The open-source protocol that separates cloud storage and private keys for simple and secure hot wallet backup.
Self-custody represents one of the foundations of financial sovereignty made possible by Bitcoin. This concept is based on the well-known principle “not your keys, not your coins”. This maxim, however, entails an enormous responsibility: the protection of private keys that control the funds. Backup management is the most critical element.
The current standard method, the 12 or 24-word seed phrase defined by the BIP39 standard, is an ingenious solution. However, for the average user, it introduces a series of risks and complexities that can completely nullify the benefits of self-custody.
A new paradigm for hot wallet backups
Recoverbull was born as a response to the pitfalls of traditional software wallet backup. It is an open-source protocol developed by Bull Bitcoin, designed to make backups secure and accessible to everyone, regardless of technical skills. Its philosophy is based on a series of principles:
- accessibility for all: the protocol is built on the assumption that users can be “uneducated, imprudent, unmotivated, inexperienced or incompetent”, without this compromising the security of their funds;
- anonymity: the protocol avoids collecting any personal data, since any information, including the simple fact of owning bitcoin, can be used by an attacker;
- simplicity and speed: the backup creation process is designed to be completed in less than two minutes, eliminating the complexity and friction that lead many users to postpone this operation;
- no possession of keys: the service that facilitates the backup never takes possession of the user’s private keys, neither in plain text nor encrypted, to avoid being classified as a regulated financial service provider;
- open standards and portability: the protocol leverages known technologies and open standards, ensuring that users are not tied to a single provider and that their backups remain portable.
How Recoverbull works
The process of using Recoverbull has been designed to be intuitive. Below are the two main operational flows: backup creation and its subsequent recovery.
The backup process
- The wallet (client) generates the 12 or 24-word seed phrase;
- from this, the client deterministically derives a secret and high-entropy backup key;
- the seed phrase is encrypted locally on the device using the backup key, creating an encrypted backup;
- this encrypted backup is inserted into a digital backup file, a simple text file that can be easily stored;
- at this point, the user is asked to choose a simple and easily memorable password;
- the client uses this password and a unique identifier from the backup file to generate an authentication key and interact with a specialized and anonymous service, the key server. The latter securely stores an encrypted version of the backup key;
- finally, the backup file is saved in the cloud storage service chosen by the user (e.g. iCloud, Google Drive). This location, normally considered insecure for a backup, becomes secure thanks to the applied encryption.
The recovery process
- When the user needs to restore their wallet on a new device, the client automatically recovers the backup file from the cloud service;
- they are asked to enter the password;
- the client uses the password and the information contained in the file (identifier) to authenticate anonymously with the key server and request the user’s encrypted backup key;
- once the key is received, the client decrypts it locally and uses it to unlock the encrypted backup contained in the file;
- the original seed phrase is revealed and the wallet is completely restored, giving the user new access to their funds.
The security of this method does not derive from the secrecy of the backup file location, but from the cryptographic separation of its two fundamental components: the file itself, stored in the cloud, and the key to decrypt it, protected by the key server.
Security model analysis
Recoverbull’s security model is based on multiple interconnected layers of defense, designed to protect the user’s funds even in the event that one of the individual components of the system is compromised. There are three pillars of the model:
- 2-of-2 segregated storage: an attacker, to succeed in stealing the funds, must gain access to both the backup file (stored in the user’s cloud) and the backup key (protected by the key server). Obtaining only one of the two elements is useless. If a hacker compromises the cloud account and obtains the file, they are faced with encrypted data that they cannot decrypt. The backup key itself is derived directly from the seed phrase using the BIP85 standard, which means that its security is equivalent to that of the seed phrase itself, making offline brute force attacks on the file as impractical as guessing the original seed phrase;
- the role of rate-limiting: the key server implements a fundamental security measure: it strictly limits the number of key recovery attempts. Bull Bitcoin’s recommendation is a limit of three attempts per day per identifier. This single measure neutralizes online brute force attacks. An attacker who has stolen the backup file cannot simply “try to guess” the user’s password. Even with a weak password like a 6-digit PIN (1 million combinations), a limit of three attempts per day makes such an attack impractical;
- anonymity as defense: the key server is designed not to collect or store any personally identifiable information: no email, no phone number, no IP address. This approach not only prevents targeted phishing attacks and vulnerable “password reset” mechanisms, but introduces a fundamental security property: the server’s inability to comply with legal requests. Since the server does not possess any information that can link a user to a specific backup, it cannot identify and hand over a user’s key upon request. Without the password or authentication key, which only the user possesses, there is no other way for the key server to identify a specific backup file.
Use cases and limits
Recoverbull was designed specifically for a certain type of use. It is not a universal solution for every custody need.
The following table clarifies which use cases Recoverbull is suitable for.
| Recoverbull is an alternative to: | Recoverbull is NOT an alternative to: |
|---|---|
| Custodial exchange accounts | Long-term cold storage |
| Custodial wallets | Hardware wallets |
| Physical backups for hot wallets | Multi-signature wallets |
| BIP39 passphrase for physical backups |
Recoverbull positions itself as a solution for the secure and daily management of funds on hot wallets (software wallets on smartphones or computers), offering an alternative to both traditional methods (paper backups) and custodial options.
Bull Bitcoin is the world’s longest-running Bitcoin-only exchange. Strictly non-custodial. Try it here.
