.. This file is part of GNU TALER. Copyright (C) 2019. 2021 Taler Systems SA TALER is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2.1, or (at your option) any later version. TALER is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with TALER; see the file COPYING. If not, see @author Christian Grothoff @author Dominik Meister @author Dennis Neufeld ========= Anastasis ========= Anastasis is a service that allows the user to securely deposit a **core secret** with an open set of escrow providers and recover it if the secret is lost. The **core secret** itself is protected from the escrow providers by encrypting it with a **master key**. The main objective of Anastasis is to ensure that the user can reliably recover the **core secret**, while making this difficult for everyone else. Furthermore, it is assumed that the user is unable to reliably remember any secret with sufficiently high entropy, so we cannot simply encrypt using some other key material in possession of the user. To uniquely identify users, an "unforgettable" **identifier** is used. This identifier should be difficult to guess for anybody but the user. However, the **identifier** is not expected to have sufficient entropy or secrecy to be cryptographically secure. Examples for such identifier would be a concatenation of the full name of the user and their social security or passport number(s). For Swiss citizens, the AHV number could also be used. The adversary model of Anastasis has two types of adversaries: weak adversaries which do not know the user's **identifier**, and strong adversaries which somehow do know a user's **identifier**. For weak adversaries the system guarantees full confidentiality. For strong adversaries, breaking confidentiality additionally requires that Anastasis escrow providers must have colluded. The user is able to specify a set of **policies** which determine which Anastasis escrow providers would need to collude to break confidentiality. These policies also set the bar for the user to recover their core secret. A **recovery document** includes all of the information a user needs to recover access to their core secret. It specifies a set of **escrow methods**, which specify how the user should convince the Anastasis server that they are "real". Escrow methods can for example include SMS-based verification, video identification or a security question. For each escrow method, the Anastasis server is provided with **truth**, that is data the Anastasis operator may learn during the recovery process to authenticate the user. Examples for truth would be a phone number (for SMS), a picture of the user (for video identification), or the (hash of) a security answer. A strong adversary is assumed to be able to learn the truth, while weak adversaries must not. In addition to a set of escrow methods and associated Anastasis server operators, the **recovery document** also specifies **policies**, which describe the combination(s) of the escrow methods that suffice to obtain access to the core secret. For example, a **policy** could say that the escrow methods (A and B) suffice, and a second policy may permit (A and C). A different user may choose to use the policy that (A and B and C) are all required. Anastasis imposes no limit on the number of policies in a **recovery document**, or the set of providers or escrow methods involved in guarding a user's secret. Weak adversaries must not be able to deduce information about a user's **recovery document** (except for its length, which may be exposed to an adversary which monitors the user's network traffic). ------------------- Anastasis DB Schema ------------------- .. image:: anastasis-db.png .. image:: anastasis_challengecode.png .. image:: anastasis_challenge_payment.png .. image:: anastasis_truth.png .. image:: anastasis_truth_payment.png ---------------------- Anastasis 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. 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_id**\ s 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. .. code-block:: none 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. 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**. .. code-block:: none ver_secret := HKDF(kdf_id, "ver", keysize) eddsa_priv := eddsa_d_to_a(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_d_to_a()**: Function which converts the ver_key to a valid EdDSA private key. Specifically, assuming the value eddsa_priv is in a 32-byte array "digest", the function clears and sets certain bits as follows: .. code-block:: none digest[0] = (digest[0] & 0x7f) | 0x40; digest[31] &= 0xf8; **eddsa_priv**: The generated EdDSA private key. **eddsa_pub**: The generated EdDSA public key. 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. .. code-block:: none (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. Key Usage ^^^^^^^^^ The keys we have generated are then used to encrypt the **recovery document** and the **key_share** of the user. 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). .. code-block:: none (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*. Nonce must contain the string "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*. Nonce must contain the string "EKS" plus the according *i*. 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: .. code-block:: none powh = POW_HASH (qsalt, answer) ekss = HKDF("Anastasis-secure-question-uuid-salting", powh, uuid); (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. Signatures ---------- The EdDSA keys are used to sign the data sent from the client to the server. Everything the client sends to server is signed. The following algorithm is equivalent for **Anastasis-Policy-Signature**. .. code-block:: none (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. When requesting policy downloads, the client must also provide a signature: .. code-block:: none (anastasis-account-signature) = eddsa_sign(version, eddsa_priv) ver_res = eddsa_verifiy(version, 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_DOWNLOAD`` (1401) (see GNUnet EdDSA signature API for the use of purpose). **version**: The version requested as a 64-bit integer, 2^64-1 for the "latest version". **ver_res**: A boolean value. True: Signature verification passed, False: Signature verification failed. 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 and adversary from using the append-only policy storage from exhausting Anastasis server capacity. ------------------ Anastasis REST API ------------------ .. _salt: .. _config: Receiving Configuration ^^^^^^^^^^^^^^^^^^^^^^^ .. http:get:: /config Obtain the configuration details of the escrow provider. **Response:** Returns an `EscrowConfigurationResponse`_. .. _EscrowConfigurationResponse: .. ts:def:: EscrowConfigurationResponse interface EscrowConfigurationResponse { // Protocol identifier, clarifies that this is an Anastasis provider. name: "anastasis"; // libtool-style representation of the Exchange protocol version, see // https://www.gnu.org/software/libtool/manual/html_node/Versioning.html#Versioning // The format is "current:revision:age". version: string; // Currency in which this provider processes payments. currency: string; // supported authorization methods methods: AuthorizationMethodConfig[]; // maximum policy upload size supported storage_limit_in_megabytes: number; // Payment required to maintain an account to store policy documents for a year. // Users can pay more, in which case the storage time will go up proportionally. annual_fee: Amount; // Payment required to upload truth. To be paid per upload. truth_upload_fee: Amount; // how long until the service expires deposited truth // (unless refreshed via another POST)? truth_lifetime: RelativeTime; // Limit on the liability that the provider is offering with // respect to the services provided. liability_limit: Amount; // salt value with 128 bits of entropy // Different providers // will use different high-entropy salt values. The resulting // **provider salt** is then used in various operations to ensure // cryptographic operations differ by provider. A provider must // never change its salt value. server_salt: string; } .. _AuthorizationMethodConfig: .. ts:def:: AuthorizationMethodConfig interface AuthorizationMethodConfig { // name of the authorization method type: string; // Fee for accessing key share using this method cost: Amount; } .. _terms: Receiving Terms of Service ^^^^^^^^^^^^^^^^^^^^^^^^^^ .. http:get:: /terms Obtain the terms of service provided by the escrow provider. **Response:** Returns the terms of service of the provider, in the best language and format available based on the client's request. .. http:get:: /privacy Obtain the privacy poliy of the service provided by the escrow provider. **Response:** Returns the privacy policy of the provider, in the best language and format available based on the client's request. .. _manage-policy: Manage policy ^^^^^^^^^^^^^ This API is used by the Anastasis client to deposit or request encrypted recovery documents with the escrow provider. Generally, a client will deposit the same encrypted recovery document with each escrow provider, but provide a different truth to each escrow provider. Operations by the client are identified and authorized by ``$ACCOUNT_PUB``, which should be kept secret from third parties. ``$ACCOUNT_PUB`` should be an account public key using the Crockford base32-encoding. In the following, UUID is always defined and used according to `RFC 4122`_. .. _`RFC 4122`: https://tools.ietf.org/html/rfc4122 .. http:get:: /policy/$ACCOUNT_PUB[?version=$NUMBER] Get the customer's encrypted recovery document. If "version" is not specified, the server returns the latest available version. If "version" is specified, returns the policy with the respective "version". The response must begin with the nonce and an AES-GCM tag and continue with the ciphertext. Once decrypted, the plaintext is expected to contain: * the escrow policy * the separately encrypted master public key Note that the key shares required to decrypt the master public key are not included, as for this the client needs to obtain authorization. The policy does provide sufficient information for the client to determine how to authorize requests for **truth**. The client MAY provide an "If-None-Match" header with an Etag. In that case, the server MUST additionally respond with an "304" status code in case the resource matches the provided Etag. **Response**: :http:statuscode:`200 OK`: The escrow provider responds with an EncryptedRecoveryDocument_ object. :http:statuscode:`304 Not modified`: The client requested the same resource it already knows. :http:statuscode:`400 Bad request`: The $ACCOUNT_PUB is not an EdDSA public key. :http:statuscode:`402 Payment Required`: The account's balance is too low for the specified operation. See the Taler payment protocol specification for how to pay. :http:statuscode:`403 Forbidden`: The required account signature was invalid. :http:statuscode:`404 Not found`: The requested resource was not found. *Anastasis-Version*: $NUMBER --- The server must return actual version of the encrypted recovery document via this header. If the client specified a version number in the header of the request, the server must return that version. If the client did not specify a version in the request, the server returns latest version of the EncryptedRecoveryDocument_. *Etag*: Set by the server to the Base32-encoded SHA512 hash of the body. Used for caching and to prevent redundancies. The server MUST send the Etag if the status code is 200 OK. *If-None-Match*: If this is not the very first request of the client, this contains the Etag-value which the client has received before from the server. The client SHOULD send this header with every request (except for the first request) to avoid unnecessary downloads. *Anastasis-Account-Signature*: The client must provide Base-32 encoded EdDSA signature over hash of body with ``$ACCOUNT_PRIV``, affirming desire to download the requested encrypted recovery document. The purpose used MUST be ``TALER_SIGNATURE_ANASTASIS_POLICY_DOWNLOAD`` (1401). .. http:post:: /policy/$ACCOUNT_PUB Upload a new version of the customer's encrypted recovery document. While the document's structure is described in JSON below, the upload should just be the bytestream of the raw data (i.e. 32-byte nonce followed by 16-byte tag followed by the encrypted document). If the request has been seen before, the server should do nothing, and otherwise store the new version. The body must begin with a nonce, an AES-GCM tag and continue with the ciphertext. The format is the same as specified for the response of the GET method. The Anastasis server cannot fully validate the format, but MAY impose minimum and maximum size limits. **Request**: :query pay: Optional argument, any non-empty value will do, suggested is ``y`` for ``yes``. The client insists on making a payment for the respective account, even if this is not yet required. The server will respond with a ``402 Payment required``, but only if the rest of the request is well-formed (account signature must match). Clients that do not actually intend to make a new upload but that only want to pay may attempt to upload the latest backup again, as this option will be checked before the ``304 Not modified`` case. :query timeout_ms=NUMBER: *Optional.* If specified, the Anastasis server will wait up to ``timeout_ms`` milliseconds for completion of the payment before sending the HTTP response. A client must never rely on this behavior, as the backend may return a response immediately. *If-None-Match*: This header MUST be present and set to the SHA512 hash (Etag) of the body by the client. The client SHOULD also set the "Expect: 100-Continue" header and wait for "100 continue" before uploading the body. The server MUST use the Etag to check whether it already knows the encrypted recovery document that is about to be uploaded. The server MUST refuse the upload with a "304" status code if the Etag matches the latest version already known to the server. *Anastasis-Policy-Signature*: The client must provide Base-32 encoded EdDSA signature over hash of body with ``$ACCOUNT_PRIV``, affirming desire to upload an encrypted recovery document. *Payment-Identifier*: Base-32 encoded 32-byte payment identifier that was included in a previous payment (see 402 status code). Used to allow the server to check that the client paid for the upload (to protect the server against DoS attacks) and that the client knows a real secret of financial value (as the **kdf_id** might be known to an attacker). If this header is missing in the client's request (or the associated payment has exceeded the upload limit), the server must return a 402 response. When making payments, the server must include a fresh, randomly-generated payment-identifier in the payment request. **Response**: :http:statuscode:`204 No content`: The encrypted recovery document was accepted and stored. "Anastasis-Version" and "Anastasis-UUID" headers indicate what version and UUID was assigned to this encrypted recovery document upload by the server. :http:statuscode:`304 Not modified`: The same encrypted recovery document was previously accepted and stored. "Anastasis-Version" header indicates what version was previously assigned to this encrypted recovery document. :http:statuscode:`400 Bad request`: The ``$ACCOUNT_PUB`` is not an EdDSA public key or mandatory headers are missing. The response body MUST elaborate on the error using a Taler error code in the typical JSON encoding. :http:statuscode:`402 Payment required`: The account's balance is too low for the specified operation. See the Taler payment protocol specification for how to pay. The response body MAY provide alternative means for payment. :http:statuscode:`403 Forbidden`: The required account signature was invalid. The response body may elaborate on the error. :http:statuscode:`413 Request entity too large`: The upload is too large *or* too small. The response body may elaborate on the error. **Details:** .. _EncryptedRecoveryDocument: .. ts:def:: EncryptedRecoveryDocument interface EncryptedRecoveryDocument { // Nonce used to compute the (iv,key) pair for encryption of the // encrypted_compressed_recovery_document. nonce: [32]; //bytearray // Authentication tag aes_gcm_tag: [16]; //bytearray // Variable-size encrypted recovery document. After decryption, // this contains a gzip compressed JSON-encoded `RecoveryDocument`. // The nonce of the HKDF for this encryption must include the // string "ERD". encrypted_compressed_recovery_document: []; //bytearray of undefined length } .. _RecoveryDocument: .. ts:def:: RecoveryDocument interface RecoveryDocument { // Account identifier at backup provider, AES-encrypted with // the (symmetric) master_key, i.e. an URL // https://sync.taler.net/$BACKUP_ID and // a private key to decrypt the backup. Anastasis is oblivious // to the details of how this is ultimately encoded. backup_account: []; //bytearray of undefined length // List of escrow providers and selected authentication method methods: EscrowMethod[]; // List of possible decryption policies policy: DecryptionPolicy[]; } .. _EscrowMethod: .. ts:def:: EscrowMethod interface EscrowMethod { // URL of the escrow provider (including possibly this Anastasis server) provider_url : string; // Type of the escrow method (e.g. security question, SMS etc.) escrow_type: string; // UUID of the escrow method (see /truth/ API below). uuid: string; // Key used to encrypt the `Truth` this `EscrowMethod` is related to. // Client has to provide this key to the server when using /truth/ truth_encryption_key: [32]; //bytearray // Salt used to encrypt the truth on the Anastasis server. truth_salt: [32]; //bytearray // The challenge to give to the user (i.e. the security question // if this is challenge-response). // (Q: as string in base32 encoding?) // (Q: what is the mime-type of this value?) // // For some methods, this value may be absent. // // The plaintext challenge is not revealed to the // Anastasis server. challenge: []; //bytearray of undefined length } .. _DecryptionPolicy: .. ts:def:: DecryptionPolicy interface DecryptionPolicy { // Salt included to encrypt master key share when // using this decryption policy. policy_salt: [32]; //bytearray // Master key, AES-encrypted with key derived from // salt and keyshares revealed by the following list of // escrow methods identified by UUID. encrypted_master_key: [32]; //bytearray // List of escrow methods identified by their uuid. uuid: string[]; } .. _Truth: Managing truth ^^^^^^^^^^^^^^ This API is used by the Anastasis client to deposit **truth** or request a (encrypted) **key share** with the escrow provider. An **escrow method** specifies an Anastasis provider and how the user should authorize themself. The **truth** API allows the user to provide the (encrypted) key share to the respective escrow provider, as well as auxiliary data required for such a respective escrow method. An Anastasis-server may store truth for free for a certain time period, or charge per truth operation using GNU Taler. .. http:post:: /truth/$UUID Upload a TruthUploadRequest_-Object according to the policy the client created before (see RecoveryDocument_). If request has been seen before, the server should do nothing, and otherwise store the new object. **Request:** :query timeout_ms=NUMBER: *Optional.* If specified, the Anastasis server will wait up to ``timeout_ms`` milliseconds for completion of the payment before sending the HTTP response. A client must never rely on this behavior, as the backend may return a response immediately. **Response:** :http:statuscode:`204 No content`: Truth stored successfully. :http:statuscode:`304 Not modified`: The same truth was previously accepted and stored under this UUID. The Anastasis server must still update the expiration time for the truth when returning this response code. :http:statuscode:`402 Payment required`: This server requires payment to store truth per item. See the Taler payment protocol specification for how to pay. The response body MAY provide alternative means for payment. :http:statuscode:`409 Conflict`: The server already has some truth stored under this UUID. The client should check that it is generating UUIDs with enough entropy. :http:statuscode:`412 Precondition failed`: The selected authentication method is not supported on this provider. **Details:** .. _TruthUploadRequest: .. ts:def:: TruthUploadRequest interface TruthUploadRequest { // Contains the information of an interface `EncryptedKeyShare`, but simply // as one binary block (in Crockford Base32 encoding for JSON). key_share_data: []; //bytearray // Key share method, i.e. "security question", "SMS", "e-mail", ... type: string; // Nonce used to compute the (iv,key) pair for encryption of the // encrypted_truth. nonce: [32]; //bytearray // Authentication tag of encrypted_truth aes_gcm_tag: [16]; //bytearray // Variable-size truth. After decryption, // this contains the ground truth, i.e. H(challenge answer), // phone number, e-mail address, picture, fingerprint, ... // **base32 encoded**. // // The nonce of the HKDF for this encryption must include the // string "ECT". encrypted_truth: [80]; //bytearray // mime type of truth, i.e. text/ascii, image/jpeg, etc. truth_mime: string; } .. http:get:: /truth/$UUID[?response=$H_RESPONSE] Get the stored encrypted key share. If ``$H_RESPONSE`` is specified by the client, the server checks if ``$H_RESPONSE`` matches the expected response specified before within the TruthUploadRequest_ (see encrypted_truth). Also, the user has to provide the correct *truth_encryption_key* with every get request (see below). When ``$H_RESPONSE`` is correct, the server responds with the encrypted key share. The encrypted key share is returned simply as a byte array and not in JSON format. **Response**: :http:statuscode:`200 OK`: EncryptedKeyShare_ is returned in body (in binary). :http:statuscode:`202 Accepted`: The escrow provider will respond out-of-band (i.e. SMS). The body may contain human-readable instructions on next steps. :http:statuscode:`208 Already Reported`: An authentication challenge was recently send, client should simply respond to the pending challenge. :http:statuscode:`303 See other`: The provider redirects for authentication (i.e. video identification/WebRTC). If the client is not a browser, it should launch a browser at the URL given in the "Location" header and allow the user to re-try the operation after successful authorization. :http:statuscode:`402 Payment required`: The service requires payment for access to truth. See the Taler payment protocol specification for how to pay. The response body MAY provide alternative means for payment. :http:statuscode:`403 Forbidden`: The server requires a valid "response" to the challenge associated with the UUID. :http:statuscode:`404 Not found`: The server does not know any truth under the given UUID. :http:statuscode:`410 Gone`: The server has not (recently) issued a challenge under the given UUID, but a reply was provided. (This does not apply for secure question.) :http:statuscode:`417 Expectation Failed`: The decrypted 'truth' does not match the expectations of the authentication backend, i.e. a phone number for sending an SMS is not a number, or an e-mail address for sending an E-mail is not a valid e-mail address. :http:statuscode:`503 Service Unavailable`: Server is out of Service. *Truth-Decryption-Key*: Key used to encrypt the **truth** (see encrypted_truth within TruthUploadRequest_) and which has to provided by the user. The key is stored with the according EscrowMethod_. The server needs this key to get the info out of TruthUploadRequest_ needed to verify the ``$RESPONSE``. **Details:** .. _EncryptedKeyShare: .. ts:def:: EncryptedKeyShare interface EncryptedKeyShare { // Nonce used to compute the decryption (iv,key) pair. nonce_i: [32]; //bytearray // Authentication tag aes_gcm_tag_i: [16]; //bytearray // Encrypted key-share in base32 encoding. // After decryption, this yields a `KeyShare`. Note that // the `KeyShare` MUST be encoded as a fixed-size binary // block (instead of in JSON encoding). // // HKDF for the key generation must include the // string "eks" as salt. // Depending on the method, // the HKDF may additionally include // bits from the response (i.e. some hash over the // answer to the security question) encrypted_key_share_i: [32]; //bytearray } .. _KeyShare: .. ts:def:: KeyShare interface KeyShare { // Key material to concatenate with policy_salt and KDF to derive // the key to decrypt the master key. key_share: [32]; //bytearray // Signature over method, uuid, and key_share. account_sig: EddsaSignature; } --------------------- Anastasis Reducer API --------------------- This section describes the Anastasis Reducer API which is used by client applications to store or load the different states the client application can have. The reducer takes a state_ in JSON syntax and returns the new state in JSON syntax. For example a **state** may take the following structure: .. code-block:: json { "backup_state": "CONTINENT_SELECTING", "continents": [ "Europe", "North_America" ] } The new state depends on the previous one and on the transition action_ with its arguments given to the reducer. A **transition argument** also is a statement in JSON syntax: .. code-block:: json { "continent": "Europe" } The new state returned by the reducer with the state and transition argument defined above would look like following for the transition action_ "select_continent": .. code-block:: json { "backup_state": "COUNTRY_SELECTING", "continents": [ "Europe", "North_America" ], "selected_continent": "Europe", "countries": [ { "code": "ch", "name": "Switzerland", "continent": "Europe", "name_i18n": { "de_DE": "Schweiz", "de_CH": "Schwiiz", "fr": "Suisse", "en": "Swiss" }, "currency": "CHF" }, { "code": "de", "name": "Germany", "continent": "Europe", "continent_i18n": { "de": "Europa" }, "name_i18n": { "de_DE": "Deutschland", "de_CH": "Deutschland", "fr": "Allemagne", "en": "Germany" }, "currency": "EUR" } ] } States ^^^^^^ Overall, the reducer knows the following states: - **ERROR**: The transition lead to an error. No further transitions are possible from this state, but the client may want to continue from a previous state. - **CONTINENT_SELECTING**: The user should specify the continent where they are living, so that we can show a list of countries to choose from. - **COUNTRY_SELECTING**: The user should specify the country where they are living, so that we can determine appropriate attributes, currencies and Anastasis providers. - **USER_ATTRIBUTES_COLLECTING**: The user should provide the country-specific personal attributes. - **AUTHENTICATIONS_EDITING**: The user should add authentication methods to be used during recovery. - **POLICIES_REVIEWING**: The user should review the recovery policies. - **SECRET_EDITING**: The user should edit the secret to be backed up. - **TRUTHS_PAYING**: The user needs to pay for one or more uploads of data associated with an authentication method. - **POLICIES_PAYING**: The user needs to pay for storing the recovery policy document. - **BACKUP_FINISHED**: A backup has been successfully generated. - **CHALLENGE_SELECTING**: The user needs to select an authorization challenge to proceed with recovery. - **CHALLENGE_PAYING**: The user needs to pay to proceed with the authorization challenge. - **CHALLENGE_SOLVING**: The user needs to solve the authorization challenge. - **RECOVERY_FINISHED**: The secret of the user has been recovered. State names: - In SELECTING-states, the user has to choose one value out of a predefined set of values (for example a continent out of a set of continents). - In COLLECTING-states, the user has to give certain values. - In EDITING-states, the user is free to choose which values he wants to give. - In REVEIWING-states, the user may make a few choices, but primarily is expected to affirm something. - in PAYING-states, the user must make a payment. - in FINISHED-states, the operation has definitively concluded. Backup Reducer ^^^^^^^^^^^^^^ .. _state: .. _action: .. figure:: anastasis_reducer_backup.png :name: fig-anastasis_reducer_backup :alt: fig-anastasis_reducer_backup :scale: 35 % :align: center Backup states and their transitions. The illustration above shows the different states the reducer can have during a backup process. Recovery Reducer ^^^^^^^^^^^^^^^^ .. figure:: anastasis_reducer_recovery.png :name: fig-anastasis_reducer_recovery :alt: fig-anastasis_reducer_recovery :scale: 35 % :align: center Recovery states and their transitions. The illustration above shows the different states the reducer can have during a recovery process. Reducer transitions ^^^^^^^^^^^^^^^^^^^ In the following, the individual transitions will be specified in more detail. Note that we only show fields added by the reducer, typically the previous state is preserved to enable "back" transitions to function smoothly. Initial state ------------- The initial states for backup and recovery processes are: **Initial backup state:** .. code-block:: json { "backup_state": "CONTINENT_SELECTING", "continents": [ "Europe", "North America" ] } **Initial recovery state:** .. code-block:: json { "recovery_state": "CONTINENT_SELECTING", "continents": [ "Europe", "North America" ] } Here, "continents" is an array of English strings with the names of the continents which contain countries for which Anastasis could function (based on having providers that are known to operate and rules being provided for user attributes from those countries). For internationalization, another field "continents_i18n" may be present. This field would be a map of language names to arrays of translated continent names: .. code-block:: json { "recovery_state": "CONTINENT_SELECTING", "continents": [ "Europe", "North America" ] "continents_i18n": { "de_DE" : [ "Europa", "Nordamerika" ], "de_CH" : [ "Europa", "Nordamerika" ] } } Translations must be given in the same order as the main English array. Common transitions ------------------ **select_continent:** Here the user specifies the continent they live on. Arguments (example): .. code-block:: json { "continent": "Europe" } The continent must be given using the English name from the "continents" array. Using a translated continent name is invalid and may result in failure. The reducer returns an updated state with a list of countries to choose from, for example: .. code-block:: json { "backup_state": "COUNTRY_SELECTING", "selected_continent": "Europe", "countries": [ { "code": "ch", "name": "Switzerland", "continent": "Europe", "name_i18n": { "de_DE": "Schweiz", "de_CH": "Schwiiz", "fr": "Suisse", "en": "Swiss" }, "currency": "CHF" }, { "code": "de", "name": "Germany", "continent": "Europe", "continent_i18n": { "de": "Europa" }, "name_i18n": { "de_DE": "Deutschland", "de_CH": "Deutschland", "fr": "Allemagne", "en": "Germany" }, "currency": "EUR" } ] } Here "countries" is an array of countries on the "selected_continent". For each country, the "code" is the ISO 3166-1 alpha-2 country code. The "continent" is only present because some countries span continents, the information is redundant and will always match "selected_continent". The "name" is the name of the country in English, internationalizations of the name may be provided in "name_i18n". The "currency" is **an** official currency of the country, if a country has multiple currencies, it may appear multiple times in the list. In this case, the user should select the entry with the currency they intend to pay with. It is also possible for users to select a currency that does not match their country, but user interfaces should by default try to use currencies that match the user's residence. **select_country:** Selects the country (via the country code) and specifies the currency. The latter is needed as some countries have more than one currency, and some use-cases may also involve users insisting on paying with foreign currency. Arguments (example): .. code-block:: json { "country_code": "de", "currency": "EUR" } The "country_code" must be an ISO 3166-1 alpha-2 country code from the array of "countries" of the reducer's state. The "currency" field must be a valid currency accepted by the Taler payment system. The reducer returns a new state with the list of attributes the user is expected to provide, as well as possible authentication providers that accept payments in the selected currency: .. code-block:: json { "backup_state": "USER_ATTRIBUTES_COLLECTING", "selected_country": "de", "currency": "EUR", "required_attributes": [ { "type": "string", "name": "full_name", "label": "Full name", "label_i18n": { "de_DE": "Vollstaendiger Name", "de_CH": "Vollstaendiger. Name", "fr": "Nom complet", "en": "Full name" }, "widget": "anastasis_gtk_ia_full_name", "uuid" : "9e8f463f-575f-42cb-85f3-759559997331" }, { "type": "date", "name": "birthdate", "label": "Birthdate", "label_i18n": { "de_DE": "Geburtsdatum", "de_CH": "Geburtsdatum", "fr": "Date de naissance", "en": "Birthdate" }, "uuid" : "83d655c7-bdb6-484d-904e-80c1058c8854" "widget": "anastasis_gtk_ia_birthdate" }, { "type": "string", "name": "tax_number", "label": "Taxpayer identification number", "label_i18n":{ "de_DE": "Steuerliche Identifikationsnummer", "de_CH": "Steuerliche Identifikationsnummer", "en": "German taxpayer identification number" }, "widget": "anastasis_gtk_ia_tax_de", "uuid": "dae48f85-e3ff-47a4-a4a3-ed981ed8c3c6", "validation-regex": "^[0-9]{11}$", "validation-logic": "DE_TIN_check" }, { "type": "string", "name": "social_security_number", "label": "Social security number", "label_i18n": { "de_DE": "Sozialversicherungsnummer", "de_CH": "Sozialversicherungsnummer", "fr": "Numéro de sécurité sociale", "en": "Social security number" }, "widget": "anastasis_gtk_ia_ssn", "validation-regex": "^[0-9]{8}[[:upper:]][0-9]{3}$", "validation-logic": "DE_SVN_check" "optional" : true } ], "authentication_providers": { "http://localhost:8089/": { "http_status": 200, "methods": [ { "type" : "question", "usage_fee" : "EUR:0.0" }, { "type" : "sms", "usage_fee" : "EUR:0.5" } ], "annual_fee": "EUR:4.99", "truth_upload_fee": "EUR:4.99", "liability_limit": "EUR:1", "currency": "EUR", "truth_lifetime": { "d_ms" : 50000000 }, "storage_limit_in_megabytes": 1, "provider_name": "Anastasis 4", "salt": "CXAPCKSH9D3MYJTS9536RHJHCW" }, "http://localhost:8088/": { "http_status": 200, "methods": [ { "type" : "question", "usage_fee" : "EUR:0.01" }, { "type" : "sms", "usage_fee" : "EUR:0.55" } ], "annual_fee": "EUR:0.99", "truth_upload_fee": "EUR:3.99", "liability_limit": "EUR:1", "currency": "EUR", "truth_lifetime": { "d_ms" : 50000000 }, "storage_limit_in_megabytes": 1, "provider_name": "Anastasis 4", "salt": "CXAPCKSH9D3MYJTS9536RHJHCW" } } } The array of "required_attributes" contains attributes about the user that must be provided includes: - **type**: the type of the attribute, for now only 'string' and 'date' are supported - **name**: the name of the attribute, this is the key under which the attribute value must be provided later. The name must be unique per response. - **label**: a human-readable description of the attribute in English. Translated descriptions may be provided under **label_i18n**. - **uuid**: a UUID that uniquely identifies identical attributes across different countries. Useful to preserve values should the user enter some attributes, and then switch to another country. Note that attributes must not be preserved if they merely have the same **name**, only the **uuid** will be identical if the semantics is identicial. - **widget**: an optional name of a widget that is known to nicely render the attribute entry in user interfaces where named widgets are supported. - **validation-regex**: an optional extended POSIX regular expression that is to be used to validate (string) inputs to ensure they are well-formed. - **validation-logic**: optional name of a function that should be called to validate the input. If the function is not known to the particular client, the respective validation can be skipped (at the expense of typos by users not being detected, possibly rendering secrets irrecoverable). - **optional**: optional boolean field that, if ``true``, indicates that this attribute is not actually required but optional and users MAY leave it blank in case they do not have the requested information. Used for common fields that apply to some large part of the population but are not sufficiently universal to be actually required. The authentication providers are listed under a key that is the base URL of the service. For each provider, the following information is provided if the provider was successfully contacted: - **http_status**: HTTP status code, always 200 on success. - **methods**: array of authentication methods supported by this provider. Includes the **type** of the authentication method and the **usage_fee** (how much the user must pay for authorization using this method during recovery). - **annual_fee**: fee the provider charges to store the recovery policy for one year. - **truth_upload_fee**: fee the provider charges to store a key share. - **truth_lifetime**: Taler-style relative time that specifies how long the provider will store truth data (key shares) after an upload. - **liability_limit**: amount the provider can be held liable for in case a key share or recovery document cannot be recovered due to provider failures. - **currency**: currency in which the provider wants to be paid, will match all of the fees. - **storage_limit_in_megabytes**: maximum size of an upload (for both recovery document and truth data) in megabytes. - **provider_name**: human-readable name of the provider's business. - **salt**: salt value used by the provider, used to derive the user's identity at this provider. Should be unique per provider, and must never change for a given provider. The salt is base32 encoded. If contacting the provider failed, the information returned is: - **http_status**: HTTP status code (if available, possibly 0 if we did not even obtain an HTTP response). - **error_code**: Taler error code, never 0. **add_provider**: This operation can be performed in state "USER_ATTRIBUTES_COLLECTING". It adds one or more Anastasis providers to the list of providers the reducer should henceforth consider. Note that removing providers is not possible at this time. Here, the client must provide an array with the base URLs of the providers to add, for example: .. code-block:: json { "urls": [ "http://localhost:8888/", "http://localhost:8089/" ] } Note that existing providers will remain in the state. The following is an example for an expected new state where the service on port 8089 is unreachable, the service on port 8088 was previously known, and service on port 8888 was now added: .. code-block:: json { "backup_state": "USER_ATTRIBUTES_COLLECTING", "authentication_providers": { "http://localhost:8089/": { "error_code": 11, "http_status": 0 }, "http://localhost:8088/": { "http_status": 200, "methods": [ { "type" : "question", "usage_fee" : "EUR:0.01" }, { "type" : "sms", "usage_fee" : "EUR:0.55" } ], "annual_fee": "EUR:0.99", "truth_upload_fee": "EUR:3.99", "liability_limit": "EUR:1", "currency": "EUR", "truth_lifetime": { "d_ms" : 50000000 }, "storage_limit_in_megabytes": 1, "provider_name": "Anastasis 4", "salt": "CXAPCKSH9D3MYJTS9536RHJHCW" } "http://localhost:8888/": { "methods": [ { "type" : "question", "usage_fee" : "EUR:0.01" }, { "type" : "sms", "usage_fee" : "EUR:0.55" } ], "annual_fee": "EUR:0.99", "truth_upload_fee": "EUR:3.99", "liability_limit": "EUR:1", "currency": "EUR", "truth_lifetime": { "d_ms" : 50000000 }, "storage_limit_in_megabytes": 1, "provider_name": "Anastasis 42", "salt": "BXAPCKSH9D3MYJTS9536RHJHCX" } } } Backup transitions ------------------ **enter_user_attributes:** This transition provides the user's personal attributes. The specific set of attributes required depends on the country of residence of the user. Some attributes may be optional, in which case they should be omitted entirely (that is, not simply be set to ``null`` or an empty string). Example arguments would be: .. code-block:: json { "identity_attributes": { "full_name": "Max Musterman", "social_security_number": "123456789", "birthdate": "2000-01-01", "birthplace": "Earth" } } Note that at this stage, the state machines between backup and recovery diverge and the ``recovery_state`` will begin to look very different from the ``backup_state``. For backups, if all required attributes are present, the reducer will transition to an ``AUTHENTICATIONS_EDITING`` state with the attributes added to it: .. code-block:: json { "backup_state": "AUTHENTICATIONS_EDITING", "identity_attributes": { "full_name": "Max Musterman", "social_security_number": "123456789", "birthdate": "2000-01-01", "birthplace": "Earth" } } If required attributes are missing, do not match the required regular expression or fail the custom validation logic, the reducer SHOULD transition to an error state indicating what was wrong about the input. A reducer that does not support some specific validation logic MAY accept the invalid input and proceed anyway. The error state will include a Taler error code that is specific to the failure, and optional details. Example: .. code-block:: json { "backup_state": "ERROR", "code": 8404, "hint": "An input did not match the regular expression.", "detail": "social_security_number" } Clients may safely repeat this transition to validate the user's inputs until they statisfy all of the constraints. This way, the user interface does not have to perform the input validation directly. **add_authentication**: This transition adds an authentication method. The method must be supported by one or more providers that are included in the current state. Adding an authentication method requires specifying the ``type`` and ``instructions`` to be given to the user. The ``challenge`` is encrypted and stored at the Anastasis provider. The specific semantics of the value depend on the ``type``. Typical challenges values are a phone number (to send an SMS to), an e-mail address (to send a PIN code to) or the answer to a security question. Note that these challenge values will still be encrypted (and possibly hashed) before being given to the Anastasis providers. Note that the ``challenge`` must be given in Crockford Base32 encoding, as it MAY include binary data (such as a photograph of the user). In the latter case, the optional ``mime_type`` field must be provided to give the MIME type of the value encoded in ``challenge``. .. code-block:: json { "authentication_method": { "type": "question", "mime_type" : "text/plain", "instructions" : "What is your favorite GNU package?", "challenge" : "E1QPPS8A", } } If the information provided is valid, the reducer will add the new authentication method to the array of authentication methods: .. code-block:: json { "backup_state": "AUTHENTICATIONS_EDITING", "authentication_methods": [ { "type": "question", "mime_type" : "text/plain", "instructions" : "What is your favorite GNU package?", "challenge" : "E1QPPS8A", }, { "type": "email", "instructions" : "E-mail to user@*le.com", "challenge": "ENSPAWJ0CNW62VBGDHJJWRVFDM50" } ] } **delete_authentication**: This transition can be used to remove an authentication method from the array of authentication methods. It simply requires the index of the authentication method to remove. Note that the array is 0-indexed: .. code-block:: json { "auth_method_index": 1 } Assuming we begin with the state from the example above, this would remove the ``email`` authentication method, resulting in the following response: .. code-block:: json { "backup_state": "AUTHENTICATIONS_EDITING", "authentication_methods": [ { "type": "question", "mime_type" : "text/plain", "instructions" : "What is your favorite GNU package?", "challenge" : "gdb", } ] } If the index is invalid, the reducer will instead transition into an ERROR state. **next** (from "AUTHENTICATIONS_EDITING"): This transition confirms that the user has finished adding (or removing) authentication methods, and that the system should now automatically compute a set of reasonable recovery policies. This transition does not take any mandatory arguments. Optional arguments can be provided to upload the recovery document only to a specific subset of the providers: .. code-block:: json { "provider_id": [ "http://localhost:8088/", "http://localhost:8089/" ] } ..note:: This is currently not supported (#6760), the current implementation of the reducer ignores this optional argument. The resulting state provides the suggested recovery policies in a way suitable for presentation to the user: .. code-block:: javascript { "backup_state": "POLICIES_REVIEWING", "policies": [ { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8088/" }, { "authentication_method": 1, "provider": "http://localhost:8089/" }, { "authentication_method": 2, "provider": "http://localhost:8087/" } ] }, { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8088/" }, { "authentication_method": 1, "provider": "http://localhost:8089/" }, { "authentication_method": 3, "provider": "http://localhost:8089/" } ] } ] } For each recovery policy, the state includes the specific details of which authentication ``methods`` must be solved to recovery the secret using this policy. The ``methods`` array specifies the index of the ``authentication_method`` in the ``authentication_methods`` array, as well as the provider that was selected to supervise this authentication. If no authentication method was provided, the reducer will transition into an ERROR state instead of suggesting policies. **add_policy**: Using this transition, the user can add an additional recovery policy to the state. The argument format is the same that is used in the existing state. An example for a possible argument would thus be: .. code-block:: javascript { "policy": [ { "auth_method_index": 1, "provider": "http://localhost:8088/" }, { "auth_method_index": 3, "provider": "http://localhost:8089/" } ] } Note that the specified providers must already be in the ``authentication_providers`` of the state. You cannot add new providers at this stage. The reducer will simply attempt to append the suggested policy to the "policies" array, returning an updated state: .. code-block:: json { "backup_state": "POLICIES_REVIEWING", "policies": [ { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8089/" }, { "authentication_method": 1, "provider": "http://localhost:8088/" } ] }, { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8089/" }, { "authentication_method": 2, "provider": "http://localhost:8088/" } ] }, { "methods": [ { "authentication_method": 1, "provider": "http://localhost:8089/" }, { "authentication_method": 2, "provider": "http://localhost:8088/" } ] }, { "methods": [ { "authentication_method": 1, "provider": "http://localhost:8088/" }, { "authentication_method": 3, "provider": "http://localhost:8089/" } ] } ] } If the new policy is invalid, for example because it add an unknown authentication method or the selected provider does not support the type of authentication, the reducer will transition into an ERROR state instead of adding the new policy. **delete_policy:** This transition allows the deletion of a recovery policy. The argument simply specifies the index of the policy to delete, for example: .. code-block:: json { "policy_index": 3 } Given as input the state from the example above, the expected new state would be: .. code-block:: json { "backup_state": "POLICIES_REVIEWING", "policies": [ { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8089/" }, { "authentication_method": 1, "provider": "http://localhost:8088/" } ] }, { "methods": [ { "authentication_method": 0, "provider": "http://localhost:8089/" }, { "authentication_method": 2, "provider": "http://localhost:8088/" } ] }, { "methods": [ { "authentication_method": 1, "provider": "http://localhost:8089/" }, { "authentication_method": 2, "provider": "http://localhost:8088/" } ] } ] } If the index given is invalid, the reducer will transition into an ERROR state instead of deleting a policy. **next** (from "POLICIES_REVIEWING"): Using this transition, the user confirms that the policies in the current state are acceptable. The transition does not take any arguments. The reducer will simply transition to the ``SECRET_EDITING`` state: .. code-block:: json { "backup_state": "SECRET_EDITING", } If the array of ``policies`` is currently empty, the reducer will transition into an ERROR state instead of allowing the user to continue. **enter_secret:** This transition provides the reducer with the actual core secret of the user that Anastasis is supposed to backup (and possibly recover). The argument is simply the Crockford-Base32 encoded ``secret``, for example: .. code-block:: javascript { "secret": "EDJP6WK5EG50" } Applications SHOULD prefix the encoded ``secret`` with some data that allows applications to recognize the application and the version of the secret. The suggested notation is ``$APPLICATION/$VERSION:$SECRET``, for example ``taler-wallet/42:KEYMATERIAL``. In case the secret is provided from a file and the application is oblivious to the details, the same syntax can be used to prefix the secret with a MIME type, for example ``image/png:IMAGE``. If the application is unaware of the format, it should use ``*/*:DATA``. ..note:: The prefixing of the secret with the mime-type or application version is currently not implemented (#6825). After adding a secret, the reducer may transition into different states depending on whether payment(s) are necessary. If payments are needed, the ``secret`` will be stored in the state under ``core_secret``. Applications should be careful when persisting the resulting state, as the ``core_secret`` is not protected in the ``PAYING`` states. The ``PAYING`` states only differ in terms of what the payments are for (key shares or the recovery document), in all cases the state simply includes an array of Taler URIs that refer to payments that need to be made with the Taler wallet. If all payments are complete, the reducer will transition into the ``BACKUP_FINISHED`` state and (if applicable) delete the ``core_secret`` as an additional safety measure. Example results are thus: .. code-block:: json { "backup_state": "TRUTHS_PAYING", "core_secret" : "DATA", "payments": [ "taler://pay/...", "taler://pay/..." ] } .. code-block:: json { "backup_state": "POLICIES_PAYING", "core_secret" : "DATA", "payments": [ "taler://pay/...", "taler://pay/..." ] } .. code-block:: json { "backup_state": "BACKUP_FINISHED", } **pay:** This transition suggests to the reducer that a payment may have been made or is immanent, and that the reducer should check with the Anastasis service provider to see if the operation is now possible. The operation takes one optional argument, which is a ``timeout`` value that specifies how long the reducer may wait (in long polling) for the payment to complete: .. code-block:: json { "timeout": { "d_ms" : 5000 }, } The specified timeout is passed on to the Anastasis service provider(s), which will wait this long before giving up. If no timeout is given, the check is done as quickly as possible without additional delays. The reducer will continue to either an updated state with the remaining payment requests, to the ``BACKUP_FINISHED`` state (if all payments have been completed and the backup finished), or into an ``ERROR`` state in case there was an irrecoverable error, indicating the specific provider and how it failed. An example for this final error state would be: .. code-block:: json { "backup_state": "ERROR", "http_status" : 500, "upload_status" : 52, "provider_url" : "https://bad.example.com/", } Here, the fields have the following meaning: - **http_status** is the HTTP status returned by the Anastasis provider - **upload_status** is the Taler error code return by the provider - **provider_url** is the base URL of the failing provider In the above example, 52 would thus imply that the Anastasis provider failed to store information into its database. Recovery transitions -------------------- **enter_user_attributes:** This transition provides the user's personal attributes. The specific set of attributes required depends on the country of residence of the user. Some attributes may be optional, in which case they should be omitted entirely (that is, not simply be set to ``null`` or an empty string). The arguments are identical to the **enter_user_attributes__ transition from the backup process. Example arguments would thus be: .. code-block:: json { "identity_attributes": { "full_name": "Max Musterman", "social_security_number": "123456789", "birthdate": "2000-01-01", "birthplace": "Earth" } } However, in contrast to the backup process, the reducer will attempt to retrieve the latest recovery document from all known providers for the selected currency given the above inputs. If a recovery document was found by any provider, the reducer will attempt to load it and transition to a state where the user can choose which challenges to satisfy: .. code-block:: json { "recovery_state": "CHALLENGE_SELECTING", "recovery_information": { "challenges": [ { "uuid": "MW2R3RCBZPHNC78AW8AKWRCHF9KV3Y82EN62T831ZP54S3K5599G", "cost": "TESTKUDOS:0", "type": "question", "instructions": "q1" }, { "uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "cost": "TESTKUDOS:0", "type": "email", "instructions": "e-mail address m?il@f*.bar" } ], "policies": [ [ { "uuid": "MW2R3RCBZPHNC78AW8AKWRCHF9KV3Y82EN62T831ZP54S3K5599G" }, { "uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0" } ] ], "provider_url": "http://localhost:8088/", "version": 1 }, "recovery_document": { ... } } The ``recovery_document`` is an internal representation of the recovery information and of no concern to the user interface. The pertinent information is in the ``recovery_information``. Here, the ``challenges`` array is a list of possible challenges the user could attempt to solve next, while ``policies`` is an array of policies, with each policy being an array of challenges. Satisfying all of the challenges of one of the policies will enable the secret to be recovered. The ``provider_url`` from where the recovery document was obtained and its ``version`` are also provided. Each challenge comes with four mandatory fields: - **uuid**: a unique identifier of the challenge; this is what the UUIDs in the policies array refer to, but also this UUID may be included in messages sent to the user. They allow the user to distinguish different PIN/TANs should say the same phone number be used for SMS-authentication with different providers. - **cost**: this is the amount the Anastasis provider will charge to allow the user to pass the challenge. - **type**: this is the type of the challenge, as a string. - **instructions**: contains additional important hints for the user to allow the user to satisfy the challenge. It typically includes an abbreviated form of the contact information or the security question. Details depend on ``type``. If a recovery document was not found, either the user never performed a backup, entered incorrect attributes or used a provider not yet in the list of Anastasis providers. Hence, the user must now either select a different provider, or go ``back`` and update the identity attributes. In the case a recovery document was not found, the transition fails, returning the error code and a human-readable error message together with a transition failure: .. code-block:: json { "recovery_state": "ERROR", "error_message": "account unknown to Anastasis server", "error_code": 9, } Here, the ``error_code`` is from the ``enum ANASTASIS_RecoveryStatus`` and describes precisely what failed about the download, while the ``error_message`` is a human-readable (English) explanation of the code. However, in general it should be sufficient to display the slightly more generic Taler error code that is returned with the new state. **change_version:** Even if a recovery document was found, it is possible that the user intended to recover a different version, or recover a backup where the recovery document is stored at a different provider. Thus, the reducer allows the user to explicitly switch to a different provider or recovery document version using the ``change_version`` transition, which takes a provider URL and policy version as arguments: .. code-block:: json { "provider_url": "https://localhost:8080/", "version": 2 } Note that using a version of 0 implies fetching ``the latest version``. The resulting states are the same as those of the ``enter_user_attributes`` transition, except that the recovery document version is not necessarily the latest available version at the provider. **select_challenge:** Selecting a challenge takes different, depending on the state of the payment. A comprehensive example for ``select_challenge`` would be: .. code-block:: json { "uuid": "80H646H5ZBR453C02Y5RT55VQSJZGM5REWFXVY0SWXY1TNE8CT30" "timeout" : { "d_ms" : 5000 }, "payment_secret": "3P4561HAMHRRYEYD6CM6J7TS5VTD5SR2K2EXJDZEFSX92XKHR4KG" } The ``uuid`` field is mandatory and specifies the selected challenge. The other fields are optional, and are needed in case the user has previously been requested to pay for the challenge. In this case, the ``payment_secret`` identifies the previous payment request, and ``timeout`` says how long the Anastasis service should wait for the payment to be completed before giving up (long polling). Depending on the type of the challenge and the need for payment, the reducer may transition into ``CHALLENGE_SOLVING`` or ``CHALLENGE_PAYING`` states. In ``CHALLENGE_SOLVING``, the new state will primarily specify the selected challenge: .. code-block:: json { "backup_state": "CHALLENGE_SOLVING", "selected_challenge_uuid": "80H646H5ZBR453C02Y5RT55VQSJZGM5REWFXVY0SWXY1TNE8CT30" } In ``CHALLENGE_PAYING``, the new state will include instructions for payment in the ``challenge_feedback``. In general, ``challenge_feedback`` includes information about attempted challenges, with the final state being ``solved``: .. code-block:: json { "recovery_state": "CHALLENGE_SELECTING", "recovery_information": { // ... } "challenge_feedback": { "80H646H5ZBR453C02Y5RT55VQSJZGM5REWFXVY0SWXY1TNE8CT30" : { "state" : "solved" } } } Challenges feedback for a challenge can have many different ``state`` values that applications must all handle. States other than ``solved`` are: - **payment**: Here, the user must pay for a challenge. An example would be: .. code-block:: json { "backup_state": "CHALLENGE_PAYING", "selected_challenge_uuid": "80H646H5ZBR453C02Y5RT55VQSJZGM5REWFXVY0SWXY1TNE8CT30", "challenge_feedback": { "80H646H5ZBR453C02Y5RT55VQSJZGM5REWFXVY0SWXY1TNE8CT30" : { "state" : "payment", "taler_pay_uri" : "taler://pay/...", "provider" : "https://localhost:8080/", "payment_secret" : "3P4561HAMHRRYEYD6CM6J7TS5VTD5SR2K2EXJDZEFSX92XKHR4KG" } } } - **instructions**: Here, the server provided human-readable instructions for how to solve the challenge. Note that the ``instructions`` provided this time are from the Anastasis provider and may differ from the ``instructions`` for the challenge under ``recovery_information``: .. code-block:: json { "recovery_state": "CHALLENGE_SOLVING", "recovery_information": { // ... } "selected_challenge_uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "challenge_feedback": { "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0": { "state": "instructions", "instructions": "Recovery TAN send to email mail@DOMAIN", "http_status": 403 } } } - **redirect**: To solve the challenge, the user must visit the indicated Web site at ``redirect_url``, for example to perform video authentication: { "recovery_state": "CHALLENGE_SOLVING", "recovery_information": { // ... } "selected_challenge_uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "challenge_feedback": { "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0": { "state": "redirect", "redirect_url": "https://videoconf.example.com/", "http_status": 303 } } } - **server-failure**: This indicates that the Anastasis provider encountered a failure and recovery using this challenge cannot proceed at this time. Examples for failures might be that the provider is unable to send SMS messages at this time due to an outage. The body includes details about the failure. The user may try again later or continue with other challenges. { "recovery_state": "CHALLENGE_SELECTING", "recovery_information": { // ... } "selected_challenge_uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "challenge_feedback": { "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0": { "state": "server-failure", "http_status": "500", "error_code": 52 } } } - **truth-unknown**: This indicates that the Anastasis provider is unaware of the specified challenge. This is typically a permanent failure, and user interfaces should not allow users to re-try this challenge. { "recovery_state": "CHALLENGE_SELECTING", "recovery_information": { // ... } "selected_challenge_uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "challenge_feedback": { "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0": { "state": "truth-unknown", "error_code": 8108 } } } - **rate-limit-exceeded**: { "recovery_state": "CHALLENGE_SELECTING", "recovery_information": { // ... } "selected_challenge_uuid": "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0", "challenge_feedback": { "TXYKGE1SJZHJ4M2FKSV1P2RZVNTHZFB9E3A79QE956D3SCAWXPK0": { "state": "rate-limit-exceeded", "error_code": 8121 } } } **pay:** With a ``pay`` transition, the application indicates to the reducer that a payment may have been made. Here, it is again possible to specify an optional ``timeout`` argument for long-polling, for example: .. code-block:: json { "payment_secret": "ABCDADF242525AABASD52525235ABABFDABABANALASDAAKASDAS" "timeout" : { "d_ms" : 5000 }, } Depending on the type of the challenge and the result of the operation, the new state may be ``CHALLENGE_SOLVING`` (if say the SMS was now sent to the user), ``CHALLENGE_SELECTING`` (if the answer to the security question was correct), ``RECOVERY_FINISHED`` (if this was the last challenge that needed to be solved) or still ``CHALLENGE_PAYING`` (if the challenge was not actually paid for). For sample messages, see the different types of ``challenge_feedback`` in the section about ``select_challenge``. **solve_challenge:** Solving a challenge takes various formats, depending on the type of the challenge and what is known about the answer. The different supported formats are: .. code-block:: json { "answer": "answer to security question" } .. code-block:: json { "pin": 1234 } .. code-block:: json { "hash": "SOMEBASE32ENCODEDHASHVALUE" } .. _anastasis-auth-methods: ---------------------- Authentication Methods ---------------------- This section describes the supported authentication methods in detail. We note that the server implements rate limiting for all authentication methods to ensure that malicious strong attackers cannot guess the values by brute-force. Typically, a user is given three attempts per hour to enter the correct code from 2^63 possible values. Transmitted codes also come with an expiration date. If the user re-requests a challenge to be sent, the same challenge may be transmitted (with the three attempts counter not increasing!) for a limited period of time (depending on the authentication method) before the service eventually rotates to a fresh random code with a fresh retry counter. Given the default value range and time intervals (which providers are at liberty to adjust), brute-force attacks against this are expected to succeed with a 50% probability after about 200000 years of attempts at the maximum permissible frequency. SMS (sms) ^^^^^^^^^ Sends an SMS with a code (prefixed with ``A-``) to the user's phone, including a UUID which identifies the challenge the code is for. The user must send this code back with his request (see ``$RESPONSE`` under `Managing truth`_). If the transmitted code is correct, the server responses with the requested encrypted key share. Email verification (email) ^^^^^^^^^^^^^^^^^^^^^^^^^^ Sends an email with a code (prefixed with ``A-``) to the user's mail address, including a UUID which identifies the challenge the code is for. The user must send this code back with his request (see ``$RESPONSE`` under `Managing truth`_). If the transmitted code is correct, the server responses with the requested encrypted key share. Video identification (vid) ^^^^^^^^^^^^^^^^^^^^^^^^^^ Requires the user to identify via video-call. In the video-call, the user is told the code (prefixed with ``A-``) needed to authenticate. The user is expected to delete all metadata revealing information about per from the images before uploading them. Since the respective images must be passed on to the video identification service in the event of password recovery, it should be ensured that no further information about the user can be derived from them. Video identification will typically result in the Anastasis provider requesting the user to be redirected to a Web site (or other URL) for the video-call. Security question (qa) ^^^^^^^^^^^^^^^^^^^^^^ Asks the user a security question. The user sends back a **salted** hash over the answer. The **question-salt** is stored encrypted as part of the recovery document and never revealed to the providers. This ensures that providers cannot derive the answer from the hash value. Furthermore, the security question itself is also only in the recovery document and never given to the Anastasis provider. A moderately expensive hash function is used to further limit strong attackers that have obtained the recovery document from brute-forcing the answer. If the hash value matches with the one the server is expecting, the server answers with the requested encrypted key share. However, unlike other encrypted key shares, the encrypted key share of a security question uses a special variation of the Anastasis encryption: Here, a different hash function over the security answer is used to provide an additional **key-salt** for the decryption of the (encrypted) **key share**. This ensures that the key share remains irrecoverable without the answer even if the Anastasis provider storing the security question is malicious. Snail mail verification (post) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Sends physical mail (snail mail) with a code (prefixed with ``A-``) to the user's mail address, including a UUID which identifies the challenge the code is for. The user must send this code back with their request (see ``$RESPONSE`` under `Managing truth`_). If the transmitted code is correct, the server responds with the requested encrypted key share.