Spring Security 7.1 JWT authentication on Spring Boot 4.1

Runnable companion for https://ankurm.com/spring-security-7-1-jwt-authentication-guide/

- login -> token issue -> OncePerRequestFilter -> SecurityContext, end to end
- HS256 and RS256 variants (RS256 publishes a real JWKS endpoint)
- the same API secured by the built-in oauth2ResourceServer().jwt(), for comparison
- 11 documentation chapters under docs/, interlinked with the code
- docs/output/ is real captured output, regenerated by scripts/run-all.sh
- 13 passing tests pinning the 401-vs-403 contract and the CSRF failure

Verified against Spring Boot 4.1.1, Spring Security 7.1.1, JDK 25.0.4.1.

scripts/rs256-demo.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# Requires: --spring.profiles.active=rs256
+set -u
+BASE="${1:-http://localhost:8080}"
+show() {
+  local out; out=$(curl -sS -D /tmp/.h -o /tmp/.b -w '%{http_code}' "$@")
+  printf 'HTTP %s\n' "$out"
+  grep -iE '^www-authenticate:' /tmp/.h | sed 's/\r$//'
+  [ -s /tmp/.b ] && { python3 -m json.tool < /tmp/.b 2>/dev/null || cat /tmp/.b; echo; }
+}
+jwt_part(){ echo "$1" | cut -d. -f$(( $2 + 1 )) | tr '_-' '/+' | sed 's/$/==/' | base64 -d 2>/dev/null | python3 -m json.tool 2>/dev/null; }
+hr(){ printf '\n%s\n' "--------------------------------------------------------------------------"; }
+
+echo "=========================================================================="
+echo " jwt-auth-demo - RS256 variant"
+echo " profiles: rs256   (private key signs, public key / JWKS verifies)"
+echo "=========================================================================="
+
+hr; echo "# 1. The public half, published as a JWK Set. No private material here -"
+echo "#    n and e only. Any number of resource servers can poll this."
+echo; show "$BASE/.well-known/jwks.json"
+
+hr; echo "# 2. Login. Same endpoint, same request, different signature algorithm."
+echo; show -X POST "$BASE/api/auth/login" -H 'Content-Type: application/json' \
+     -d '{"username":"root","password":"root-password"}'
+TOK=$(python3 -c "import json;print(json.load(open('/tmp/.b'))['accessToken'])")
+
+hr; echo "# 3. The JOSE header now carries alg=RS256 and the kid that selects the key."
+echo; jwt_part "$TOK" 0
+
+hr; echo "# 4. Token length. RS256 signatures are 256 bytes; HS256 signatures are 32."
+echo
+printf '  RS256 access token: %s characters\n' "${#TOK}"
+printf '  signature segment : %s characters\n' "$(echo "$TOK" | cut -d. -f3 | wc -c)"
+
+hr; echo "# 5. It works exactly the same from the caller's side."
+echo; show "$BASE/api/me" -H "Authorization: Bearer $TOK"
+
+hr; echo "# 6. Tampered payload, original signature -> 401, same as HS256."
+HDR=$(echo "$TOK" | cut -d. -f1); SIG=$(echo "$TOK" | cut -d. -f3)
+FORGED=$(echo "$TOK" | cut -d. -f2 | tr '_-' '/+' | sed 's/$/==/' | base64 -d 2>/dev/null \
+  | sed 's/"sub":"root"/"sub":"mallory"/' | base64 -w0 | tr '/+' '_-' | tr -d '=')
+echo; show "$BASE/api/me" -H "Authorization: Bearer $HDR.$FORGED.$SIG"
+
+hr; echo "# end"