SIMD-0258: Signatures sysvar for signature introspection

proposals/0258-transaction-signatures-introspection.md

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+---
+simd: '0258'
+title: Signatures sysvar for signature introspection
+authors:
+  - Ahmad Abbasi (Syndica)
+category: Standard
+type: Core
+status: Review
+created: 2025-03-14
+
+---
+
+## Summary
+
+Allow the ability to introspect a transaction's signatures within an
+instruction via a Signatures sysvar.
+
+## Motivation
+
+As we develop the [Sig validator](https://github.com/Syndica/sig), we've
+experimented with novel ways an on-chain protocol can be designed. Many of
+those designs require some source of sequencing. Along with the need for a
+sequencing mechanism, the ability to query previous sequence "entries" is
+also important. Today, the only viable way to accomplish retrieving previous
+sequence entries is to use `getSignaturesForAddress` RPC and then parse through
+transactions individually to find the ones which create the next/previous
+"entry." If you have access to a transaction's signature within an
+instruction, you're able to save that into an account which can be used down
+the line to retrieve past entries.
+
+## New Terminology
+
+- Signatures sysvar: A Solana runtime sysvar which provides the ability to read
+a transaction's signatures within an instruction.
+
+## Detailed Design
+
+In order to introspect transaction signatures within an instruction, a
+transaction MUST include the Signatures sysvar
+`SysvarSignatures111111111111111111111111111` in the accounts list. The runtime
+will recognize this special sysvar, pass the raw transaction signatures to the
+instruction so that the bpf program can extract them and use as needed.
+
+A helper function to extract the signatures can be used:
+
+```rust
+fn load_signatures(
+        signatures_sysvar_account_info: &AccountInfo
+    ) -> Result<&[Signature]>;
+```
+
+## Alternatives Considered
+
+Currently, no alternative exists to introspect a transaction's signature
+from within an instruction.
+
+## Impact
+
+The impact of this will allow for unique and bespoke protocols to emerge on
+Solana enabling a new subset of sequence-based and hybrid (on/off-chain)
+applications to exist.
+
+## Security Considerations
+
+Because Sysvar(s) are inherently read-only, no major security concerns would
+arise from this feature. We already allow instruction data introspection via
+`Sysvar1nstructions1111111111111111111111111` sysvar.