flynn/noise has improper nonce handling yielding potential state DoS

The Go package github.com/flynn/noise, a Noise Protocol implementation, has two bugs in nonce handling in versions prior to v1.0.0.

Issue 1: Potential nonce overflow

If 2⁶⁴ (~18.4 quintillion) or more messages are encrypted with Encrypt after handshaking, the nonce counter will wrap around, causing multiple messages to be encrypted with the same key and nonce, resulting in a potentially catastrophic weakening of the security properties of the symmetric cipher.

This has been resolved in the patched version by returning ErrMaxNonce from the CipherState Encrypt and Decrypt methods before the reserved maximum nonce is reached. If this error is encountered, the program should handshake again to start with a fresh CipherState.

Issue 2: Potential denial of service via invalid ciphertext

If an attacker sends an invalid ciphertext into one peer’s Decrypt, the nonce is incremented unconditionally. This causes a desync of the CipherState due to a nonce mismatch between the peers, resulting in a failure to decrypt all subsequent messages. A new handshake will be required to establish a new CipherState.

This has been resolved in the patched version by returning authentication errors from Decrypt before incrementing the nonce.

Patches

Fixed in https://github.com/flynn/noise/pull/44, tagged as v1.0.0.

Acknowledgements

These issues were discovered during an audit of a user of this package (dnstt). Thanks to UC Berkley for commissioning the audit, and to David Fifield and Nathan Brown for their collaboration on the fixes. The fixed issues are noted in the audit as:

• UCB-02-003 Potential nonce overflow in Noise protocol
• UCB-02-006 DoS due to unconditional nonce increment