Add 'paddle' crate

Should verify a Paddle webhook. This ensures that the request really
does come from Paddle.

In order to verify the request, we need to sign the serialized POST
parameters. And, weirdly, Paddle requires you to serialize parameters in
PHP's serialization format.

Using this Gist by 'drewmccormack' as a reference for the format:
https://gist.github.com/drewmccormack/a51b18ffeda8f596a11a8623481344d8

4 files changed, 53 insertions, 1 deletions

diff --git a/license-generator/Cargo.lock b/license-generator/Cargo.lock
index cee7dd9..d615538 100644
--- a/license-generator/Cargo.lock
+++ b/license-generator/Cargo.lock
@@ -147,6 +147,10 @@ dependencies = [
 ]
 
 [[package]]
+name = "paddle"
+version = "0.0.1"
+
+[[package]]
 name = "pkg-config"
 version = "0.3.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/license-generator/Cargo.toml b/license-generator/Cargo.toml
index 55d1a3f..99c9a3c 100644
--- a/license-generator/Cargo.toml
+++ b/license-generator/Cargo.toml
@@ -6,4 +6,7 @@ version = "0.0.1"
 fastcgi = "1.0.0"
 
 [workspace]
-members = ["aquatic-prime"]
+members = [
+	"aquatic-prime",
+	"paddle",
+]
diff --git a/license-generator/paddle/Cargo.toml b/license-generator/paddle/Cargo.toml
new file mode 100644
index 0000000..0a4f677
--- /dev/null
+++ b/license-generator/paddle/Cargo.toml
@@ -0,0 +1,5 @@
+[package]
+name = "paddle"
+version = "0.0.1"
+
+[dependencies]
diff --git a/license-generator/paddle/src/lib.rs b/license-generator/paddle/src/lib.rs
new file mode 100644
index 0000000..a1db417
--- /dev/null
+++ b/license-generator/paddle/src/lib.rs
@@ -0,0 +1,40 @@
+// https://paddle.com/docs/reference-verifying-webhooks/
+fn verify_signature<'a, I>(params: I) -> bool
+where I: ExactSizeIterator<Item = (&'a str, &'a str)> {
+    false
+}
+
+fn php_serialize<'a, I>(pairs: I) -> String
+where I: ExactSizeIterator<Item = (&'a str, &'a str)> {
+    let mut serialized = String::with_capacity(500);
+
+    serialized.push_str(
+        &format!("a:{pairs_count}:{{", pairs_count = pairs.len())
+    );
+
+    for (key, value) in pairs {
+        serialized.push_str(
+            &format!(
+                "s:{key_length}:\"{key}\";s:{value_length}:\"{value}\";",
+                key_length = key.len(),
+                key = key,
+                value_length = value.len(),
+                value = value
+            )
+        );
+    }
+
+    serialized.push_str("}");
+
+    serialized
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn php_serialize_serializes_key_values_as_php_string() {
+    }
+}