypt parameters N = 15, r = 8, and p = 1.
- The file is encrypted or decrypted using the derived key and the chunked encryption format.
Chunked File Format
Files may be too large to fit entirely into memory, so they are split into into encrypted and authenticated chunks.
Each chunk has a chunk number starting from zero and incrementing sequentially (0, 1, 2, 3, ...).
The chunk number is also used as the nonce for the encryption function. Keys are fresh for each message ensure that the nonce does not repeat.
Chunks are 65,536 bytes (64k) in size.
The last chunk has an authenticated last chunk indicator signifying that this is the last chunk in the message. The chunk number MUST increase sequentially and MUST contain only a single last chunk indicator.
This design ensures that chunks in a message CANNOT be re-ordered, modified, removed, duplicated, or truncated.
Messages CANNOT be modified or truncated without detection.