Add Double Public Key Signing Function Oracle Attack on ed25519-dalek (#1744)

Closes #1360

crates/ed25519-dalek/RUSTSEC-0000-0000.md

@@ -0,0 +1,29 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "ed25519-dalek"
+date = "2022-06-11"
+categories = ["crypto-failure"]
+url = "https://github.com/MystenLabs/ed25519-unsafe-libs"
+
+[versions]
+patched = [">= 2"]
+```
+
+# Double Public Key Signing Function Oracle Attack on `ed25519-dalek`
+
+Versions of `ed25519-dalek` prior to v2.0 model private and public keys as
+separate types which can be assembled into a `Keypair`, and also provide APIs
+for serializing and deserializing 64-byte private/public keypairs.
+
+Such APIs and serializations are inherently unsafe as the public key is one of
+the inputs used in the deterministic computation of the `S` part of the signature,
+but not in the `R` value. An adversary could somehow use the signing function as
+an oracle that allows arbitrary public keys as input can obtain two signatures
+for the same message sharing the same `R` and only differ on the `S` part.
+
+Unfortunately, when this happens, one can easily extract the private key.
+
+Revised public APIs in v2.0 of `ed25519-dalek` do NOT allow a decoupled
+private/public keypair as signing input, except as part of specially labeled
+"hazmat" APIs which are clearly labeled as being dangerous if misused.