1.8. Wallet Backup and Synchronization Service API

The wallet backup and synchronization service uses an EdDSA wallet key to identify the “account” of the user. The wallet key is Crockford Base32-encoded in the URI to access the data and used to sign requests as well as to encrypt the contents (see below). These signatures are provided in detached from as HTTP headers.

Once the user activates backup or synchronization, the wallet should display the wallet key as a QR code as well as in text format together with the synchronization service’s URL and ask the user to print this key material and keep it safe.

The actual format of the wallet database is not relevant for the backup and synchronization service, as the service must only ever see a padded and encrypted version of the data.

However, there are a few general rules that will apply to any version of the wallet database. Still, except for the 32 byte minimum upload size, the synchronization service itself cannot not enforce these rules.

  • First, the database should be compressed (i.e. gzip), then padded to a power of 2 in kilobytes or a multiple of megabytes, then encrypted and finally protected with an HDKF.
  • The encryption should use an ephemeral Curve25519 point that is prefixed to the actual database, and combined with the private wallet key via ECDH to create a symmetric secret. With every revision of the wallet (but only real revisions or merge operations), a fresh ephemeral must be used to ensure that the symmetric secret differs every time. HKDFs are used to derive symmetric key material for authenticated encryption (encrypt-then-mac or a modern AEAD-cipher like Keccak). Given that AES is more easily available and will likey increase the code of the wallet less, AES plus a SHA-512 HMAC should suffice for now.
  • The wallet must enable merging databases in a way that is associative and commutative. For most activities, this implies merging lists, applying expirations, dropping duplicates and sorting the result. For deletions (operations by which the user removed records prior to their scheduled expiration), it means keeping a summarizing log of all deletion operations and applying the deletions after each merge. A summarizing log of a deletion operation would combine two deletion operations of the form “delete all transactions smaller than amount X before time T” and “delete all transactions smaller than amount Y before time T” into “delete all transactions smaller than amount max(X,Y) before time T”. Similar summarizations should be applied to all deletion operations supported by the wallet. Deletion operations themselves are associated with an expiration time reflecting the expiration of the longest lasting record that they explicitly deleted. Purchases do not have an expiration time, thus they create a challenge if an indivdiual purchase is deleted. Thus, when an individual purchase is deleted, the wallet is to keep track of the deletion with a deletion record. The deletion record still includes the purchase amount and purchase date. Thus, when purchases are deleted “in bulk” in a way that would have covered the individual deletion, such deletion records may still be subsumed by a more general deletion clause. In addition to the date and amount, the deletion record should only contain a salted hash of the original purchase record’s primary key, so as to minimize information leakage.
  • The database should contain a “last modified” timestamp to ensure we do not go backwards in time if the synchronization service is malicious. Merging two databases means taking the max of the “last modified” timestamps, not setting it to the current time. The wallet should reject a “fast forward” database update if the result would imply going back in time. If the wallet receives a database with a timestamp into the future, it must still increment it by the smallest possible amount when uploading an update.

It is assumed that the synchronization service is only ever accessed over TLS, and that the synchronization service is trusted to not build user’s location profiles by linking client IP addresses and wallet keys.

1.8.1. Receiving Terms of Service

GET /terms

Obtain the terms of service provided by the storage service.

Response:

Returns a SyncTermsOfServiceResponse.

interface SyncTermsOfServiceResponse {
  // maximum wallet database backup size supported
  storage_limit_in_megabytes: number;

  // maximum number of sync requests per day (per account)
  daily_sync_limit: number;

  // how long after an account (or device) becomes dormant does the
  // service expire the respective records?
  inactive_expiration: relative-time;

  // Fee for an account, per year.
  annual_fee: Amount;

}
GET /salt

Obtain the salt used by the storage service.

Response:

Returns a SaltResponse.

interface SaltResponse {
  // salt value, at least 128 bits of entropy
  salt: string;
}
GET /$WALLET-KEY

Download latest version of the wallet database. The returned headers must include “Etags” based on the hash of the (encrypted) database. The server must check the client’s caching headers and only return the full database if it has changed since the last request of the client.

This method is generally only performed once per device when the private key and URL of a synchronization service are first given to the wallet on the respective device. Once a wallet has a database, it should always use the POST method.

A signature is not required, as (1) the wallet-key should be reasonably private and thus unauthorized users should not know how to produce the correct request, and (2) the information returned is encrypted to the private key anyway and thus virtually useless even to an attacker who somehow managed to obtain the public key.

Response

Status Codes
  • 200 OK – The body contains the current version of the wallet’s database as known to the server.
  • 204 No content – This is a fresh account, no previous wallet data exists at the server.
  • 402 Payment required – The synchronization service requires payment before the account can continue to be used. The fulfillment URL should be the /$WALLET-KEY URL, but can be safely ignored by the wallet. The contract should be shown to the user in the canonical dialog, possibly in a fresh tab.
  • 410 Gone – The backup service has closed operations. The body will contain the latest version still available at the server. The body may be empty if no version is available. The user should be urged to find another provider.
  • 429 Too many requests – This account has exceeded daily thresholds for the number of requests. The wallet should try again later, and may want to decrease its synchronization frequency.

Note

“200 OK” responses include an HTTP header “X-Taler-Sync-Signature” with the signature of the wallet from the orginal upload, and an “X-Taler-Sync-Previous” with the version that was being updated (unless this is the first revision). “X-Taler-Sync-Previous” is only given to enable signature validation.

POST /$WALLET-KEY

Upload a new version of the wallet’s database, or download the latest version. The request must include the “Expect: 100 Continue” header. The client must wait for “100 Continue” before proceeding with the upload, regardless of the size of the upload.

Request

The request must include a “If-Match” header indicating the latest version of the wallet’s database known to the client. If the server knows a more recent version, it will respond with a “409 conflict” and return the server’s version in the response. The client must then merge the two versions before retrying the upload. Note that a “409 Conflict” response will typically be given before the upload, (instead of “100 continue”), but may also be given after the upload, for example due to concurrent activities from other wallets on the same account!

The request must also include an “X-Taler-Sync-Signature” signing the “If-Match” SHA-512 value and the SHA-512 hash of the body with the wallet private key.

Finally, the SHA-512 hash of the body must also be given in an “E-tag” header of the request (so that the signature can be verified before the upload is allowed to proceed). We note that the use of “E-tag” in HTTP requests is non-standard, but in this case logical.

The uploaded body must have at least 32 bytes of payload (see suggested upload format beginning with an ephemeral key).

Response

Status Codes
  • 204 No content – The transfer was successful, and the server has registered the new version.
  • 304 Not modified – The server is already aware of this version of the wallet. Returned before 100 continue to avoid upload.
  • 400 Bad request – Most likely, the uploaded body is too short (less than 32 bytes).
  • 401 Unauthorized – The signature is invalid or missing (or body does not match).
  • 402 Payment required – The synchronization service requires payment before the account can continue to be used. The fulfillment URL should be the /$WALLET-KEY URL, but can be safely ignored by the wallet. The contract should be shown to the user in the canonical dialog, possibly in a fresh tab.
  • 409 Conflict – The server has a more recent version than what is given in “If-Match”. The more recent version is returned. The client should merge the two versions and retry using the given response’s “E-Tag” in the next attempt in “If-Match”.
  • 410 Gone – The backup service has closed operations. The body will contain the latest version still available at the server. The body may be empty if no version is available. The user should be urged to find another provider.
  • 411 Length required – The client must specify the “Content-length” header before attempting upload. While technically optional by the HTTP specification, the synchronization service may require the client to provide the length upfront.
  • 413 Request Entity Too Large – The requested upload exceeds the quota for the type of account. The wallet should suggest to the user to migrate to another backup and synchronization service (like with “410 Gone”).
  • 429 Too many requests – This account has exceeded daily thresholds for the number of requests. The wallet should try again later, and may want to decrease its synchronization frequency.

Note

Responses with a body include an HTTP header “X-Taler-Sync-Signature” with the signature of the wallet from the orginal upload, and an “X-Taler-Sync-Previous” with the version that was being updated (unless this is the first revision). “X-Taler-Sync-Previous” is only given to enable signature validation.

1.8.2. Special constraints for Tor

We might introduce the notion of a “constraint” into the wallet’s database that states that the database is a “Tor wallet”. Then, synchronizing a “Tor-wallet” with a non-Tor wallet should trigger a stern warning and require user confirmation (as otherwise cross-browser synchronization may weaken the security of Tor browser users).

1.8.3. Discovery of backup and synchronization services

The wallet should keep a list of “default” synchronization services per currency (by the currency the synchronization service accepts for payment). If a synchronization service is entirely free, it should be kept in a special list that is always available.

Extending (or shortening) the list of synchronization services should be possible using the same mechanism that is used to add/remove auditors or exchanges.

The wallet should urge the user to make use of a synchronization service upon first withdrawal, suggesting one that is free or accepts payment in the respective currency. If none is available, the wallet should warn the user about the lack of availalable backups and synchronization and suggest to the user to find a reasonable service. Once a synchronization service was selected, the wallet should urge the user to print the respective key material.

When the wallet starts the first time on a new device, it should ask the user if he wants to synchronize with an existing wallet, and if so, ask the user to enter the respective key and the (base) URL of the synchronization service.

1.8.4. Synchronization frequency

Generally, the wallet should attempt to synchronize at a randomized time interval between 30 and 300 seconds of being started, unless it already synchronized less than two hours ago already. Afterwards, the wallet should synchronize every two hours, or after purchases exceed 5 percent of the last bulk amount that the user withdrew. In all cases the exact time of synchronization should be randomized between 30 and 300 seconds of the specified event, both to minimize obvious correlations and to spread the load.

If the two hour frequency would exceed half of the rate budget offered by the synchronization provider, it should be reduced to remain below that threshold.

1.8.5. Synchronization user experience

The menu should include three entries for synchronization:

  • “synchronize” to manually trigger synchronization,
    insensitive if no synchronization provider is available
  • “export backup configuration” to re-display (and possibly
    print) the synchronization and backup parameters (URL and private key), insensitive if no synchronization provider is available, and
  • “import backup configuration” to:
    • import another devices’ synchronization options (by specifying URL and private key, or possibly scanning a QR code), or
    • select a synchronization provider from the list, including manual specification of a URL; here confirmation should only be possible if the provider is free or can be paid for; in this case, the wallet should trigger the payment interaction when the user presses the “select” button.
    • a special button to “disable synchronization and backup”

One usability issue here is that we are asking users to deal with a private key. It is likely better to map private keys to trustwords (PEP-style). Also, when putting private keys into a QR code, there is the danger of the QR code being scanned and interpreted as a “public” URL. Thus, the QR code should use the schema “taler-sync://$SYNC-DOMAIN/$SYNC-PATH#private-key” where “$SYNC-DOMAIN” is the domainname of the synchronization service and $SYNC-PATH the (usually empty) path. By putting the private key after “#”, we may succeed in disclosing the value even to eager Web-ish interpreters of URLs. Note that the actual synchronization service must use the HTTPS protocol, which means we can leave out this prefix.