[blog] Blog post on the Libsodium vulnerability regarding edd25519 point validation

content/blog/libsodium-vulnerability-ed25519-valid-points.md

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+title = "A vulnerability in libsodium's validation of ed25519 elliptic curve points: You are likely not affected"
+date = 2025-12-21T12:40:00
+updated = 2025-12-21T12:40:00
+draft = false # Leave this as true so that it is not published
+template = "blog/page.html"
+
+[extra]
+lead = "Libsodium's author Frank Denis has found a missing check for validating elliptic curve points in a low-level function not exposed by libsodium-bindings"
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+
+On December 30th 2025, [Libsodium][Libsodium] author Frank Denis has published the existence of a vulnerability regarding a [missing validation of elliptic curve points for the `crypto_core_ed25519_is_valid_point`
+function](https://00f.net/2025/12/30/libsodium-vulnerability/) of the Ed25519 public signature API. Fortunately, the fallout is mitigated in great part by two factors:
+
+1. The [`libsodium-bindings`][libsodium-bindings] library does not export this low-level function;
+2. If you do not use this function directly, but instead keep on using the [`CryptoSign`][CryptoSign] high-level API, you are safe.
+
+## Affected versions
+
+If you still need to comply with an audit, know that you are bound to be marked as "vulnerable" if you use a released version lower or equal to 1.0.20, or a version of libsodium released before December 30, 2025 (date of the publication of this vulnerabilty).
+
+## Recommendations
+
+For finite field arithmetic, an advanced use-case of the C library, The Libsodium project advises to switch to [Ristretto255][Ristretto255], available since Libsodium 1.0.18.
+At this time, [libsodium-bindings][libsodium-bindings] does not export bindings to this particular API, although a contribution in this direction would be greatly appreciated if you or your
+organisation might depend on such operations.
+
+---
+
+The Haskell Cryptography Group is affiliated with the [Haskell Foundation][Haskell Foundation]. [Get in touch](mailto:sponsorship@haskell.foundation) to see how you can best support our work.
+
+Please consider [sponsoring] the Libsodium project in order to ensure it can provide the best-in-class cryptographic safety for its end-users.
+
+[Libsodium]: https://libsodium.gitbook.io/doc
+[CryptoSign]: https://hackage-content.haskell.org/package/libsodium-bindings/docs/LibSodium-Bindings-CryptoSign.html
+[libsodium-bindings]: https://github.com/haskell-cryptography/libsodium-bindings/tree/main/libsodium-bindings
+[Ristretto255]: https://libsodium.gitbook.io/doc/advanced/point-arithmetic/ristretto
+[sponsoring]: https://opencollective.com/libsodium/contribute
+[Haskell Foundation]: https://haskell.foundation/