diff --git a/backend/.env.example b/backend/.env.example
index 24ee6133..48386f43 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -9,3 +9,6 @@ REPUTATION_CONTRACT_ID=TODO_after_deploy
 JOB_REGISTRY_CONTRACT_ID=TODO_after_deploy
 PORT=3001
 RUST_LOG=backend=debug,tower_http=debug
+# SIWS auth
+APP_DOMAIN=localhost:3001
+SESSION_SECRET=TODO_replace_with_32_random_bytes_hex
diff --git a/backend/Cargo.toml b/backend/Cargo.toml
index 91a00219..5f7cd8db 100644
--- a/backend/Cargo.toml
+++ b/backend/Cargo.toml
@@ -27,6 +27,9 @@ bytes        = { workspace = true }
 base64       = "0.22"
 sha2         = "0.10"
 ed25519-dalek = { version = "2", features = ["rand_core"] }
+rand         = "0.8"
+hex          = "0.4"
+base32       = "0.4"
 
 [dev-dependencies]
 axum-test    = "16.0"
diff --git a/backend/src/db.rs b/backend/src/db.rs
index 09602aad..8c2d08dc 100644
--- a/backend/src/db.rs
+++ b/backend/src/db.rs
@@ -1,20 +1,28 @@
 use crate::services::judge::JudgeService;
 use crate::services::stellar::StellarService;
 use sqlx::PgPool;
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+
+/// In-memory nonce store: address → (nonce, issued_at).
+/// Nonces are consumed on first use (one-time).
+pub type NonceStore = Arc<Mutex<HashMap<String, (String, String)>>>;
 
 #[derive(Clone)]
 pub struct AppState {
     pub pool: PgPool,
-    pub judge: std::sync::Arc<JudgeService>,
-    pub stellar: std::sync::Arc<StellarService>,
+    pub judge: Arc<JudgeService>,
+    pub stellar: Arc<StellarService>,
+    pub nonces: NonceStore,
 }
 
 impl AppState {
     pub fn new(pool: PgPool) -> Self {
         Self {
             pool,
-            judge: std::sync::Arc::new(JudgeService::from_env()),
-            stellar: std::sync::Arc::new(StellarService::from_env()),
+            judge: Arc::new(JudgeService::from_env()),
+            stellar: Arc::new(StellarService::from_env()),
+            nonces: Arc::new(Mutex::new(HashMap::new())),
         }
     }
 }
diff --git a/backend/src/routes/auth.rs b/backend/src/routes/auth.rs
new file mode 100644
index 00000000..3444ce38
--- /dev/null
+++ b/backend/src/routes/auth.rs
@@ -0,0 +1,100 @@
+//! SIWS authentication routes.
+//!
+//! POST /api/v1/auth/nonce  — issue a one-time nonce for an address
+//! POST /api/v1/auth/verify — verify a signed SIWS message, return a session token
+
+use axum::{extract::State, Json};
+use chrono::Utc;
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    db::AppState,
+    error::{AppError, Result},
+    services::siws,
+};
+
+// ── Request / Response types ─────────────────────────────────────────────────
+
+#[derive(Deserialize)]
+pub struct NonceRequest {
+    pub address: String,
+}
+
+#[derive(Serialize)]
+pub struct NonceResponse {
+    pub nonce: String,
+    pub issued_at: String,
+}
+
+#[derive(Deserialize)]
+pub struct VerifyRequest {
+    pub address: String,
+    pub signature: String, // hex-encoded 64-byte ed25519 signature
+}
+
+#[derive(Serialize)]
+pub struct VerifyResponse {
+    pub token: String, // opaque session token (hex-encoded random bytes)
+}
+
+// ── Handlers ─────────────────────────────────────────────────────────────────
+
+/// Issue a fresh nonce for the given Stellar address.
+/// Replaces any previously stored nonce for that address.
+pub async fn nonce(
+    State(state): State<AppState>,
+    Json(req): Json<NonceRequest>,
+) -> Result<Json<NonceResponse>> {
+    if req.address.is_empty() {
+        return Err(AppError::BadRequest("address is required".into()));
+    }
+
+    let nonce = siws::generate_nonce();
+    let issued_at = Utc::now().to_rfc3339();
+
+    state
+        .nonces
+        .lock()
+        .unwrap()
+        .insert(req.address.clone(), (nonce.clone(), issued_at.clone()));
+
+    Ok(Json(NonceResponse { nonce, issued_at }))
+}
+
+/// Verify a SIWS signature.
+/// Consumes the stored nonce (one-time use) and returns a session token on success.
+pub async fn verify(
+    State(state): State<AppState>,
+    Json(req): Json<VerifyRequest>,
+) -> Result<Json<VerifyResponse>> {
+    // Consume the nonce — remove it regardless of outcome to prevent replay.
+    let entry = state
+        .nonces
+        .lock()
+        .unwrap()
+        .remove(&req.address);
+
+    let (nonce, issued_at) = entry
+        .ok_or_else(|| AppError::BadRequest("no pending nonce for this address".into()))?;
+
+    let domain = std::env::var("APP_DOMAIN").unwrap_or_else(|_| "lance.app".into());
+    let message = siws::build_message(&domain, &req.address, &nonce, &issued_at);
+
+    siws::verify(&req.address, &message, &req.signature)
+        .map_err(|e| AppError::BadRequest(e.to_string()))?;
+
+    // Issue a simple random session token.
+    // In production, replace with a signed JWT or encrypted cookie.
+    let token = siws::generate_nonce(); // reuse the 32-byte random hex generator
+
+    Ok(Json(VerifyResponse { token }))
+}
+
+// ── Router ───────────────────────────────────────────────────────────────────
+
+pub fn router() -> axum::Router<AppState> {
+    use axum::routing::post;
+    axum::Router::new()
+        .route("/nonce", post(nonce))
+        .route("/verify", post(verify))
+}
diff --git a/backend/src/routes/mod.rs b/backend/src/routes/mod.rs
index 233b820c..c5ba9a84 100644
--- a/backend/src/routes/mod.rs
+++ b/backend/src/routes/mod.rs
@@ -1,4 +1,5 @@
 pub mod appeals;
+pub mod auth;
 pub mod bids;
 pub mod deliverables;
 pub mod disputes;
@@ -15,12 +16,11 @@ use axum::{routing::get, Router};
 
 pub fn api_router() -> Router<AppState> {
     Router::new()
-        // health check — outside versioned prefix so load balancers can reach it
         .route("/health", get(health::health))
-        // v1 API routes
         .nest(
             "/v1",
             Router::new()
+                .nest("/auth", auth::router())
                 .nest("/jobs", jobs::router())
                 .nest("/disputes", disputes::router())
                 .nest("/appeals", appeals::router())
diff --git a/backend/src/services/mod.rs b/backend/src/services/mod.rs
index f99224cc..19d2beaf 100644
--- a/backend/src/services/mod.rs
+++ b/backend/src/services/mod.rs
@@ -1,3 +1,4 @@
 pub mod ipfs;
 pub mod judge;
+pub mod siws;
 pub mod stellar;
diff --git a/backend/src/services/siws.rs b/backend/src/services/siws.rs
new file mode 100644
index 00000000..93ffe4f5
--- /dev/null
+++ b/backend/src/services/siws.rs
@@ -0,0 +1,112 @@
+//! Sign-In With Stellar (SIWS) — nonce generation and signature verification.
+//!
+//! Message format (plain-text, signed by the wallet):
+//! ```
+//! <domain> wants you to sign in with your Stellar account:
+//! <address>
+//!
+//! Nonce: <hex-nonce>
+//! Issued At: <iso8601>
+//! ```
+//!
+//! The wallet signs the UTF-8 bytes of this message with its ed25519 key.
+//! The backend verifies the signature against the public key encoded in the
+//! Stellar G-address (strkey, version byte 6 << 3 = 0x30).
+
+use ed25519_dalek::{Signature, VerifyingKey};
+use rand::RngCore;
+
+const STRKEY_VERSION_ACCOUNT: u8 = 6 << 3; // 0x30
+
+/// Generate a cryptographically random 32-byte hex nonce.
+pub fn generate_nonce() -> String {
+    let mut bytes = [0u8; 32];
+    rand::thread_rng().fill_bytes(&mut bytes);
+    hex::encode(bytes)
+}
+
+/// Build the canonical SIWS message that the wallet must sign.
+pub fn build_message(domain: &str, address: &str, nonce: &str, issued_at: &str) -> String {
+    format!(
+        "{domain} wants you to sign in with your Stellar account:\n\
+         {address}\n\
+         \n\
+         Nonce: {nonce}\n\
+         Issued At: {issued_at}"
+    )
+}
+
+/// Decode a Stellar G-address into its raw 32-byte ed25519 public key.
+fn decode_stellar_address(address: &str) -> anyhow::Result<[u8; 32]> {
+    // Stellar strkey: base32(version_byte || payload || crc16)
+    let decoded = base32::decode(base32::Alphabet::RFC4648 { padding: false }, address)
+        .ok_or_else(|| anyhow::anyhow!("invalid base32 in address"))?;
+
+    // minimum: 1 version + 32 payload + 2 crc = 35 bytes
+    if decoded.len() < 35 {
+        anyhow::bail!("address too short");
+    }
+    if decoded[0] != STRKEY_VERSION_ACCOUNT {
+        anyhow::bail!("not an account address (wrong version byte)");
+    }
+
+    let payload = &decoded[1..decoded.len() - 2];
+    if payload.len() != 32 {
+        anyhow::bail!("unexpected payload length");
+    }
+
+    let mut key = [0u8; 32];
+    key.copy_from_slice(payload);
+    Ok(key)
+}
+
+/// Verify a SIWS signature.
+///
+/// * `address`   — Stellar G-address of the signer
+/// * `message`   — the canonical SIWS message (as produced by [`build_message`])
+/// * `signature` — hex-encoded 64-byte ed25519 signature
+pub fn verify(address: &str, message: &str, signature_hex: &str) -> anyhow::Result<()> {
+    let key_bytes = decode_stellar_address(address)?;
+    let verifying_key = VerifyingKey::from_bytes(&key_bytes)
+        .map_err(|e| anyhow::anyhow!("invalid public key: {e}"))?;
+
+    let sig_bytes = hex::decode(signature_hex)
+        .map_err(|_| anyhow::anyhow!("signature is not valid hex"))?;
+    if sig_bytes.len() != 64 {
+        anyhow::bail!("signature must be 64 bytes");
+    }
+    let mut sig_arr = [0u8; 64];
+    sig_arr.copy_from_slice(&sig_bytes);
+    let signature = Signature::from_bytes(&sig_arr);
+
+    verifying_key
+        .verify_strict(message.as_bytes(), &signature)
+        .map_err(|_| anyhow::anyhow!("signature verification failed"))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn nonce_is_64_hex_chars() {
+        let n = generate_nonce();
+        assert_eq!(n.len(), 64);
+        assert!(n.chars().all(|c| c.is_ascii_hexdigit()));
+    }
+
+    #[test]
+    fn message_contains_expected_fields() {
+        let msg = build_message("example.com", "GABC", "deadbeef", "2026-01-01T00:00:00Z");
+        assert!(msg.contains("example.com wants you to sign in"));
+        assert!(msg.contains("GABC"));
+        assert!(msg.contains("Nonce: deadbeef"));
+        assert!(msg.contains("Issued At: 2026-01-01T00:00:00Z"));
+    }
+
+    #[test]
+    fn rejects_bad_signature_hex() {
+        let result = verify("GABC", "msg", "not-hex");
+        assert!(result.is_err());
+    }
+}