Add testutils to build ed25519 keypairs (#764)

### What

Test utils to build ed25519 keypairs from a Stellar public key and
secret key.

### Why

There currently is no way to build signatures for existing ed25519
keypairs, so if I wanted to generate signatures in tests or in a script
to use them later with the CLI, I'd have to build the keypair and strkey
decoding myself. With this PR users are provided with a function to
build the keypair from a Stellar public and secret key (as you can see
in tests).

### Known limitations

N/A

Co-authored-by: Leigh McCulloch <351529+leighmcculloch@users.noreply.github.com>

soroban-auth/Cargo.toml

@@ -26,6 +26,7 @@ rand = { version = "0.7.3", optional = true }
 
 [dev_dependencies]
 soroban-sdk = { workspace = true, features = ["testutils"] }
+stellar-strkey = { workspace = true }
 ed25519-dalek = { version = "1.0.1" }
 rand = { version = "0.7.3" }
 

soroban-auth/src/tests/test_ed25519.rs

@@ -1,13 +1,15 @@
 extern crate std;
 
+use core::str::FromStr;
+
 use soroban_sdk::{
     contractimpl, symbol,
     testutils::{Ledger, LedgerInfo},
     BytesN, Env,
 };
 
 use crate::{
-    testutils::ed25519::{generate, sign},
+    testutils::ed25519::{generate, sign, signer},
     verify, Signature,
 };
 
@@ -58,3 +60,41 @@ fn test() {
 
     client.examplefn(&sig, &1, &2);
 }
+
+#[test]
+fn test_build_keypair() {
+    let env = Env::default();
+    let contract_id = BytesN::from_array(&env, &[0; 32]);
+    env.register_contract(&contract_id, ExampleContract);
+    let client = ExampleContractClient::new(&env, &contract_id);
+
+    env.ledger().set(LedgerInfo {
+        base_reserve: 0,
+        network_passphrase: "soroban-auth test".as_bytes().to_vec(),
+        protocol_version: 0,
+        sequence_number: 0,
+        timestamp: 0,
+    });
+
+    std::println!("network: {:?}", env.ledger().network_passphrase());
+    std::println!("contract id: {:?}", contract_id);
+    std::println!("name: {:?}", symbol!("examplefn"));
+
+    let key = stellar_strkey::StrkeyPrivateKeyEd25519::from_str(
+        "SC24O4H2LT4PVOYCWMKUSD2DL4UL26IYGPFKANDH7S4MU6JVQEFOS7DC",
+    )
+    .unwrap();
+    let (id, signer) = signer(&env, &key.0);
+    std::println!("signer: {:?}", signer);
+    std::println!("id: {:?}", id);
+    let sig = sign(
+        &env,
+        &signer,
+        &contract_id,
+        symbol!("examplefn"),
+        (&id, &1, &2),
+    );
+    std::println!("signature: {:?}", sig);
+
+    client.examplefn(&sig, &1, &2);
+}

soroban-auth/src/testutils.rs

@@ -38,6 +38,25 @@ pub mod ed25519 {
         (signer.identifier(env), signer)
     }
 
+    /// Create a signer from a ed25519 secret key.
+    pub fn signer(
+        env: &Env,
+        secret_key: &[u8; 32],
+    ) -> (
+        IdentifierValue,
+        impl Identifier + Sign<SignaturePayload, Signature = [u8; 64]> + Debug,
+    ) {
+        let secret = ed25519_dalek::SecretKey::from_bytes(secret_key).unwrap();
+        let public = ed25519_dalek::PublicKey::from(&secret);
+        let signer = ed25519_dalek::Keypair { secret, public };
+        (signer.identifier(env), signer)
+    }
+
+    /// Create an identifier from a ed25519 public key.
+    pub fn identifier(env: &Env, public_key: &[u8; 32]) -> IdentifierValue {
+        IdentifierValue::Ed25519(public_key.into_val(env))
+    }
+
     /// Sign a [`SignaturePayload`] constructed using the arguments.
     ///
     /// The returned [`Signature`] can be verified by [`verify`](crate::verify)