# sBTC Signers

### Overview

The [sBTC Signers contract](https://github.com/stacks-network/sbtc/blob/main/contracts/contracts/sbtc-bootstrap-signers.clar) (`sbtc-bootstrap-signers.clar`) manages the signer set for the sBTC system. It handles rotation of signer keys and provides utilities for generating multisig addresses.

**Constants**

* `key-size`: The required length of public keys (33 bytes).

**Error Constants**

* `ERR_KEY_SIZE_PREFIX`: Prefix for key size errors in batch processing.
* `ERR_KEY_SIZE` (u200): Indicates that a provided key is not the correct length.
* `ERR_INVALID_CALLER` (u201): Signifies that the function caller is not the current signer principal.
* `ERR_SIGNATURE_THRESHOLD` (u202): Indicates an invalid signature threshold (must be >50% and ≤100% of total signer keys).

#### Public Functions

**`rotate-keys-wrapper`**

Rotates the keys of the signers. Called when the signer set is updated.

* Parameters:
  * `new-keys`: `(list 128 (buff 33))` - List of new signer public keys
  * `new-aggregate-pubkey`: `(buff 33)` - New aggregate public key
  * `new-signature-threshold`: `uint` - New signature threshold
* Returns: `(response (buff 33) uint)`

Function flow:

{% stepper %}
{% step %}
**Validate signature threshold**

Ensure the new signature threshold is valid (must be >50% and ≤100% of total signer keys).
{% endstep %}

{% step %}
**Verify caller**

Verify that the caller is the current signer principal.
{% endstep %}

{% step %}
**Validate keys**

Check the length of each new key and the aggregate public key (must be 33 bytes).
{% endstep %}

{% step %}
**Update registry**

Call the sBTC Registry contract to update the keys and address.
{% endstep %}
{% endstepper %}

#### Read-only Functions

**`pubkeys-to-spend-script`**

Generates the p2sh redeem script for a multisig.

* Parameters:
  * `pubkeys`: `(list 128 (buff 33))` - List of public keys
  * `m`: `uint` - Number of required signatures
* Returns: `(buff 1024)` - The p2sh redeem script

**`pubkeys-to-hash`**

Computes the hash160 of the p2sh redeem script.

* Parameters:
  * `pubkeys`: `(list 128 (buff 33))` - List of public keys
  * `m`: `uint` - Number of required signatures
* Returns: `(buff 20)` - The hash160 of the redeem script

**`pubkeys-to-principal`**

Generates a principal (Stacks address) from a set of pubkeys and an m-of-n threshold.

* Parameters:
  * `pubkeys`: `(list 128 (buff 33))` - List of public keys
  * `m`: `uint` - Number of required signatures
* Returns: `principal` - The generated Stacks address

**`pubkeys-to-bytes`**

Concatenates a list of pubkeys into a buffer with length prefixes.

* Parameters:
  * `pubkeys`: `(list 128 (buff 33))` - List of public keys
* Returns: `(buff 510)` - Concatenated pubkeys with length prefixes

**`concat-pubkeys-fold`**

Concatenates a pubkey buffer with a length prefix.

* Parameters:
  * `pubkey`: `(buff 33)` - A single public key
  * `iterator`: `(buff 510)` - Accumulator for concatenation
* Returns: `(buff 510)` - Updated concatenated buffer

**`bytes-len`**

Returns the length of a byte buffer as a single byte.

* Parameters:
  * `bytes`: `(buff 33)` - Input byte buffer
* Returns: `(buff 1)` - Length as a single byte

**`uint-to-byte`**

Converts a uint to a single byte.

* Parameters:
  * `n`: `uint` - Input number
* Returns: `(buff 1)` - Number as a single byte

#### Private Functions

**`signer-key-length-check`**

Checks that the length of each key is exactly 33 bytes.

* Parameters:
  * `current-key`: `(buff 33)` - Public key to check
  * `helper-response`: `(response uint uint)` - Accumulator for error handling
* Returns: `(response uint uint)` - Updated accumulator or error

#### Constants

**`BUFF_TO_BYTE`**

A constant list mapping uint values (0-255) to their corresponding byte representations.

Interactions with Other Contracts

* `.sbtc-registry`: Calls `get-current-signer-data` and `rotate-keys` to manage signer data.

Security Considerations

{% hint style="warning" %}

* Access Control: Only the current signer principal can call the key rotation function.
* Key Validation: Ensures all provided keys are the correct length.
* Signature Threshold: Enforces a minimum threshold of over 50% of signers and a maximum of 100%.
* Multisig Generation: Provides utilities for secure generation of multisig addresses.
  {% endhint %}