> For the complete documentation index, see [llms.txt](https://bootcamp.openguild.wtf/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://bootcamp.openguild.wtf/building-a-blockchain-with-polkadot-sdk/polkadot-sdk/substrate/adding-a-custom-logic-to-runtime/extensions/signed-extensions.md).

# Signed Extensions

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Substrate provides the concept of **signed extensions** to extend an extrinsic with additional data, provided by the [`SignedExtension`](https://paritytech.github.io/substrate/master/sp_runtime/traits/trait.SignedExtension.html) trait.

The transaction queue regularly calls signed extensions to keep checking that a transaction is valid before it gets put in the ready queue. This is a useful safeguard for verifying that transactions won't fail in a block. They are commonly used to enforce validation logic to protect the transaction pool from spam and replay attacks.

### Reference

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