refactor(webapi,types)!: make challenge type extensive

We may allow more challenge types other than PoW in the future, eg.
captcha. So make the relevent types more generic.

Now the challenge is returned in JSON response as a individual top-level
field `register_challenge` instead of in HTTP headers.

blah-types/benches/crypto_ops.rs

@@ -2,7 +2,7 @@
 use std::hint::black_box;
 use std::time::Instant;
 
-use blah_types::msg::{ChatPayload, UserRegisterPayload};
+use blah_types::msg::{ChatPayload, UserRegisterChallengeResponse, UserRegisterPayload};
 use blah_types::{get_timestamp, Id, PubKey, SignExt, Signee, UserKey};
 use criterion::{criterion_group, criterion_main, Criterion};
 use ed25519_dalek::SigningKey;
@@ -24,7 +24,7 @@ fn bench_register_pow(c: &mut Criterion) {
         id_key: id_key.clone(),
         server_url: "http://some.example.com".parse().unwrap(),
         id_url: "http://another.example.com".parse().unwrap(),
-        challenge_nonce: rng.gen(),
+        challenge: Some(UserRegisterChallengeResponse::Pow { nonce: rng.gen() }),
     };
     let mut signee = Signee {
         nonce: 0,

blah-types/src/msg.rs

@@ -58,7 +58,17 @@ pub struct UserRegisterPayload {
     pub server_url: Url,
     pub id_url: IdUrl,
     pub id_key: PubKey,
-    pub challenge_nonce: u32,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub challenge: Option<UserRegisterChallengeResponse>,
+}
+
+/// The server-specific challenge data for registration.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum UserRegisterChallengeResponse {
+    /// Proof of work challenge containing the same nonce from server challenge request.
+    /// The whole msg signee hash should have enough prefix zero bits.
+    Pow { nonce: u32 },
 }
 
 // FIXME: `deny_unknown_fields` breaks this.

blah-types/src/server.rs

@@ -6,9 +6,6 @@ use url::Url;
 use crate::msg::{Id, MemberPermission, RoomAttrs, SignedChatMsgWithId};
 use crate::PubKey;
 
-pub const X_BLAH_NONCE: &str = "x-blah-nonce";
-pub const X_BLAH_DIFFICULTY: &str = "x-blah-difficulty";
-
 /// Metadata about the version and capabilities of a Chat Server.
 ///
 /// It should be relatively stable and do not change very often.
@@ -37,6 +34,18 @@ pub struct ServerCapabilities {
     pub allow_public_register: bool,
 }
 
+/// Registration challenge information.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum UserRegisterChallenge {
+    /// Proof-of-work (PoW) challenge.
+    Pow { nonce: u32, difficulty: u8 },
+
+    /// A catch-all unknown challenge type.
+    #[serde(other, skip_serializing)]
+    Unknown,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct RoomMetadata {
     /// Room id.