# Signed Messages

The **Bitcoin protocol** allows users to sign any type of data (not just transactions) using a **Bitcoin private key**. The same counts for BSV. This process is primarily used for **message authentication**—proving that the signer has access to a specific BSV address at the time of signing.

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### Private and Public Key Generation

The process of generating **private and public keys** is the same as in BSV transactions.\
Some wallets also provide a **message signing feature**, enabling you to sign data with your private key.

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### How a Message is Signed

1. The message is **prepended** with the string:\
   `"Bitcoin Signed Message:\n"` along with the message length.
2. It is then **double-hashed** with **SHA-256**.
3. The result is signed using **ECDSA**.
4. The **signature** is produced as a tuple `(i, r, s)` where:
   * `i` = 1 byte (parity byte)
   * `r` = 32 bytes
   * `s` = 32 bytes
5. Finally, the tuple is **base64 encoded**.

<figure><img src="/files/3VZ1K17LexnRTxEIBA1z" alt=""><figcaption></figcaption></figure>

### How a Message is Verified

1. The **address** is transmitted with the **original message** and the **signed message**.
2. Using the **parity byte** `i`, the system derives between **0–4 candidate public keys**.
3. The verification process repeats the same steps:
   * Prepend `"Bitcoin Signed Message:\n"`
   * **Double-hash** the message with SHA-256
   * Run the **verification algorithm** for each candidate key
4. If one of the candidate keys returns **True**, the signature is valid.
5. The valid public key is then converted into an **address** and checked against the sender’s provided address.

<figure><img src="/files/eREKO9TPrld5CW8zfxsf" alt=""><figcaption></figcaption></figure>

### Address Reminder

Recall: a **BSV address** is derived as follows:

* Start with the **public key**
* Apply **SHA-256**, then **RIPEMD-160**
* Encode the result as a **Base58 string**

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### Key Takeaway

* **Signed messages** prove control of a BSV private key without moving coins.
* The process ensures **authenticity** (message truly from the signer) and **integrity** (message unchanged).
* Both **signing** and **verification** rely on **ECDSA**, **double hashing**, and the **Base58-encoded address** structure.


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