[#CKey-SignSchnorr]
= xref:CKey.adoc[CKey]::SignSchnorr
:relfileprefix: ../
:mrdocs:

Create a BIP‐340 Schnorr signature, for the xonly‐pubkey corresponding to *this, optionally tweaked by *merkle_root. Additional nonce entropy is provided through aux.

== Synopsis

Declared in `<key.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
bool
SignSchnorr(
    xref:uint256.adoc[uint256] const& hash,
    std::span<unsigned char> sig,
    xref:uint256.adoc[uint256] const* merkle_root,
    xref:uint256.adoc[uint256] const& aux) const;
----

== Description

merkle_root is used to optionally perform tweaking of the private key, as specified in BIP341:

* If merkle_root == nullptr: no tweaking is done, sign with key directly (this is used for signatures in BIP342 script).
* If merkle_root‐>IsNull(): sign with key + H_TapTweak(pubkey) (this is used for key path spending when no scripts are present).
* Otherwise: sign with key + H_TapTweak(pubkey || *merkle_root) (this is used for key path spending, with specific Merkle root of the script tree).

== Return Value

true if signing succeeded.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *hash*
| The 32‐byte message hash to sign.
| *sig*
| Output span (64 bytes) receiving the Schnorr signature.
| *merkle_root*
| Optional Merkle root controlling BIP341 tweaking (see above).
| *aux*
| Additional nonce entropy.
|===


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