4. Cryptography

When a user needs to interact with Anastasis, the system first derives some key material, but not the master secret, from the user’s identifier using different HKDFs. These HKDFs are salted using the respective escrow provider’s server salt, which ensures that the accounts for the same user cannot be easily correlated across the various Anastasis servers.

Each Anastasis server uses an EdDSA account key to identify the account of the user. The account private key is derived from the user’s identifier using a computationally expensive cryptographic hash function. Using an expensive hash algorithm is assumed to make it infeasible for a weak adversary to determine account keys by brute force (without knowing the user’s identifier). However, it is assumed that a strong adversary performing a targeted attack can compute the account key pair.

The public account key is Crockford base32-encoded in the URI to identify the account, and used to sign requests. These signatures are also provided in base32-encoding and transmitted using the HTTP header Anastasis-Account-Signature.

When confidential data is uploaded to an Anastasis server, the respective payload is encrypted using AES-GCM with a symmetric key and initialization vector derived from the identifier and a high-entropy nonce. The nonce and the GCM tag are prepended to the ciphertext before being uploaded to the Anastasis server. This is done whenever confidential data is stored with the server.

The core secret of the user is (AES) encrypted using a symmetric master key. Recovering this master key requires the user to satisfy a particular policy. Policies specify a set of escrow methods, each of which leads the user to a key share. Combining those key shares (by hashing) allows the user to obtain a policy key, which can be used to decrypt the master key. There can be many policies, satisfying any of these will allow the user to recover the master key. A recovery document contains the encrypted core secret, a set of escrow methods and a set of policies.

4.1. Key derivations

EdDSA and ECDHE public keys are always points on Curve25519 and represented using the standard 256 bit Ed25519 compact format. The binary representation is converted to Crockford Base32 when transmitted inside JSON or as part of URLs.

To start, a user provides their private, unique and unforgettable identifier as a seed to identify their account. For example, this could be a social security number together with their full name. Specifics may depend on the cultural context, in this document we will simply refer to this information as the identifier.

This identifier will be first hashed with Argon2, to provide a kdf_id which will be used to derive other keys later. The Hash must also include the respective server_salt. This also ensures that the kdf_id is different on each server. The use of Argon2 and the respective server_salt is intended to make it difficult to brute-force kdf_id values and help protect the user’s privacy. Also this ensures that the kdf_ids on every server differs. However, we do not assume that the identifier or the kdf_id cannot be determined by an adversary performing a targeted attack, as a user’s identifier is likely to always be known to state actors and may likely also be available to other actors.

kdf_id := Argon2( identifier, server_salt, keysize )

identifier: The secret defined from the user beforehand.

server_salt: The salt from the Server.

keysize: The desired output size of the KDF, here 32 bytes.

4.1.1. Verification

For users to authorize “policy” operations we need an EdDSA key pair. As we cannot assure that the corresponding private key is truly secret, such policy operations must never be destructive: Should an adversary learn the private key, they could access (and with the kdf_id, decrypt) the user’s policy (but not the core secret), or upload a new version of the encrypted recovery document (but not delete an existing version).

For the generation of the private key we use the kdf_id as the entropy source, hash it to derive a base secret which will then be processed to fit the requirements for EdDSA private keys. From the private key we can then generate the corresponding public key. Here, “ver” is used as a salt for the HKDF to ensure that the result differs from other cases where we hash kdf_id.

ver_secret := HKDF(kdf_id, "ver", keysize)
eddsa_priv := ver_secret
eddsa_pub := get_EdDSA_Pub(eddsa_priv)

HKDF(): The HKDF-function uses two phases: First we use HMAC-SHA512 for the extraction phase, then HMAC-SHA256 is used for expansion phase.

kdf_id: Hashed identifier.

key_size: Size of the output, here 32 bytes.

ver_secret: Derived key from the kdf_id, serves as intermediate step for the generation of the private key.

eddsa_priv: The generated EdDSA private key.

eddsa_pub: The generated EdDSA public key.

4.1.2. Encryption

For symmetric encryption of data we use AES256-GCM. For this we need a symmetric key and an initialization vector (IV). To ensure that the symmetric key changes for each encryption operation, we compute the key material using an HKDF over a nonce and the kdf_id.

(iv,key) := HKDF(kdf_id, nonce, keysize + ivsize)

HKDF(): The HKDF-function uses two phases: First we use HMAC-SHA512 for the extraction phase, then HMAC-SHA256 is used for expansion phase.

kdf_id: Hashed identifier.

keysize: Size of the AES symmetric key, here 32 bytes.

ivsize: Size of the AES GCM IV, here 12 bytes.

prekey: Original key material.

nonce: 32-byte nonce, must never match “ver” (which it cannot as the length is different). Of course, we must avoid key reuse. So, we have to use different nonces to get different keys and IVs (see below).

key: Symmetric key which is later used to encrypt the documents with AES256-GCM.

iv: IV which will be used for AES-GCM.

4.2. Key Usage

The keys we have generated are then used to encrypt the recovery document and the key_share of the user.

4.2.1. Encryption

Before every encryption a 32-byte nonce is generated. From this the symmetric key is computed as described above. We use AES256-GCM for the encryption of the recovery document and the key_share. To ensure that the key derivation for the encryption of the recovery document differs fundamentally from that of an individual key share, we use different salts (“erd” and “eks”, respectively).

(iv0, key0) := HKDF(key_id, nonce0, "erd", keysize + ivsize)
(encrypted_recovery_document, aes_gcm_tag) := AES256_GCM(recovery_document, key0, iv0)
(iv_i, key_i) := HKDF(key_id, nonce_i, "eks", [optional data], keysize + ivsize)
(encrypted_key_share_i, aes_gcm_tag_i) := AES256_GCM(key_share_i, key_i, iv_i)

encrypted_recovery_document: The encrypted recovery document which contains the escrow methods, policies and the encrypted core secret.

nonce0: Nonce which is used to generate key0 and iv0 which are used for the encryption of the recovery document. This key derivation must be done using the salt “erd”.

optional data: Key material that optionally is contributed from the authentication method to further obfuscate the key share from the escrow provider.

encrypted_key_share_i: The encrypted key_share which the escrow provider must release upon successful authentication. Here, i must be a positive number used to iterate over the various key shares used for the various escrow methods at the various providers.

nonce_i: Nonce which is used to generate key_i and iv_i which are used for the encryption of the key share. i must be the same number as specified above for encrypted_key_share_i. Key derivation must be done using the salt “eks”.

As a special rule, when a security question is used to authorize access to an encrypted_key_share_i, then the salt “eks” is replaced with an (expensive) hash of the answer to the security question as an additional way to make the key share inaccessible to those who do not have the answer:

powh := POW_HASH (qsalt, answer)
ekss := HKDF("Anastasis-secure-question-uuid-salting",
(iv_i, key_i) := HKDF(key_id, nonce_i, ekss, [optional data], keysize + ivsize)

qsalt: Salt value used to hash answer to satisfy the challenge to prevent the provider from determining the answer via guessing.

answer: Answer to the security question, in UTF-8, as entered by the user.

powh: Result of the (expensive, proof-of-work) hash algorithm.

uuid: UUID of the challenge associated with the security question and the encrypted key share.

ekss: Replacement salt to be used instead of “eks” when deriving the key to encrypt/decrypt the key share.

4.2.2. Signatures

The EdDSA keys are used to sign the data sent from the client to the server. This signature ensures that an adversary that observes the upload is not able to upload a new version of the policy without knowing the user’s identity attributes. The signature is made over a hash of the request body. The following algorithm is equivalent for Anastasis-Policy-Signature.

(anastasis-account-signature) := eddsa_sign(h_body, eddsa_priv)
ver_res := eddsa_verifiy(h_body, anastasis-account-signature, eddsa_pub)

anastasis-account-signature: Signature over the SHA-512 hash of the body using the purpose code TALER_SIGNATURE_ANASTASIS_POLICY_UPLOAD (1400) (see GNUnet EdDSA signature API for the use of purpose).

h_body: The hashed body.

ver_res: A boolean value. True: Signature verification passed, False: Signature verification failed.

4.3. Availability Considerations

Anastasis considers two main threats against availability. First, the Anastasis server operators must be protected against denial-of-service attacks where an adversary attempts to exhaust the operator’s resources. The API protects against these attacks by allowing operators to set fees for all operations. Furthermore, all data stored comes with an expiration logic, so an attacker cannot force servers to store data indefinitely.

A second availability issue arises from strong adversaries that may be able to compute the account keys of some user. While we assume that such an adversary cannot successfully authenticate against the truth, the account key does inherently enable these adversaries to upload a new policy for the account. This cannot be prevented, as the legitimate user must be able to set or change a policy using only the account key. To ensure that an adversary cannot exploit this, policy uploads first of all never delete existing policies, but merely create another version. This way, even if an adversary uploads a malicious policy, a user can still retrieve an older version of the policy to recover access to their data. This append-only storage for policies still leaves a strong adversary with the option of uploading many policies to exhaust the Anastasis server’s capacity. We limit this attack by requiring a policy upload to include a reference to a payment identifier from a payment made by the user. Thus, a policy upload requires both knowledge of the identity and making a payment. This effectively prevents an adversary from using the append-only policy storage from exhausting Anastasis server capacity.