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+..
+ This file is part of GNU TALER.
+ Copyright (C) 2019 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 <http://www.gnu.org/licenses/>
+
+ @author Christian Grothoff
+ @author Dominik Meister
+ @author Dennis Neufeld
+
+=========
+Anastasis
+=========
+
+**Anastasis** is a service that allows the user to securely deposit a master
+secret with an open set of escrow providers and recover it if it is lost. To
+uniquely identify users, an "unforgettable" **identifier** is used. This
+identifier should be difficult to guess for anybody but the user, but 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. Some key material, but not the
+master secret, is then derived from this **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.
+
+The Anastasis service 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 H_1. Using an
+expensive hash algorithm is supposed to make it difficult for an adversary to
+determine account keys by brute force without knowing the user's identifier.
+However, it is assumed that an 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 provided in base32
+encoding using the HTTP header "Anastasis-Account-Signature".
+
+Payloads are encrypted using AES-GCM with a symmetric key and IV derived from
+the identifier and a nonce. The nonce and the GCM tag are then pre-pended to
+the resulting ciphertext and uploaded to the Anastasis server. This is done
+whenever encrypted data is stored with the server.
+
+The **core secret** of the user is encrypted using a symmetric **master key**.
+Recovering the 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 then
+ultimately 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.
+
+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 the respective authorization method.
+
+
+---------------
+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 **user_identifier**.
+
+This user_identifier will be first hashed with SCrypt, 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 SCrypt and the respective server_salt is intended
+to make it difficult to brute-force **kdf_id** values and help protect user's
+privacy. Also this ensures that the kdf_ids on every server differs. However,
+we do not assume that the **user_identifier** or the **kdf_id** cannot be
+determined by an adversary performing a targeted attack, as a user's
+**user_identifier** is likely to always be known to state actors and may
+likely also be available to other actors.
+
+
+.. code-block:: tsref
+
+ kdf_id := SCrypt( user_identifier, server_salt, keysize )
+
+**user_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 policy (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:: tsref
+
+ 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 to phases: First we use HMAC-SHA512 for the extraction phase, then HMAC-SHA256 is used for expansion phase.
+
+**kdf_id**: Hashed user_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:: tsref
+
+ 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:: tsref
+
+ (iv,key) := HKDF(kdf_id, nonce, keysize + ivsize)
+
+**HKDF()**: The HKDF-function uses to phases: First we use HMAC-SHA512 for the extraction phase, then HMAC-SHA256 is used for expansion phase.
+
+**kdf_id**: Hashed user_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).
+
+**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 now 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
+key_share.
+
+.. code-block:: tsref
+
+ (encrypted_recovery_document, aes_gcm_tag) = AES256_GCM(recovery_document, key, iv)
+ (encrypted_key_share, aes_gcm_tag) = AES256_GCM(key_share, key, iv)
+
+**encrypted_recovery_document**: The encrypted RecoveryDocument (recovery_document) which contains the policies.
+
+**encrypted_key_share**: The encrypted KeyShare (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:: tsref
+
+ (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 hash of body.
+
+**h_body**: The hashed body.
+
+**ver_res**: A boolean value. True: Verification passed, False: Verification failed.
+
+
+
+-------------
+Anastasis API
+-------------
+
+.. _salt:
+
+
+Obtain salt
+^^^^^^^^^^^
+
+.. http:get:: /salt
+
+ Obtain the salt used by the escrow provider. 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.
+
+
+ **Response:**
+
+ Returns a `SaltResponse`_.
+
+ .. _SaltResponse:
+ .. _tsref-type-SaltResponse:
+ .. code-block:: tsref
+
+ interface SaltResponse {
+ // salt value, at least 128 bits of entropy
+ server_salt: string;
+ }
+
+.. _terms:
+
+
+Receiving Terms of Service
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. http:get:: /terms
+
+ Obtain the terms of service provided by the escrow provider.
+
+ **Response:**
+
+ Returns a `SyncTermsOfServiceResponse`_.
+
+ .. _SyncTermsOfServiceResponse:
+ .. _tsref-type-SyncTermsOfServiceResponse:
+ .. code-block:: tsref
+
+ interface SyncTermsOfServiceResponse {
+ // maximum key database backup size supported
+ storage_limit_in_megabytes: number;
+
+ // maximum number of sync requests per day (per account)
+ daily_sync_limit: number;
+
+ // minimum supported protocol version
+ min_version: number;
+
+ // maximum supported protocol version
+ max_version: number;
+
+ // supported authentication methods
+ auth_methods: string[];
+
+ // how long the service expire the deposited truth?
+ truth_expiration: relative-time;
+
+ // Fee per transaction.
+ transaction_fee: Amount;
+
+ }
+
+.. _escrow:
+
+
+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
+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. $ACCOUNT_PUB should be an account
+public key using the Crockford base32-encoding.
+
+
+.. http:get:: /policy/$ACCOUNT_PUB[?version=$NUMBER]
+
+ Get the customer's policy and encrypted master key share data. If "version"
+ is not specified, 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-not-modified-since" 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.
+
+ :status 200 OK:
+ The escrow provider responds with an `EncryptedRecoveryDocument`_ object.
+ :status 304 Not modified:
+ The client requested the same ressource he already owns.
+ :status 400 Bad request:
+ The $ACCOUNT_PUB is not an EdDSA public key.
+ :status 402 Payment Required:
+ The account's balance is too low for the specified operation.
+ See the Taler payment protocol specification for how to pay.
+ :status 403 Forbidden:
+ The required account signature was invalid.
+ :status 404 Not Found:
+ The requested resource was not found.
+
+ *Anastasis-Version*: $NUMBER --- The server must return actual version number in header;
+ the client specifies version number in the header of the request (if not specified in request, the server returns latest version of EncryptedRecoveryDocument_ ).
+
+ *Etag*: Etag, hash over the body for caching and to prevent redundancies. If status is 200 OK, the server must send the Etag.
+
+ *If-modified-since*: If the client has previously received an Etag from the server, he has to send it with this request (to avoid unnecessary downloads).
+
+ *If-None-Match*: If this is not the very first request of the client, this contains the Etag-Value which the client has reveived before from the server.
+ The client must send this header with every request (except for the very first request).
+
+ *Anastasis-Account-Signature*: The client must provide Base-32 encoded EdDSA signature over hash of body with $ACCOUNT_PRIV, affirming desire to download the requested encrypted recovery document.
+
+.. http:post:: /policy/$ACCOUNT_PUB
+
+ Upload a new version of the customer's policy and encrypted master key share data.
+ If 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 validate the format, but MAY impose
+ minimum and maximum size limits.
+
+ :status 204 No Content:
+ The policy was accepted and stored. "Anastasis-Version" and "Anastasis-UUID" headers
+ incidate what version and UUID was assigned to this policy upload by the server.
+ :status 304 Not modified:
+ The same encrypted recovery document was previously accepted and stored. "Anastasis-Version" header
+ incidates what version was previously assigned to this encrypted recovery document.
+ :status 400 Bad request:
+ The $ACCOUNT_PUB is not an EdDSA public key. The response body may elaborate on the error.
+ :status 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 SHOULD provide various means for payment.
+ :status 403 Forbidden:
+ The required account signature was invalid. The response body may elaborate on the error.
+ :status 413 Request Entity Too Large:
+ The upload is too large *or* too small. The response body may elaborate on the error.
+
+
+ *Anastasis-Version*: $NUMBER --- The server must return the actual version number it determined.
+ Only generated if the status is 204 or 304.
+
+ *If-not-modified-since*: The client must provide an Etag with the hash over the body (to avoid unnecessary re-uploads).
+
+ *Anastasis-Policy-Signature*: The client must provide Base-32 encoded EdDSA signature over hash of body with $ACCOUNT_PRIV, affirming desire to upload an encrypted recovery document.
+
+ *Payment-Identifier*: Base-32 encoded 32-byte payment identifier that was included in a previous payment (see 402 status code). Used to allow the server to check that the client paid for the upload (to protect the server against DoS attacks) and that the client knows a real secret of financial value (as the kdf_id might be known to an attacker). If this header is missing in the client's request (or the associated payment has exceeded the upload limit), the server must return a 402 response. When making payments, the server must include a fresh, randomly-generated payment-identifier in the payment request.
+
+ **Details:**
+
+ .. _EncryptedRecoveryDocument:
+ .. code-block:: tsref
+
+ interface EncryptedRecoveryDocument {
+ // Nonce used to compute the (iv,key) pair for encryption of the
+ // encrypted_compressed_recovery_document.
+ nonce: byte[32];
+
+ // Authentication tag
+ aes_gcm_tag: byte[16];
+
+ // Variable-size encrypted recovery document. After decryption,
+ // this contains a gzip compressed JSON-encoded `RecoveryDocument`_.
+ // The salt of the HKDF for this encryption must include the
+ // string "EDR".
+ encrypted_compressed_recovery_document: byte[]
+
+ }
+
+ .. _RecoveryDocument:
+ .. code-block:: tsref
+
+ interface RecoveryDocument {
+ // Account identifier at backup provider, AES-encrypted with
+ // the (symmetric) master_key, i.e. an URL
+ // https://sync.taler.net/$BACKUP_ID and
+ // a private key to decrypt the backup. Anastasis is oblivious
+ // to the details of how this is ultimately encoded.
+ backup_account: byte[];
+
+ // List of escrow providers and selected authentication method
+ methods: EscrowMethod[];
+
+ // List of possible decryption policies
+ policy: EscrowPolicy[];
+
+ }
+
+ .. _EscrowMethod:
+ .. code-block:: tsref
+
+ interface EscrowMethod {
+ // URL of the escrow provider (including possibly this Anastasis server)
+ provider_url : string;
+
+ // Name of the escrow method (e.g. security question, SMS etc.)
+ escrow_method: string;
+
+ // UUID of the escrow method (see /truth/ API below).
+ uuid: uuid;
+
+ // Salt used to encrypt the truth on the Anastasis server.
+ truth_salt: byte[32];
+
+ // 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: byte[];
+
+ }
+
+ .. _EscrowPolicy:
+ .. code-block:: tsref
+
+ interface DecryptionPolicy {
+ // Salt included to encrypt master key share when
+ // using this decryption policy.
+ policy_salt: byte[32];
+
+ // Master key, AES-encrypted with key derived from
+ // salt and secrets revealed by the following list of
+ // escrow methods identified by UUID.
+ encrypted_master_key: byte[32];
+
+ // List of escrow methods identified by their uuid
+ uuid: uuid[];
+
+ }
+
+
+.. _truth:
+
+
+Managing truth
+^^^^^^^^^^^^^^
+
+This API is used by the Anastasis client to deposit or request **truth** with
+the escrow provider. As with the policy, the user may be identified and
+authorized by $ACCOUNT_PUB. Note that authentification of the user is
+optional when uploading truth and depends on the server. An Anastasis-server
+may agree to store truth for free for a certain time period, or charge per
+truth without associating the truth with an account. Hence the "account"
+argument and signature may be optional.
+
+.. http:post:: /truth/$UUID[?account=$ACCOUNT_PUB]
+
+ :status 204 No content:
+ Truth stored successfully.
+ :status 304 Not modified:
+ The same truth was previously accepted and stored under this UUID.
+ :status 400 Bad request:
+ The $ACCOUNT_PUB is not an EdDSA public key. The response body may elaborate on the error.
+ :status 402 Payment Required:
+ The account's balance is too low for the specified operation (or the server
+ requires payment to store truth per item).
+ See the Taler payment protocol specification for how to pay.
+ The response body SHOULD provide various means for payment.
+ :status 403 Forbidden:
+ The required account signature was invalid. The response body may elaborate on the error.
+ :status 409 Conflict:
+ The server already has some truth stored under this UUID. The client should check that it
+ is generating UUIDs with enough entropy.
+ :status 412 Precondition Failed:
+ The selected authentication method is not supported on this provider.
+
+ *Anastasis-Account-Signature*: The client must provide Base-32 encoded EdDSA signature over hash of body with $ACCOUNT_PRIV, affirming the desire to upload the truth; only present if "account" is specified in the URL.
+
+ **Details:**
+
+ .. _Truth:
+ .. code-block:: tsref
+
+ interface Truth {
+ // Key share method, i.e. "security question", "SMS", "e-mail", ...
+ method: String;
+
+ // The explicit key material to reveal (Q: as string in base32 encoding?)
+ // Contains a KeyShare_, but in compact binary encoding.
+ //
+ // The salt of the HKDF for the encryption of this
+ // value must include the string "EKS". Depending
+ // on the method, the HKDF may additionally include
+ // bits from the response (i.e. some hash over the
+ // answer to the security question)
+ encrypted_key_share: byte[];
+
+ // Nonce used to generate the (iv,key) from kdf_id to AES-GCM encrypt the truth.
+ nonce: byte[32];
+
+ // Authentication tag over the encrypted_key_share
+ key_share_aes_gcm_tag: byte[32];
+
+ // ground truth, i.e. H(challenge answer),
+ // phone number, e-mail address, picture, fingerprint, ...
+ // base32 encoded
+ //
+ // The truth MUST NOT be revealed to the user, even
+ // after successful authentication (of course the user
+ // was originally aware when establishing the truth).
+ truth: string;
+
+ // mime type of truth, i.e. text/ascii, image/jpeg, etc.
+ truth_mime: string;
+
+ }
+
+
+.. http:get:: /truth/$UUID[?response=$RESPONSE]
+
+ :status 200 OK:
+ EncryptedKeyShare_ is returned in body (in binary).
+ :status 202 Accepted:
+ The escrow provider will respond out-of-band (i.e. SMS).
+ The body may contain human-readable instructions on next steps.
+ :status 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.
+ :status 402 Payment Required:
+ The account's balance is too low for the specified operation (or the server
+ requires payment to store truth per item).
+ See the Taler payment protocol specification for how to pay.
+ The response body SHOULD provide various means for payment.
+ :status 403 Forbidden:
+ The server requires a valid "response" to the challenge associated with the UUID.
+ :status 404 Not Found:
+ The server does not know any truth under the given UUID.
+ :status 412 Precondition Failed:
+ The escrow provider responds with an EscrowChallenge_ object containing
+ details on the challenge the user has to satisfy (see below).
+ :status 503 Service Unavailable:
+ Server is out of Service.
+
+ **Details:**
+
+ .. _EncryptedKeyShare:
+ .. code-block:: tsref
+
+ interface EncryptedKeyShare {
+ // Nonce used to compute the decryption (iv,key) pair.
+ nonce: byte[32];
+
+ // Authentication tag
+ aes_gcm_tag: byte[32];
+
+ // 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).
+ //
+ // The salt of the HKDF for the encryption of this
+ // value must include the string "EKS". 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: byte[];
+
+ }
+
+ .. _KeyShare:
+ .. code-block:: tsref
+
+ interface KeyShare {
+ // Key material to concatenate with policy_salt and KDF to derive
+ // the key to decrypt the master key.
+ key_share: byte[32];
+
+ // Signature over method, uuid, and key_share.
+ account_sig: EdDSA-Signature;
+
+ }
+
+ .. _EscrowChallenge:
+ .. code-block:: tsref
+
+ interface EscrowChallenge {
+ // ground truth, i.e. challenge question,
+ // phone number, e-mail address, picture, fingerprint, ...
+ truth: byte[];
+
+ // mime type of truth, i.e. text/ascii, image/jpeg, etc.
+ truth_mime: string;
+
+ }