Aegis persists the user's tokens to a file. This file is referred to as the vault. Users can configure the app to store the vault in plain text or to encrypt it with a password.
This document describes Aegis' security design and file format. It's split up into two parts. First, the cryptographic primitives and use of them for encryption are discussed. The second section documents the details of the file format of the vault.
Two cryptographic primitives were selected for use in Aegis. An Authenticated Encryption with Associated Data (AEAD) cipher and a Key Derivation Function (KDF).
AES-256 in GCM mode is used as the AEAD cipher to ensure the confidentility, integrity and authenticity of the vault contents.
It requires a unique 96-bit nonce for each invocation with the same key. However, it is not possible to use a monotically increasing counter for this in this case, because a future use case could involve using the vault on multiple devices simultaneously, which would almost certainly result in nonce reuse. This is suboptimal, because 96 bits is not large enough to comfortably generate an unlimited amount of random numbers without getting collisions at some point either. As a repeat of the nonce would have catastrophic consequences for the confidentiality and integrity of the ciphertext, NIST strongly recommends not exceeding 232 invocations when using random nonces with GCM. As such, the security of the Aegis vault also relies on the assumption that this limit is never exceeded. In the case of Aegis, this is a reasonable assumption to make, as it's highly unlikely that a user will ever come close to saving the vault 232 times.
Switching to a nonce misuse-resistant cipher like AES-GCM-SIV or a cipher with a larger (192 bits) nonce like XChaCha-Poly1305 will be explored in the future.
scrypt is used as the KDF to derive a key from a user-provided password, with the following parameters:
| Parameter | Value |
|---|---|
| N | 215 |
| r | 8 |
| p | 1 |
These are the same parameters as Android itself uses to derive a key for full-disk encryption. Because of the memory limitations Android apps have, it's not possible to increase them without running into OOM conditions on most devices.
Argon2 is a more modern KDF that provides an advantage over scrypt because it allows tweaking the memory-hardness parameter and CPU-hardness parameter separately, whereas scrypt ties those together into one cost parameter (N). As many applications have started using Argon2 in production, it seems that it has withstood the test of time. It will be considered as an alternative option to switch to in the future.
When a vault is first created, a random 256-bit key is generated that is used to encrypt the contents with AES in GCM mode. This key is referred to as the master key.
Aegis supports unlocking a vault with multiple different credentials. The main credential is a key derived from a user-provided password. In addition to that, users can also add a key backed by the Android KeyStore (authorized by biometrics) as a credential.
Each credential that should be able to encrypt/decrypt the contents of a vault has its own slot. Every slot contains a copy of the master key that is encrypted with its credential. The process of encrypting a key with another key is known as key wrapping. This allows obtaining the master key by providing any of the credentials. An important consequence is that the master key is only as secure as the weakest credential.
This design is similar to and largely inspired by LUKS' key slot system.
Because of the use of an AEAD for encryption, the vault contents and encrypted master keys in the slots are checked for integrity and authenticity. The rest of the file is not.
An attempt was made to create a clear overview of the encryption system.
The vault is stored in JSON and encoded in UTF-8. The upper-level structure is shown below:
{
"version": 1,
"header": {},
"db": {}
}It starts with a version number and a header. If a backwards
incompatible change is introduced to the content format, the version number will
be incremented. The vault contents are stored under db. Its value depends on
wheter the vault is encrypted or not. If it is, the value is a string containing
the Base64 encoded (with padding) ciphertext of the vault contents. Otherwise,
the value is a JSON object.
Full examples of a plain text vault and an encrypted vault are available in the testdata folder. There's also a Python script that can decrypt an Aegis vault given the password: scripts/decrypt.py.
The header starts with the list of slots. It also has a params object
that holds the nonce and tag that were produced during encryption,
encoded as a hexadecimal string.
Setting slots and params to null indicates that the vault is not
encrypted and Aegis will try to parse it as such.
{
"slots": [],
"params": {
"nonce": "0123456789abcdef01234567",
"tag": "0123456789abcdef0123456789abcdef"
}
}The different slot types are identified with a numerical ID.
| Type | ID |
|---|---|
| Raw | 0x00 |
| Biometric | 0x01 |
| Password | 0x02 |
This slot type is used for raw AES key credentials. It is not used directly in the app, but all other slots are based on this slot type, so this section applies to all of them.
Each slot transforms its credential in a way that it can be used to encrypt the
master key with AES-256 in GCM mode. The nonce, tag and encrypted
key are encoded as a hexadecimal string and stored together. Slots also have
a unique randomly generated UUID (version 4).
{
"type": 0,
"uuid": "01234567-89ab-cdef-0123-456789abcdef",
"key": "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
"key_params": {
"nonce": "0123456789abcdef01234567",
"tag": "0123456789abcdef0123456789abcdef"
}
}The structure of the Biometric slot is exactly the same as the Raw slot. The difference is that the wrapper key is backed by the Android KeyStore, whereas Raw slots don't imply use of a particular storage type.
As noted earlier, scrypt is used to derive a 256-bit key from a user-provided
password. A random 256-bit salt is generated and passed to scrypt to protect
against rainbow table attacks. Its stored along with the N, r and p
parameters.
{
"type": 1,
"uuid": "01234567-89ab-cdef-0123-456789abcdef",
"key": "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef",
"key_params": {
"nonce": "0123456789abcdef01234567",
"tag": "0123456789abcdef0123456789abcdef"
},
"n": 32768,
"r": 8,
"p": 1,
"salt": "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
}The content is a JSON object encoded in UTF-8.
{
"version": 1,
"entries": []
}It has a version number and a list of entries. If a backwards
incompatible change is introduced to the content format, the version number will
be incremented.
Each entry has a unique randomly generated UUID (version 4), as well as a
name and issuer to idenfity the account name and service that the token
is for. Entries can also have an icon. These are JPEG's encoded in Base64 with
padding. The info object holds information specific to the OTP type. The
secret is encoded in Base32 without padding.
There are a number of supported types:
| Type | ID |
|---|---|
| TOTP | "totp" |
| HOTP | "hotp" |
| Steam | "steam" |
There is no specification available for Steam's OTP algorithm. It's essentially the same as TOTP, but it uses a different final encoding step. Aegis's implementation of it can be found in crypto/otp/OTP.java.
The following algorithms are supported for all OTP types:
| Algorithm | ID |
|---|---|
| SHA-1 | "SHA1" |
| SHA-256 | "SHA256" |
| SHA-512 | "SHA512" |
Example of a TOTP entry:
{
"type": "totp",
"uuid": "01234567-89ab-cdef-0123-456789abcdef",
"name": "Bob",
"issuer": "Google",
"icon": null,
"info": {
"secret": "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567",
"algo": "SHA1",
"digits": 6,
"period": 30
}
}