Bitmask Cryptography Details

You asked for encryption details, you get encryption details. Here we try to document all the crypto used by Bitmask, and some of the thinking behind these decisions. For more details, inspect the source or browse our technical documentation.

Authentication – Secure Remote Password

Bitmask uses Secure Remote Password (SRP) to authenticate with a service provider. SRP is a type of zero-knowledge-proof for authentication via username and password that does not give the server a copy of the actual password. Typically, password systems work by sending a cleartext copy of the password to the server, which then hashes this password and saves the hash. With SRP, the client and server negotiate a “password verifier” after several round trips. The server never has access to the cleartext of the password.

One additional benefit of SRP is that both parties authenticate each other. With traditional hashed passwords, the server can say that the password was correct, even if it has no idea what the real password is. With SRP, the user authenticates with the server, but the server also authenticates with the user.

Currently we use 1024-bit discrete-log parameters. We are exploring increasing this to 2048-bit.

There are some limitations with SRP. A compromised or nefarious provider can attempt to brute force crack a password by trying millions of combinations, just like with normal hashed passwords. For this reason, it is still important to pick a strong password. In practice, however, users are horrible at picking strong passwords.

A second limitation is with the web application. It also uses SRP, but the SRP javascript code is loaded from the provider. If the provider is compromised or nefarious, they could load some javascript to capture the user’s password.

We have three plans for the future to overcome these potential problems:

1. Allow the use of an additional long random key that is required as part of the authentication process (optionally). For example, each device a user has Bitmask installed on could have a “device key” and the user would need to authorize these device keys before they could run Bitmask on that new device.
2. We also plan to include with Bitmask a bloom filter of the top 10,000 most commonly used passwords. By some accounts, 98.8% of all users pick a password in the top 10,000. A bloom filter of these passwords is relatively small, and we can simply forbid the user from selecting any of these (albeit with some false positives).
3. Allow providers to forbid authentication via the web application. Authentication would happen via the Bitmask app, which would then load the website with the session token it obtained. This way, the critical SRP authentication code is never loaded from the provider.

For more information, see:

• srp.stanford.edu
• en.wikipedia.org/wiki/Secure_Remote_Password_protocol
• xato.net/passwords/more-top-worst-passwords

Transport – TLS

The Bitmask client frequently makes various connections using TLS to the provider. For example, to check to see if there is an update to the list of VPN gateways.

When a service provider is first added by Bitmask, the CA certificate from the provider is downloaded via a normal TLS connection authenticated using existing x.509 CA system. This is the only moment that Bitmask relies on the CA system.

All subsequent connections with that provider use the provider-specific CA to authenticate the TLS connection. Essentially, this is a form of certificate pinning and TOFU. In order for an outside attacker to impersonate a provider, they would need to present a false x.509 server certificate authenticated by a Certificate Authority, and then intercept and rewrite all subsequent traffic between the Bitmask client and provider.

If a provider has been pre-seeded with the Bitmask application, then the fingerprint of the provider-specific CA certificate is known in advance. In these cases, the x.509 CA system is never relied upon.

The provider-specific CA certificates use 4096 bit RSA with SHA256 digest, by default. The server certificates use 4096 bit RSA with SHA256 digest, by default. These defaults are easily changed.

All TLS connections use PFS ciphers.

Encrypted Tunnel – OpenVPN

OpenVPN has three settings that control what ciphers it uses (there is a fourth, --tls-auth, but we cannot use this in a public multi-user environment). Every provider can easily choose whatever options they want for these. Below are the current defaults that come with the leap_platform.

tls-cipher

The --tls-cipher option governs the session authentication process of OpenVPN. If this is compromised, you could be communicating with a MiTM attacker. The TLS part of OpenVPN authenticates the server and client with each other, and negotiates the random material used in the packet authentication digest and the packet encryption.

Instead of allowing many options, Bitmask only supports a single cipher (to prevent rollback attacks).

For the moment, we have chosen DHE-RSA-AES128-SHA. The most important thing is to choose a cipher that supports PFS, as all the DHE ciphers do.

We have chosen AES-128 because there are known weaknesses with the AES-192 and AES-256 key schedules. There is no known weakness to brute force attacks against full 14 round AES-256, but weakness of AES-256 using other round counts is sufficient to recommend AES-128 over AES-256 generally. For more information, see Bruce Scheier’s post Another New AES Attack.

We would prefer to use ECC over RSA, and plan to eventually. It is a bit more complicated and involves changes to our TLS code in many places (recompiling openvpn, and changing certificate generation libraries used by sysadmins and the provider API).

The current default for client and server x.509 certificates used by OpenVPN is 2048 bit RSA and 4096 bit RSA (respectively) with SHA256 digest. This is also easily configurable by the provider (to see all the options, run leap inspect provider.json).

auth

The --auth option determines what hashing digest is used to to authenticate each packet of traffic using HMAC.

We have chosen to keep the SHA1 the default digest rather than go with SHA256. If an attacker can break a SHA1 HMAC on each packet in real time, you have bigger problems than your VPN.

cipher

The --cipher option determines how actual traffic packets are encrypted. We have chosen AES-128-CBC.

The OpenVPN default is probably actually better than AES-128, since it’s Blowfish. We have chosen AES-128 because the TLS cipher is already relying on AES-128. We would normally prefer cipher mode OFB over CBC, but the OpenVPN manual says that “CBC is recommended and CFB and OFB should be considered advanced modes”.

obfsproxy

Obfsproxy is optionally used to make VPN traffic not appear as VPN traffic to someone who is monitoring the network. Obfsproxy uses modules called pluggable transports to obfuscate underlying traffic. Different transports may or may not use encryption and have different implementation and choices over encryption schemes.

We have chosen the Scramblesuit pluggable transport that uses Uniform Diffie-Hellman for the initial handshake and AES-CTR 256 for application data.

Other

OpenSSH

By default, all servers use RSA key host keys instead of ECDSA. If a host has a ECDSA key, the platform will prompt the sysadmin to switch to RSA. In the future, when Curve255219 is better supported, the platform will encourage switching to 25519.