diff --git a/src/core/mod.rs b/src/core/mod.rs
index 173d73d..7b3f598 100644
--- a/src/core/mod.rs
+++ b/src/core/mod.rs
@@ -4,5 +4,7 @@
 //! this layer straightforward to test in isolation.
 
 pub mod crypto;
+pub mod pow;
+pub mod session;
 
-// session, pow, address, and grin_confirmation are wired in as they land.
+// address and grin_confirmation are wired in as they land.
diff --git a/src/core/pow.rs b/src/core/pow.rs
new file mode 100644
index 0000000..1646a88
--- /dev/null
+++ b/src/core/pow.rs
@@ -0,0 +1,154 @@
+//! Proof-of-work signup gate (ALTCHA v2, PBKDF2/SHA-256).
+//!
+//! Makes mass account creation *expensive* rather than perfectly Sybil-proof:
+//! a legitimate signup pays ~1-2s of client CPU; the server verifies in <2ms.
+//! Feature-gated via config (`FEATURE_POW`); the HMAC key is required when
+//! enabled (config validation rejects a missing/placeholder key — there is no
+//! source-visible fallback). Wire protocol matches the wallet's `altcha-lib`.
+
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use altcha::{
+    create_challenge, verify_solution, CreateChallengeOptions, Payload, VerifySolutionOptions,
+};
+
+use crate::config::PowConfig;
+use crate::error::AppError;
+
+/// Challenge TTL: long enough for a slow phone, short enough to bound an
+/// attacker's pre-solve window before the HMAC key rotates.
+const CHALLENGE_TTL_SECONDS: u64 = 600;
+
+/// Whether a registration from this BTC pubkey hash must present a valid PoW
+/// solution: when the global gate is on, or this pubkey is individually opted in.
+pub fn required_for(cfg: &PowConfig, pubkey_hash: &str) -> bool {
+    cfg.required || cfg.required_for_pubkeys.iter().any(|p| p == pubkey_hash)
+}
+
+/// Issue a fresh challenge. The HMAC signature embeds an expiry, so the gate is
+/// stateless: no issued-challenge store is needed for expiry.
+pub fn issue_challenge(cfg: &PowConfig) -> Result<altcha::Challenge, AppError> {
+    let now = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|d| d.as_secs())
+        .unwrap_or(0);
+    let opts = CreateChallengeOptions {
+        algorithm: "PBKDF2/SHA-256".to_string(),
+        cost: cfg.cost as u32,
+        expires_at: Some(now + CHALLENGE_TTL_SECONDS),
+        hmac_signature_secret: Some(cfg.hmac_key.clone()),
+        ..Default::default()
+    };
+    create_challenge(opts)
+        .map_err(|e| AppError::Internal(format!("PoW challenge creation failed: {}", e)))
+}
+
+/// Verify a client-submitted solution. `Ok(())` only on a valid, unexpired,
+/// correctly-signed solution.
+pub fn verify_payload(cfg: &PowConfig, payload: &Payload) -> Result<(), AppError> {
+    let opts =
+        VerifySolutionOptions::new(&payload.challenge, &payload.solution, cfg.hmac_key.as_str());
+    let result = verify_solution(opts)
+        .map_err(|e| AppError::ValidationError(format!("PoW verify failed: {}", e)))?;
+    if !result.verified {
+        let reason = if result.expired {
+            "challenge expired (re-fetch the challenge)"
+        } else if result.invalid_signature.unwrap_or(false) {
+            "invalid signature"
+        } else if result.invalid_solution.unwrap_or(false) {
+            "wrong solution"
+        } else {
+            "rejected"
+        };
+        return Err(AppError::ValidationError(format!(
+            "PoW solution rejected ({})",
+            reason
+        )));
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use altcha::{solve_challenge, SolveChallengeOptions};
+
+    fn test_cfg() -> PowConfig {
+        PowConfig {
+            enabled: true,
+            hmac_key: "test-hmac-key-32-bytes-of-secret!".to_string(),
+            required: true,
+            cost: 100, // tiny so tests stay fast
+            required_for_pubkeys: vec![],
+        }
+    }
+
+    fn solve(cfg: &PowConfig) -> Payload {
+        let challenge = issue_challenge(cfg).expect("issue");
+        let solution = solve_challenge(SolveChallengeOptions::new(&challenge))
+            .expect("solve")
+            .expect("solution found");
+        Payload {
+            challenge,
+            solution,
+        }
+    }
+
+    #[test]
+    fn issue_solve_verify_roundtrips() {
+        let cfg = test_cfg();
+        verify_payload(&cfg, &solve(&cfg)).expect("valid solution verifies");
+    }
+
+    #[test]
+    fn tampered_signature_rejected() {
+        let cfg = test_cfg();
+        let mut p = solve(&cfg);
+        p.challenge.signature = Some("ff".repeat(32));
+        assert!(verify_payload(&cfg, &p).is_err());
+    }
+
+    #[test]
+    fn wrong_secret_rejected() {
+        let cfg = test_cfg();
+        let p = solve(&cfg);
+        let attacker = PowConfig {
+            hmac_key: "a-completely-different-secret-key!".to_string(),
+            ..test_cfg()
+        };
+        assert!(verify_payload(&attacker, &p).is_err());
+    }
+
+    #[test]
+    fn solution_not_replayable_across_challenges() {
+        let cfg = test_cfg();
+        let a = solve(&cfg);
+        let b_challenge = issue_challenge(&cfg).expect("issue b");
+        let crossed = Payload {
+            challenge: b_challenge,
+            solution: a.solution,
+        };
+        assert!(verify_payload(&cfg, &crossed).is_err());
+    }
+
+    #[test]
+    fn missing_signature_rejected() {
+        let cfg = test_cfg();
+        let mut p = solve(&cfg);
+        p.challenge.signature = None;
+        assert!(verify_payload(&cfg, &p).is_err());
+    }
+
+    #[test]
+    fn required_for_respects_global_and_per_pubkey() {
+        let mut cfg = PowConfig {
+            required: false,
+            required_for_pubkeys: vec!["abcd1234".to_string()],
+            ..test_cfg()
+        };
+        assert!(required_for(&cfg, "abcd1234"));
+        assert!(!required_for(&cfg, "deadbeef"));
+        cfg.required = true;
+        assert!(required_for(&cfg, "deadbeef"));
+    }
+}
diff --git a/src/core/session.rs b/src/core/session.rs
new file mode 100644
index 0000000..24c1ff6
--- /dev/null
+++ b/src/core/session.rs
@@ -0,0 +1,323 @@
+//! Session and JWT management.
+//!
+//! Access and refresh tokens are HS256 JWTs. Verification is strict: the
+//! algorithm is pinned to HS256 (no `alg` confusion), `exp`/`sub`/`aud` are
+//! required, clock leeway is zero, and access vs refresh tokens carry distinct
+//! audiences so one can never be presented as the other. Refresh tokens are
+//! also tracked in the database (by a peppered hash) for rotation/revocation.
+
+use chrono::{Duration, Utc};
+use hmac::{Hmac, Mac};
+use jsonwebtoken::{decode, encode, Algorithm, DecodingKey, EncodingKey, Header, Validation};
+use serde::{Deserialize, Serialize};
+use sha2::Sha256;
+use uuid::Uuid;
+
+use crate::error::AppError;
+
+/// Audience for access tokens. Distinct from the refresh audience so a refresh
+/// token can never be accepted on an access-protected route, and vice versa.
+const ACCESS_AUD: &str = "smirk:access";
+/// Audience for refresh tokens.
+const REFRESH_AUD: &str = "smirk:refresh";
+
+/// JWT claims for access tokens.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AccessTokenClaims {
+    /// Subject (user ID).
+    pub sub: String,
+    /// Audience (token type).
+    pub aud: String,
+    pub iat: i64,
+    pub exp: i64,
+    /// Client kind: extension | web | nostr.
+    pub platform: String,
+}
+
+/// JWT claims for refresh tokens.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct RefreshTokenClaims {
+    /// Subject (user ID).
+    pub sub: String,
+    /// Audience (token type).
+    pub aud: String,
+    /// Session ID (for revocation).
+    pub sid: String,
+    pub iat: i64,
+    pub exp: i64,
+}
+
+/// Decoded access-token information.
+#[derive(Debug, Clone)]
+pub struct TokenInfo {
+    pub user_id: Uuid,
+    pub platform: String,
+}
+
+/// Session token pair.
+#[derive(Debug, Clone, Serialize)]
+pub struct TokenPair {
+    pub access_token: String,
+    pub refresh_token: String,
+    pub expires_in: i64,
+}
+
+/// Client kind a session was created from.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Platform {
+    Extension,
+    Web,
+    Nostr,
+}
+
+impl Platform {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Platform::Extension => "extension",
+            Platform::Web => "web",
+            Platform::Nostr => "nostr",
+        }
+    }
+}
+
+impl std::fmt::Display for Platform {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.as_str())
+    }
+}
+
+impl std::str::FromStr for Platform {
+    type Err = AppError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.to_lowercase().as_str() {
+            "extension" => Ok(Platform::Extension),
+            "web" => Ok(Platform::Web),
+            "nostr" => Ok(Platform::Nostr),
+            _ => Err(AppError::ValidationError(format!(
+                "Invalid platform: {}",
+                s
+            ))),
+        }
+    }
+}
+
+/// Challenge for website ("Sign in with your wallet") auth.
+#[derive(Debug, Clone)]
+pub struct WebChallenge {
+    pub nonce: String,
+    pub created_at: chrono::DateTime<Utc>,
+    pub origin: String,
+    pub expires_at: chrono::DateTime<Utc>,
+}
+
+impl WebChallenge {
+    pub fn new(origin: String) -> Self {
+        let now = Utc::now();
+        let nonce_bytes: [u8; 32] = rand::random();
+        Self {
+            nonce: hex::encode(nonce_bytes),
+            created_at: now,
+            origin,
+            expires_at: now + Duration::minutes(5),
+        }
+    }
+
+    pub fn is_expired(&self) -> bool {
+        Utc::now() > self.expires_at
+    }
+
+    pub fn message(&self) -> String {
+        format!(
+            "Smirk Authentication Challenge\nNonce: {}\nOrigin: {}\nTimestamp: {}",
+            self.nonce,
+            self.origin,
+            self.created_at.timestamp()
+        )
+    }
+}
+
+/// Creates and verifies session JWTs.
+#[derive(Clone)]
+pub struct SessionManager {
+    encoding_key: EncodingKey,
+    decoding_key: DecodingKey,
+    access_validation: Validation,
+    refresh_validation: Validation,
+    access_token_expiry_hours: i64,
+    refresh_token_expiry_days: i64,
+}
+
+fn strict_validation(audience: &str) -> Validation {
+    let mut v = Validation::new(Algorithm::HS256);
+    v.leeway = 0;
+    v.validate_exp = true;
+    v.set_required_spec_claims(&["exp", "sub", "aud"]);
+    v.set_audience(&[audience]);
+    v
+}
+
+impl SessionManager {
+    /// `jwt_secret` must be >= 32 bytes (enforced by config validation).
+    pub fn new(jwt_secret: &str, access_token_expiry_hours: u64) -> Self {
+        Self {
+            encoding_key: EncodingKey::from_secret(jwt_secret.as_bytes()),
+            decoding_key: DecodingKey::from_secret(jwt_secret.as_bytes()),
+            access_validation: strict_validation(ACCESS_AUD),
+            refresh_validation: strict_validation(REFRESH_AUD),
+            access_token_expiry_hours: access_token_expiry_hours as i64,
+            refresh_token_expiry_days: 30,
+        }
+    }
+
+    /// Mint an access + refresh token pair. `session_id` ties the refresh token
+    /// to a revocable DB session row.
+    pub fn create_token_pair(
+        &self,
+        user_id: Uuid,
+        platform: Platform,
+        session_id: Uuid,
+    ) -> Result<TokenPair, AppError> {
+        let now = Utc::now();
+        let access_exp = now + Duration::hours(self.access_token_expiry_hours);
+        let refresh_exp = now + Duration::days(self.refresh_token_expiry_days);
+        let header = Header::new(Algorithm::HS256);
+
+        let access_claims = AccessTokenClaims {
+            sub: user_id.to_string(),
+            aud: ACCESS_AUD.to_string(),
+            iat: now.timestamp(),
+            exp: access_exp.timestamp(),
+            platform: platform.to_string(),
+        };
+        let access_token = encode(&header, &access_claims, &self.encoding_key)
+            .map_err(|e| AppError::Internal(format!("Failed to create access token: {}", e)))?;
+
+        let refresh_claims = RefreshTokenClaims {
+            sub: user_id.to_string(),
+            aud: REFRESH_AUD.to_string(),
+            sid: session_id.to_string(),
+            iat: now.timestamp(),
+            exp: refresh_exp.timestamp(),
+        };
+        let refresh_token = encode(&header, &refresh_claims, &self.encoding_key)
+            .map_err(|e| AppError::Internal(format!("Failed to create refresh token: {}", e)))?;
+
+        Ok(TokenPair {
+            access_token,
+            refresh_token,
+            expires_in: self.access_token_expiry_hours * 3600,
+        })
+    }
+
+    /// Verify an access token. The error message is a literal (never the inner
+    /// `jsonwebtoken` Display) so it cannot become an auth oracle.
+    pub fn verify_access_token(&self, token: &str) -> Result<TokenInfo, AppError> {
+        let data = decode::<AccessTokenClaims>(token, &self.decoding_key, &self.access_validation)
+            .map_err(|_| AppError::AuthError("Invalid or expired token".into()))?;
+        let user_id = Uuid::parse_str(&data.claims.sub)
+            .map_err(|_| AppError::AuthError("Invalid or expired token".into()))?;
+        Ok(TokenInfo {
+            user_id,
+            platform: data.claims.platform,
+        })
+    }
+
+    /// Verify a refresh token, returning `(user_id, session_id)`.
+    pub fn verify_refresh_token(&self, token: &str) -> Result<(Uuid, Uuid), AppError> {
+        let data =
+            decode::<RefreshTokenClaims>(token, &self.decoding_key, &self.refresh_validation)
+                .map_err(|_| AppError::AuthError("Invalid or expired token".into()))?;
+        let user_id = Uuid::parse_str(&data.claims.sub)
+            .map_err(|_| AppError::AuthError("Invalid or expired token".into()))?;
+        let session_id = Uuid::parse_str(&data.claims.sid)
+            .map_err(|_| AppError::AuthError("Invalid or expired token".into()))?;
+        Ok((user_id, session_id))
+    }
+
+    pub fn refresh_token_expiry(&self) -> Duration {
+        Duration::days(self.refresh_token_expiry_days)
+    }
+}
+
+/// Peppered hash of a refresh token for storage. HMAC-SHA256 with the server
+/// pepper makes a stolen DB row non-reversible and non-correlatable without the
+/// pepper. We store the hash, never the token.
+pub fn hash_refresh_token(token: &str, pepper: &str) -> String {
+    let mut mac =
+        Hmac::<Sha256>::new_from_slice(pepper.as_bytes()).expect("HMAC accepts any key length");
+    mac.update(token.as_bytes());
+    hex::encode(mac.finalize().into_bytes())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SECRET: &str = "test-secret-key-at-least-32-bytes-long!!";
+
+    #[test]
+    fn create_and_verify_access_and_refresh() {
+        let m = SessionManager::new(SECRET, 24);
+        let uid = Uuid::new_v4();
+        let sid = Uuid::new_v4();
+        let pair = m.create_token_pair(uid, Platform::Extension, sid).unwrap();
+
+        let info = m.verify_access_token(&pair.access_token).unwrap();
+        assert_eq!(info.user_id, uid);
+        assert_eq!(info.platform, "extension");
+
+        let (u, s) = m.verify_refresh_token(&pair.refresh_token).unwrap();
+        assert_eq!(u, uid);
+        assert_eq!(s, sid);
+    }
+
+    #[test]
+    fn access_token_is_not_accepted_as_refresh() {
+        let m = SessionManager::new(SECRET, 24);
+        let pair = m
+            .create_token_pair(Uuid::new_v4(), Platform::Web, Uuid::new_v4())
+            .unwrap();
+        // Distinct audiences: an access token must fail refresh validation.
+        assert!(m.verify_refresh_token(&pair.access_token).is_err());
+    }
+
+    #[test]
+    fn refresh_token_is_not_accepted_as_access() {
+        let m = SessionManager::new(SECRET, 24);
+        let pair = m
+            .create_token_pair(Uuid::new_v4(), Platform::Nostr, Uuid::new_v4())
+            .unwrap();
+        assert!(m.verify_access_token(&pair.refresh_token).is_err());
+    }
+
+    #[test]
+    fn tampered_access_token_rejected() {
+        let m = SessionManager::new(SECRET, 24);
+        let pair = m
+            .create_token_pair(Uuid::new_v4(), Platform::Extension, Uuid::new_v4())
+            .unwrap();
+        let mut t = pair.access_token;
+        t.pop();
+        t.push(if t.ends_with('a') { 'b' } else { 'a' });
+        assert!(m.verify_access_token(&t).is_err());
+    }
+
+    #[test]
+    fn peppered_hash_is_deterministic_and_keyed() {
+        let a = hash_refresh_token("token", "pepper-A");
+        assert_eq!(a, hash_refresh_token("token", "pepper-A"));
+        assert_ne!(a, "token");
+        // Different pepper -> different hash (DB compromise without pepper is useless).
+        assert_ne!(a, hash_refresh_token("token", "pepper-B"));
+    }
+
+    #[test]
+    fn platform_roundtrips() {
+        use std::str::FromStr;
+        for p in [Platform::Extension, Platform::Web, Platform::Nostr] {
+            assert_eq!(Platform::from_str(p.as_str()).unwrap(), p);
+        }
+        assert!(Platform::from_str("telegram").is_err());
+    }
+}