diff --git a/AGENT.MD b/AGENT.MD
index a5ea6d0..b645726 100644
--- a/AGENT.MD
+++ b/AGENT.MD
@@ -67,3 +67,14 @@ STATUS.md            完成度总表    AGENT.md 本文件
 - 补全(2026-08-14):`pssh ssh -W host:port`（stdio/netcat 转发,目标 TCP 隧道桥接本地 stdio）、`-N`（仅端口转发不执行远程命令,配 `-L/-R/-D`）。
 
 已知限制:`-J` 接受逗号分隔的多跳语法,但多级级联跳板在同步 I/O 架构下的应用数据阶段交互不稳定,实测多跳握手可完成但握手后易卡;故当前对多跳打印警告并**退化为仅第一跳的稳定单跳**。单跳与 `-W`/`-N` 均已实测验证。
+
+### 4. secp521r1（TLS P-521）（✅ 已完成 2026-08-14）
+
+复用 P-384 全链路套路新增 P-521：
+- 曲线 `src/crypto/p521.c` + `include/paze/p521.h`（域 p=2^521-1,标量/仿射点 66/133 字节,常量时间标量乘;注意 P-521 非整字节对齐,标量乘须按大端位 `byte=65-t/8, bit=t&7` 遍历）。
+- ECDSA:`ecdsa.c/h` 增 `paze_ecdsa_p521_sign/verify`(RFC 6979,配 SHA-512)与 RFC 5915 私钥解析。
+- X.509:`x509.c/h` 支持 secp521r1 证书解析/验签/签发(`ecdsa_p521_pub[133]`、`build_spki521`、signer `ecdsa_d521[66]`、ecdsa-with-SHA512)。
+- TLS:`PAZE_TLS_GROUP_SECP521R1(0x0019)` / `PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512(0x0603)`,kex 增 `P521`,ECDH 按 SEC1 x 坐标 66 字节,supported_groups/key_share/CertificateVerify/HRR/TLS 1.2 ECDHE 均支持。
+- CLI:`s_client -key_share P-521`。
+- 验证:曲线(n·G=∞、3G=2G+G)、ECDSA 双向、X.509 证书 OpenSSL verify=OK、TLS 端点(密钥交换 hrr=YES+证书链/主机名验证)全部通过;既有自测无回归。
+
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28a1238..7ff8188 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,20 @@
 
 本项目 (PazeSSL / PazeSSH) 的变更记录。格式参考 [Keep a Changelog](https://keepachangelog.com/zh-CN/1.0.0/)；版本号按里程碑独立标注（仓库未打 tag，以日期与功能阶段为准）。
 
+## [开发主线] — 2026-08-14
+
+### 新增 —— TLS secp521r1 (P-521) 完整链路
+
+- 新增 P-521 椭圆曲线：`src/crypto/p521.c` + `include/paze/p521.h`（域 p = 2^521-1，标量/仿射点 66/133 字节，常量时间标量乘；非整字节对齐，标量乘按大端位遍历）。
+- ECDSA 支持 P-521 签名/验签（RFC 6979 确定性 k，配合 SHA-512）与 RFC 5915 私钥解析（`paze_ecdsa_p521_sign/verify`、`paze_ecdsa_p521_priv_from_pem/der`）。
+- X.509 支持 secp521r1 证书解析、验签与签发（`pk_kind==ECDSA_P521`、`ecdsa_p521_pub[133]`、`paze_x509_build_spki521`、签发者 `ecdsa_d521[66]`、ecdsa-with-SHA512）。
+- TLS 1.3 支持 secp521r1 命名组与签名算法：
+  - 组 `0x0019`（`PAZE_TLS_GROUP_SECP521R1`）纳入 key_share / supported_groups / ECDH（按 SEC1 x 坐标，66 字节） / HelloRetryRequest / 服务端组选择。
+  - 签名算法 `PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512 (0x0603)` 纳入 CertificateVerify 签发与验签。
+- TLS 1.2 ECDHE 亦支持 P-521 组。
+- CLI：`pazessl s_client -key_share P-521`。
+- 验证：曲线（n·G=∞、3G=2G+G）、ECDSA 双向（OpenSSL 验我的签名 + 我验 OpenSSL 签名）、X.509 P-521 自签名证书 OpenSSL `verify=OK`、TLS 端点 P-521 密钥交换（hrr=YES）+ 证书链/主机名验证全部通过；既有自测无回归。
+
 ## [开发主线] — 2026-08-13
 
 ### 新增 —— TLS secp384r1 (P-384) 完整链路
diff --git a/STATUS.md b/STATUS.md
index 2a0bcec..4da2c88 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -16,7 +16,7 @@
 
 - 哈希：SHA-1 / SHA-256 / SHA-512 / MD5 / HMAC / HKDF / PBKDF2
 - 对称：AES-128/192/256、AES-CBC/GCM、ChaCha20、Poly1305、ChaCha20-Poly1305 AEAD
-- 曲线：X25519、Ed25519、**P-256、P-384（secp384r1）**、ECDSA（大数运算自研：模幂、模逆、Montgomery）
+- 曲线：X25519、Ed25519、**P-256、P-384（secp384r1）、P-521（secp521r1）**、ECDSA（大数运算自研：模幂、模逆、Montgomery）
 - 非对称：RSA（密钥生成/加解密/签名）、X.509 证书解析与构建、PEM/DER 编解码
 - 其他：ASN.1 DER、base64、hex、常量时间准则
 
@@ -32,6 +32,7 @@
 - ✅ **0-RTT 数据路径**（RFC 8446 §4.2.10/§4.5/§8）：early_data 扩展、client_early_traffic_secret 密钥派生、握手前早数据收发、EndOfEarlyData（early key 加密且计入 transcript）；`pazessl s_client/s_server -early` 端到端验证（早数据接收 + early accepted）
 - ✅ **HRR（HelloRetryRequest）**（RFC 8446 §4.1.3/§4.1.4）：服务端 key_share 不匹配时发 HRR（随机数=固定魔数）、transcript 合成 MessageHash 重放、客户端 CH2（仅选定组 share）响应、HRR 后放弃 0-RTT；`-key_share` 选项触发，`hrr=YES` 互操作验证通过
 - ✅ **secp384r1（TLS P-384）**：新增 P-384 曲线点运算（`p384.c`，Jacobian，常量时间标量乘）；ECDSA 支持 P-384 签名/验签（RFC 6979 确定性 k,`paze_ecdsa_p384_*`）；X.509 支持 secp384r1 证书解析/验签/签发（含 ecdsa-with-SHA384 与 P-384 公钥,`paze_x509_build_spki384`）；TLS 1.3 支持 secp384r1 组(key_share/supported_groups/ECDH 按 SEC1 x 坐标)、签名算法 ECDSA_SECP384R1_SHA384；TLS 1.2 ECDHE 亦支持 P-384。`s_client -key_share P-384` 端到端验证（含 HRR 触发与 P-384 证书链/主机名验证）；曲线与 ECDSA 经 OpenSSL 双向交叉验证
+- ✅ **secp521r1（TLS P-521）**：新增 P-521 曲线点运算（`p521.c`，域 p=2^521-1，标量/点 66/133 字节，常量时间标量乘）；ECDSA 支持 P-521 签名/验签（RFC 6979，配 SHA-512，`paze_ecdsa_p521_*`）；X.509 支持 secp521r1 证书解析/验签/签发（含 ecdsa-with-SHA512,`paze_x509_build_spki521`）；TLS 1.3 支持 secp521r1 组与签名算法 ECDSA_SECP521R1_SHA512（0x0603），TLS 1.2 ECDHE 亦支持。`s_client -key_share P-521` 端到端验证（HRR + P-521 证书链/主机名验证，应用数据收发）；曲线(n·G=∞)、ECDSA 双向、X.509 证书均经 OpenSSL 交叉验证
 - ✅ **修复**：ECDSA `bits2int` 输入短于群阶时错误左移（影响 P-384+SHA-256 验签）；TLS 1.3/1.2 对端证书 `issuer_dn/subject_dn` 等指针指向被记录层复用缓冲导致的链验证失败（`paze_tls_store_peer_cert` 拷贝 DER）
 
 ## PazeSSH — SSH-2（✅ 完成）
diff --git a/apps/pazessl/main.c b/apps/pazessl/main.c
index a8fcf1e..18683be 100644
--- a/apps/pazessl/main.c
+++ b/apps/pazessl/main.c
@@ -72,7 +72,7 @@ static int usage(void) {
       "PazeSSL v0.1 (libc-only TLS 1.3 + 1.2)\n"
       "Usage:\n"
      "  pazessl s_client  -connect host:port [-CAfile roots.pem] [-servername SNI] [-tls1_2]\n"
-     "                     [-sess_in file] [-sess_out file] [-msg text] [-early] [-key_share X25519|P-256|P-384]\n"
+     "                     [-sess_in file] [-sess_out file] [-msg text] [-early] [-key_share X25519|P-256|P-384|P-521]\n"
      "  pazessl s_server  -accept port -cert cert.pem -key key.pem [-tls1_2] [-reuse N] [-early]\n"
       "  pazessl verify    -CAfile roots.pem cert.pem\n"
       "  pazessl genrsa    -out key.pem [2048]\n"
@@ -164,7 +164,8 @@ int main(int argc, char **argv) {
             if (strcmp(key_share, "X25519") == 0) g = PAZE_TLS_GROUP_X25519;
             else if (strcmp(key_share, "P-256") == 0) g = PAZE_TLS_GROUP_SECP256R1;
             else if (strcmp(key_share, "P-384") == 0) g = PAZE_TLS_GROUP_SECP384R1;
-            else { fprintf(stderr, "bad -key_share (X25519|P-256|P-384)\n"); return 1; }
+            else if (strcmp(key_share, "P-521") == 0) g = PAZE_TLS_GROUP_SECP521R1;
+            else { fprintf(stderr, "bad -key_share (X25519|P-256|P-384|P-521)\n"); return 1; }
             if (paze_tls_config_set_kex_group(cfg, g) != PAZE_OK) return 1;
         }
         if (sni) paze_tls_config_set_sni(cfg, sni);
diff --git a/include/paze/ecdsa.h b/include/paze/ecdsa.h
index 2aff97d..3e92a27 100644
--- a/include/paze/ecdsa.h
+++ b/include/paze/ecdsa.h
@@ -83,6 +83,21 @@ paze_status_t paze_ecdsa_p384_priv_from_der(uint8_t d384[48],
                                             const uint8_t *der, size_t len);
 paze_status_t paze_ecdsa_p384_priv_from_pem(uint8_t d384[48], const char *pem_text);
 
+/* P-521 (secp521r1) 签名/验签(确定 k, RFC 6979;配 ecdsa_secp521r1_SHA512 用
+ * PAZE_RSA_HASH_SHA512)。d_bytes 66 字节大端, pub_affine 133 字节 0x04||X||Y。 */
+paze_status_t paze_ecdsa_p521_sign(const uint8_t d_bytes[66],
+                                   paze_rsa_hash_t hash_alg,
+                                   const uint8_t *mhash, size_t hlen,
+                                   uint8_t *sig, size_t *siglen);
+paze_status_t paze_ecdsa_p521_verify(const uint8_t pub_affine[133],
+                                     paze_rsa_hash_t hash_alg,
+                                     const uint8_t *mhash, size_t hlen,
+                                     const uint8_t *sig, size_t siglen);
+/* P-521 (RFC 5915) EC PRIVATE KEY 解析:校验 secp521r1,写 66 字节标量。 */
+paze_status_t paze_ecdsa_p521_priv_from_der(uint8_t d521[66],
+                                            const uint8_t *der, size_t len);
+paze_status_t paze_ecdsa_p521_priv_from_pem(uint8_t d521[66], const char *pem_text);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/include/paze/p521.h b/include/paze/p521.h
new file mode 100644
index 0000000..d06e40c
--- /dev/null
+++ b/include/paze/p521.h
@@ -0,0 +1,54 @@
+/* p521.h —— NIST P-521 (secp521r1) 椭圆曲线
+ *
+ * 域算术复用 bignum (mod p),点运算用 Jacobian 坐标,公式与 p256/p384 相同。
+ * 曲线: y^2 = x^3 - 3x + b (mod p),  p = 2^521 - 1
+ * 域元素/标量 66 字节,未压缩仿射点 133 字节 (0x04 || X || Y)。
+ * 与 p256.h/p384.h 接口保持一致。
+ */
+#ifndef PAZE_P521_H
+#define PAZE_P521_H
+
+#include "paze/paze_types.h"
+#include "paze/bignum.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Jacobian 坐标点:(X:Y:Z),Z=0 表示无穷远。 */
+typedef struct {
+    paze_bn_t X, Y, Z;
+} paze_p521_point_t;
+
+/* 域/群阶常量(大端 66 字节) */
+extern const uint8_t paze_p521_p_be[66];
+extern const uint8_t paze_p521_n_be[66];
+extern const uint8_t paze_p521_gx_be[66];
+extern const uint8_t paze_p521_gy_be[66];
+
+void paze_p521_load_p(paze_bn_t *p);
+void paze_p521_load_n(paze_bn_t *n);
+void paze_p521_load_generator(paze_p521_point_t *G);
+
+void paze_p521_set_inf(paze_p521_point_t *P);
+int  paze_p521_is_inf(const paze_p521_point_t *P);
+void paze_p521_copy(paze_p521_point_t *dst, const paze_p521_point_t *src);
+
+paze_status_t paze_p521_from_affine(paze_p521_point_t *P, const uint8_t *in, size_t len);
+paze_status_t paze_p521_to_affine(const paze_p521_point_t *P, uint8_t *out, size_t len);
+
+paze_status_t paze_p521_add(paze_p521_point_t *R,
+                            const paze_p521_point_t *P,
+                            const paze_p521_point_t *Q);
+paze_status_t paze_p521_double(paze_p521_point_t *R, const paze_p521_point_t *P);
+
+paze_status_t paze_p521_scalar_mult(paze_p521_point_t *R,
+                                    const uint8_t k_be[66],
+                                    const paze_p521_point_t *P);
+
+paze_status_t paze_p521_get_x_mod_n(paze_bn_t *x, const paze_p521_point_t *P);
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* PAZE_P521_H */
diff --git a/include/paze/tls.h b/include/paze/tls.h
index e7eaa08..22d0f8c 100644
--- a/include/paze/tls.h
+++ b/include/paze/tls.h
@@ -50,11 +50,13 @@ extern "C" {
 /* 命名组 (NamedGroup, key exchange) */
 #define PAZE_TLS_GROUP_SECP256R1  0x0017
 #define PAZE_TLS_GROUP_SECP384R1  0x0018
+#define PAZE_TLS_GROUP_SECP521R1  0x0019
 #define PAZE_TLS_GROUP_X25519     0x001D
 
 /* 签名算法 (SignatureScheme) */
 #define PAZE_TLS_SIG_ECDSA_SECP256R1_SHA256 0x0403
 #define PAZE_TLS_SIG_ECDSA_SECP384R1_SHA384 0x0503
+#define PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512 0x0603
 #define PAZE_TLS_SIG_RSA_PSS_RSAE_SHA256    0x0804
 #define PAZE_TLS_SIG_RSA_PSS_RSAE_SHA384    0x0805
 #define PAZE_TLS_SIG_RSA_PKCS1_SHA256       0x0401
diff --git a/include/paze/x509.h b/include/paze/x509.h
index f1b3c44..9d08a5c 100644
--- a/include/paze/x509.h
+++ b/include/paze/x509.h
@@ -34,7 +34,8 @@ typedef enum {
     PAZE_X509_PK_UNKNOWN    = 0,
     PAZE_X509_PK_RSA        = 1,
     PAZE_X509_PK_ECDSA_P256 = 2,
-    PAZE_X509_PK_ECDSA_P384 = 3
+    PAZE_X509_PK_ECDSA_P384 = 3,
+    PAZE_X509_PK_ECDSA_P521 = 4
 } paze_x509_pk_kind_t;
 
 /* 证书签名算法 */
@@ -45,7 +46,8 @@ typedef enum {
     PAZE_X509_SIG_SHA384_RSA   = 3,
     PAZE_X509_SIG_SHA512_RSA   = 4,
     PAZE_X509_SIG_ECDSA_SHA256 = 5,  /* P-256 */
-    PAZE_X509_SIG_ECDSA_SHA384 = 6   /* P-384 */
+    PAZE_X509_SIG_ECDSA_SHA384 = 6,  /* P-384 */
+    PAZE_X509_SIG_ECDSA_SHA512 = 7   /* P-521 */
 } paze_x509_sig_alg_t;
 
 #define PAZE_X509_SAN_MAX 8
@@ -69,6 +71,8 @@ typedef struct {
     paze_ecdsa_pub_t ecdsa_pub;     /* pk_kind==ECDSA_P256 时有效 */
     /* pk_kind==ECDSA_P384 时,主体公钥为未压缩点 0x04||X||Y (97 字节) */
     uint8_t          ecdsa_p384_pub[97];
+    /* pk_kind==ECDSA_P521 时,主体公钥为未压缩点 0x04||X||Y (133 字节) */
+    uint8_t          ecdsa_p521_pub[133];
 
     /* 签名值(BIT STRING 内容,已去掉 unused-bits 前导 0x00) */
     const uint8_t *signature; size_t signature_len;
@@ -138,6 +142,7 @@ typedef struct {
     paze_rsa_priv_t rsa;        /* kind==RSA 时有效 */
     uint8_t ecdsa_d[32];        /* kind==ECDSA_P256 时,私钥标量大端 */
     uint8_t ecdsa_d384[48];     /* kind==ECDSA_P384 时,私钥标量大端 */
+    uint8_t ecdsa_d521[66];     /* kind==ECDSA_P521 时,私钥标量大端 */
 } paze_x509_signer_t;
 
 /* 构造 SubjectPublicKeyInfo DER。
@@ -150,6 +155,10 @@ paze_status_t paze_x509_build_spki(uint8_t *out, size_t *outlen,
  * ecPublicKey + secp384r1 + pub_affine(97 字节 0x04||X||Y) */
 paze_status_t paze_x509_build_spki384(uint8_t *out, size_t *outlen,
                                       const uint8_t pub_affine[97]);
+/* 构造 secp521r1 的 SubjectPublicKeyInfo DER:
+ * ecPublicKey + secp521r1 + pub_affine(133 字节 0x04||X||Y) */
+paze_status_t paze_x509_build_spki521(uint8_t *out, size_t *outlen,
+                                      const uint8_t pub_affine[133]);
 
 /* 构造仅含 commonName 的 Name DER (UTF8String)。 */
 paze_status_t paze_x509_build_dn_cn(uint8_t *out, size_t *outlen, const char *cn);
diff --git a/src/crypto/ecdsa.c b/src/crypto/ecdsa.c
index c3d2a44..4dd42da 100644
--- a/src/crypto/ecdsa.c
+++ b/src/crypto/ecdsa.c
@@ -12,6 +12,7 @@
  */
 #include "paze/ecdsa.h"
 #include "paze/p384.h"
+#include "paze/p521.h"
 #include "paze/hmac.h"
 #include "paze/hash.h"
 #include "paze/asn1.h"
@@ -52,22 +53,35 @@ static void int2octets(uint8_t *out, size_t outlen, const paze_bn_t *x) {
 }
 
 /* ============================================================
- * 曲线抽象:p256 / p384 的差异点提取为这组小助手。
- * 点(paze_bn_t X,Y,Z)两种曲线 ABI 相同,用 paze_bn_t[3] 承载。
+ * 曲线抽象:p256 / p384 / p521 的差异点提取为这组小助手。
+ * 点(paze_bn_t X,Y,Z)三种曲线 ABI 相同,用 paze_bn_t[3] 承载。
  * ============================================================ */
 
-typedef enum { ECDSA_CURVE_P256, ECDSA_CURVE_P384 } curve_t;
+typedef enum { ECDSA_CURVE_P256, ECDSA_CURVE_P384, ECDSA_CURVE_P521 } curve_t;
 
 static void curve_load_n(curve_t c, paze_bn_t *n) {
-    if (c == ECDSA_CURVE_P384) paze_p384_load_n(n);
+    if (c == ECDSA_CURVE_P521) paze_p521_load_n(n);
+    else if (c == ECDSA_CURVE_P384) paze_p384_load_n(n);
     else                       paze_p256_load_n(n);
 }
-static int curve_scalar_len(curve_t c) { return (c == ECDSA_CURVE_P384) ? 48 : 32; }
-static int curve_pub_len(curve_t c)    { return (c == ECDSA_CURVE_P384) ? 97 : 65; }
+static int curve_scalar_len(curve_t c) {
+    return (c == ECDSA_CURVE_P521) ? 66 : ((c == ECDSA_CURVE_P384) ? 48 : 32);
+}
+static int curve_pub_len(curve_t c) {
+    return (c == ECDSA_CURVE_P521) ? 133 : ((c == ECDSA_CURVE_P384) ? 97 : 65);
+}
 
 /* R = k*G;若 P!=NULL 则 R = k*P。pt 为 3 个 paze_bn_t 的数组。 */
 static paze_status_t curve_point_mult(curve_t c, paze_bn_t pt[3],
                                       const uint8_t *k, const paze_bn_t P[3]) {
+    if (c == ECDSA_CURVE_P521) {
+        paze_p521_point_t R, base;
+        if (P) { paze_bn_copy(&base.X, &P[0]); paze_bn_copy(&base.Y, &P[1]); paze_bn_copy(&base.Z, &P[2]); }
+        paze_status_t st = paze_p521_scalar_mult(&R, k, P ? &base : NULL);
+        if (st != PAZE_OK) return st;
+        paze_bn_copy(&pt[0], &R.X); paze_bn_copy(&pt[1], &R.Y); paze_bn_copy(&pt[2], &R.Z);
+        return PAZE_OK;
+    }
     if (c == ECDSA_CURVE_P384) {
         paze_p384_point_t R, base;
         if (P) { paze_bn_copy(&base.X, &P[0]); paze_bn_copy(&base.Y, &P[1]); paze_bn_copy(&base.Z, &P[2]); }
@@ -87,7 +101,13 @@ static paze_status_t curve_point_mult(curve_t c, paze_bn_t pt[3],
 /* R = P1 + P2 (两个 3-bn 点) */
 static void curve_point_add(curve_t c, paze_bn_t R[3],
                             const paze_bn_t P1[3], const paze_bn_t P2[3]) {
-    if (c == ECDSA_CURVE_P384) {
+    if (c == ECDSA_CURVE_P521) {
+        paze_p521_point_t a, b, r;
+        a.X = P1[0]; a.Y = P1[1]; a.Z = P1[2];
+        b.X = P2[0]; b.Y = P2[1]; b.Z = P2[2];
+        paze_p521_add(&r, &a, &b);
+        R[0] = r.X; R[1] = r.Y; R[2] = r.Z;
+    } else if (c == ECDSA_CURVE_P384) {
         paze_p384_point_t a, b, r;
         a.X = P1[0]; a.Y = P1[1]; a.Z = P1[2];
         b.X = P2[0]; b.Y = P2[1]; b.Z = P2[2];
@@ -108,6 +128,11 @@ static int curve_point_is_inf(curve_t c, const paze_bn_t P[3]) {
 }
 
 static paze_status_t curve_point_x_mod_n(curve_t c, paze_bn_t *x, const paze_bn_t P[3]) {
+    if (c == ECDSA_CURVE_P521) {
+        paze_p521_point_t pt;
+        pt.X = P[0]; pt.Y = P[1]; pt.Z = P[2];
+        return paze_p521_get_x_mod_n(x, &pt);
+    }
     if (c == ECDSA_CURVE_P384) {
         paze_p384_point_t pt;
         pt.X = P[0]; pt.Y = P[1]; pt.Z = P[2];
@@ -122,6 +147,13 @@ static paze_status_t curve_point_x_mod_n(curve_t c, paze_bn_t *x, const paze_bn_
 static paze_status_t curve_point_from_affine(curve_t c, paze_bn_t P[3],
                                              const uint8_t *in, size_t len) {
     (void)len;
+    if (c == ECDSA_CURVE_P521) {
+        paze_p521_point_t pt;
+        paze_status_t st = paze_p521_from_affine(&pt, in, 133);
+        if (st != PAZE_OK) return st;
+        P[0] = pt.X; P[1] = pt.Y; P[2] = pt.Z;
+        return PAZE_OK;
+    }
     if (c == ECDSA_CURVE_P384) {
         paze_p384_point_t pt;
         paze_status_t st = paze_p384_from_affine(&pt, in, 97);
@@ -155,14 +187,14 @@ static paze_status_t rfc6979_k(curve_t c, uint8_t *k_be,
     int slen = curve_scalar_len(c);
 
     /* int2octets(d): 直接拷贝已定长大端标量 */
-    uint8_t x_oct[48];
+    uint8_t x_oct[66];
     memcpy(x_oct, d_bytes, (size_t)slen);
 
     /* bits2octets(h1): z1 = bits2int(h1); z2 = z1 mod n; int2octets(z2, slen) */
     paze_bn_t z1, z2;
     bits2int(&z1, h1, h1len, qlen_bits);
     paze_bn_mod(&z2, &z1, &n);
-    uint8_t h1_oct[48];
+    uint8_t h1_oct[66];
     int2octets(h1_oct, (size_t)slen, &z2);
 
     /* V = 0x01 * hlen, K = 0x00 * hlen */
@@ -170,8 +202,10 @@ static paze_status_t rfc6979_k(curve_t c, uint8_t *k_be,
     memset(V, 0x01, hlen);
     memset(K, 0x00, hlen);
 
-    uint8_t buf[64 + 1 + 48 + 48];
-    uint8_t T[64];
+    /* buf 需容纳 hlen+1+slen+slen(最大 SHA-512=64 + 1 + 66 + 66 = 197);
+     * T 需容纳 |qlen/8| 的连续 HMAC 块(P-521 用 SHA-512 时 2 轮 = 128 字节)。 */
+    uint8_t buf[197];
+    uint8_t T[132];
 
     /* K = HMAC_K(V || 0x00 || x || h1_oct) */
     memcpy(buf, V, hlen); buf[hlen] = 0x00;
@@ -305,7 +339,7 @@ static paze_status_t ecdsa_sign_internal(curve_t c,
     bits2int(&z, mhash, hlen, paze_bn_bit_len(&n));
 
     paze_bn_t r, s;
-    uint8_t k_be[48];
+    uint8_t k_be[66];
     paze_status_t rc;
 
     int got = 0;
@@ -383,7 +417,7 @@ static paze_status_t ecdsa_verify_internal(curve_t c,
     paze_bn_modmul(&u2, r, &w, &n);
 
     int slen = curve_scalar_len(c);
-    uint8_t u1_be[48], u2_be[48];
+    uint8_t u1_be[66], u2_be[66];
     int2octets(u1_be, (size_t)slen, &u1);
     int2octets(u2_be, (size_t)slen, &u2);
 
@@ -444,6 +478,29 @@ paze_status_t paze_ecdsa_p384_verify(const uint8_t pub_affine[97],
                                  mhash, hlen, &r, &s);
 }
 
+paze_status_t paze_ecdsa_p521_sign(const uint8_t d_bytes[66],
+                                   paze_rsa_hash_t hash_alg,
+                                   const uint8_t *mhash, size_t hlen,
+                                   uint8_t *sig, size_t *siglen) {
+    if (!sig || !siglen || !d_bytes) return PAZE_ERR_INVALID;
+    paze_bn_t r, s;
+    paze_status_t rc = ecdsa_sign_internal(ECDSA_CURVE_P521, d_bytes, hash_alg,
+                                           mhash, hlen, &r, &s);
+    if (rc != PAZE_OK) return rc;
+    return paze_ecdsa_sig_to_der(&r, &s, sig, siglen);
+}
+
+paze_status_t paze_ecdsa_p521_verify(const uint8_t pub_affine[133],
+                                     paze_rsa_hash_t hash_alg,
+                                     const uint8_t *mhash, size_t hlen,
+                                     const uint8_t *sig, size_t siglen) {
+    if (!sig || !pub_affine) return PAZE_ERR_INVALID;
+    paze_bn_t r, s;
+    if (paze_ecdsa_sig_from_der(&r, &s, sig, siglen) != PAZE_OK) return PAZE_ERR_FORMAT;
+    return ecdsa_verify_internal(ECDSA_CURVE_P521, pub_affine, hash_alg,
+                                 mhash, hlen, &r, &s);
+}
+
 /* ============================================================
  * PEM / DER 私钥与公钥解析 (P-256,既有)
  * ============================================================ */
@@ -575,3 +632,48 @@ paze_status_t paze_ecdsa_p384_priv_from_pem(uint8_t d384[48], const char *pem_te
     free(der);
     return st;
 }
+
+/* ============================================================
+ * P-521 (RFC 5915) 私钥解析:校验 secp521r1,写 66 字节标量。
+ * ============================================================ */
+paze_status_t paze_ecdsa_p521_priv_from_der(uint8_t d521[66],
+                                            const uint8_t *der, size_t len) {
+    static const uint8_t OID_SECP521R1[] = {0x2B, 0x81, 0x04, 0x00, 0x23};
+    if (!d521 || !der) return PAZE_ERR_INVALID;
+    paze_asn1_reader_t r, body;
+    paze_asn1_r_init(&r, der, len);
+    if (paze_asn1_enter_seq(&r, &body) != PAZE_OK) return PAZE_ERR_FORMAT;
+
+    uint8_t tag; const uint8_t *b; size_t blen;
+    if (paze_asn1_read_tl(&body, &tag, &b, &blen) != PAZE_OK) return PAZE_ERR_FORMAT;
+    if (tag != ASN1_INTEGER) return PAZE_ERR_FORMAT;   /* version */
+
+    if (paze_asn1_read_tl(&body, &tag, &b, &blen) != PAZE_OK) return PAZE_ERR_FORMAT;
+    if (tag != ASN1_OCTETSTRING || blen != 66) return PAZE_ERR_FORMAT;   /* 非 P-521 标量 */
+    memcpy(d521, b, 66);
+
+    if (!paze_asn1_at_end(&body)) {
+        paze_asn1_read_tl(&body, &tag, &b, &blen);
+        if (tag == ASN1_CTX0) {
+            if (!paze_asn1_at_end(&body)) {
+                paze_asn1_read_tl(&body, &tag, &b, &blen);
+            }
+            if (tag == ASN1_OID) {
+                if (blen != sizeof(OID_SECP521R1) ||
+                    memcmp(b, OID_SECP521R1, sizeof(OID_SECP521R1)) != 0)
+                    return PAZE_ERR_NOTSUP;
+            }
+        }
+    }
+    return PAZE_OK;
+}
+
+paze_status_t paze_ecdsa_p521_priv_from_pem(uint8_t d521[66], const char *pem_text) {
+    if (!d521 || !pem_text) return PAZE_ERR_INVALID;
+    size_t dlen = 0;
+    uint8_t *der = paze_pem_decode(pem_text, "EC PRIVATE KEY", &dlen);
+    if (!der) return PAZE_ERR_FORMAT;
+    paze_status_t st = paze_ecdsa_p521_priv_from_der(d521, der, dlen);
+    free(der);
+    return st;
+}
diff --git a/src/crypto/p521.c b/src/crypto/p521.c
new file mode 100644
index 0000000..839416f
--- /dev/null
+++ b/src/crypto/p521.c
@@ -0,0 +1,211 @@
+/* p521.c —— NIST P-521 椭圆曲线 (Jacobian 坐标, bignum 域算术)
+ *
+ * 曲线: y^2 = x^3 - 3x + b (mod p),  p = 2^521 - 1
+ * 域元素/标量固定 66 字节大端(最高字节仅 1 位有效),仿射点 133 字节。
+ * 公式 (dbl-2009-l / add-2007-bl, a=-3) 与 p256.c/p384.c 相同。
+ * 常量来源: OpenSSL ecparam -name secp521r1 -param_enc explicit 权威输出。
+ */
+#include "paze/p521.h"
+#include "paze/mem.h"
+#include "paze/error.h"
+#include <string.h>
+
+/* ---- 曲线常量(大端 66 字节; P 最高字节 0x01,低位部分) ---- */
+
+const uint8_t paze_p521_p_be[66] = {
+    0x01,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
+    0xFF,0xFF
+};
+const uint8_t paze_p521_n_be[66] = {
+    0x01,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
+    0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
+    0xff,0xfa,0x51,0x86,0x87,0x83,0xbf,0x2f,0x96,0x6b,0x7f,0xcc,0x01,0x48,0xf7,0x09,
+    0xa5,0xd0,0x3b,0xb5,0xc9,0xb8,0x89,0x9c,0x47,0xae,0xbb,0x6f,0xb7,0x1e,0x91,0x38,
+    0x64,0x09
+};
+const uint8_t paze_p521_gx_be[66] = {
+    0x00,0xC6,0x85,0x8E,0x06,0xB7,0x04,0x04,0xE9,0xCD,0x9E,0x3E,0xCB,0x66,0x23,0x95,
+    0xB4,0x42,0x9C,0x64,0x81,0x39,0x05,0x3F,0xB5,0x21,0xF8,0x28,0xAF,0x60,0x6B,0x4D,
+    0x3D,0xBA,0xA1,0x4B,0x5E,0x77,0xEF,0xE7,0x59,0x28,0xFE,0x1D,0xC1,0x27,0xA2,0xFF,
+    0xA8,0xDE,0x33,0x48,0xB3,0xC1,0x85,0x6A,0x42,0x9B,0xF9,0x7E,0x7E,0x31,0xC2,0xE5,
+    0xBD,0x66
+};
+const uint8_t paze_p521_gy_be[66] = {
+    0x01,0x18,0x39,0x29,0x6A,0x78,0x9A,0x3B,0xC0,0x04,0x5C,0x8A,0x5F,0xB4,0x2C,0x7D,
+    0x1B,0xD9,0x98,0xF5,0x44,0x49,0x57,0x9B,0x44,0x68,0x17,0xAF,0xBD,0x17,0x27,0x3E,
+    0x66,0x2C,0x97,0xEE,0x72,0x99,0x5E,0xF4,0x26,0x40,0xC5,0x50,0xB9,0x01,0x3F,0xAD,
+    0x07,0x61,0x35,0x3C,0x70,0x86,0xA2,0x72,0xC2,0x40,0x88,0xBE,0x94,0x76,0x9F,0xD1,
+    0x66,0x50
+};
+/* b(补前导 0x00 成 66 字节) */
+static const uint8_t P521_B_BE[66] = {
+    0x00,0x51,0x95,0x3E,0xB9,0x61,0x8E,0x1C,0x9A,0x1F,0x92,0x9A,0x21,0xA0,0xB6,0x85,
+    0x40,0xEE,0xA2,0xDA,0x72,0x5B,0x99,0xB3,0x15,0xF3,0xB8,0xB4,0x89,0x91,0x8E,0xF1,
+    0x09,0xE1,0x56,0x19,0x39,0x51,0xEC,0x7E,0x93,0x7B,0x16,0x52,0xC0,0xBD,0x3B,0xB1,
+    0xBF,0x07,0x35,0x73,0xDF,0x88,0x3D,0x2C,0x34,0xF1,0xEF,0x45,0x1F,0xD4,0x6B,0x50,
+    0x3F,0x00
+};
+
+void paze_p521_load_p(paze_bn_t *p) { paze_bn_from_bytes(p, paze_p521_p_be, 66); }
+void paze_p521_load_n(paze_bn_t *n) { paze_bn_from_bytes(n, paze_p521_n_be, 66); }
+
+void paze_p521_load_generator(paze_p521_point_t *G) {
+    paze_bn_from_bytes(&G->X, paze_p521_gx_be, 66);
+    paze_bn_from_bytes(&G->Y, paze_p521_gy_be, 66);
+    paze_bn_set_u32(&G->Z, 1);
+}
+
+void paze_p521_set_inf(paze_p521_point_t *P) {
+    paze_bn_zero(&P->X);
+    paze_bn_zero(&P->Y);
+    paze_bn_zero(&P->Z);
+}
+int paze_p521_is_inf(const paze_p521_point_t *P) { return paze_bn_is_zero(&P->Z); }
+void paze_p521_copy(paze_p521_point_t *dst, const paze_p521_point_t *src) {
+    paze_bn_copy(&dst->X, &src->X);
+    paze_bn_copy(&dst->Y, &src->Y);
+    paze_bn_copy(&dst->Z, &src->Z);
+}
+
+static void fe_mul(paze_bn_t *r, const paze_bn_t *a, const paze_bn_t *b, const paze_bn_t *p) { paze_bn_modmul(r, a, b, p); }
+static void fe_sqr(paze_bn_t *r, const paze_bn_t *a, const paze_bn_t *p) { paze_bn_modmul(r, a, a, p); }
+static void fe_add(paze_bn_t *r, const paze_bn_t *a, const paze_bn_t *b, const paze_bn_t *p) { paze_bn_modadd(r, a, b, p); }
+static void fe_sub(paze_bn_t *r, const paze_bn_t *a, const paze_bn_t *b, const paze_bn_t *p) { paze_bn_modsub(r, a, b, p); }
+
+paze_status_t paze_p521_double(paze_p521_point_t *R, const paze_p521_point_t *P) {
+    if (paze_p521_is_inf(P)) { paze_p521_set_inf(R); return PAZE_OK; }
+    paze_bn_t p; paze_p521_load_p(&p);
+    paze_bn_t delta, gamma, beta, alpha, t, X3, Y3, Z3;
+    fe_sqr(&delta, &P->Z, &p);
+    fe_sqr(&gamma, &P->Y, &p);
+    fe_mul(&beta, &P->X, &gamma, &p);
+    fe_sub(&t, &P->X, &delta, &p);
+    paze_bn_t t2; fe_add(&t2, &P->X, &delta, &p);
+    fe_mul(&alpha, &t, &t2, &p);
+    paze_bn_t three; paze_bn_set_u32(&three, 3);
+    paze_bn_modmul(&alpha, &alpha, &three, &p);
+    fe_sqr(&X3, &alpha, &p);
+    paze_bn_t beta8; paze_bn_t eight; paze_bn_set_u32(&eight, 8);
+    paze_bn_modmul(&beta8, &beta, &eight, &p);
+    fe_sub(&X3, &X3, &beta8, &p);
+    paze_bn_t yz; fe_add(&yz, &P->Y, &P->Z, &p);
+    fe_sqr(&Z3, &yz, &p); fe_sub(&Z3, &Z3, &gamma, &p); fe_sub(&Z3, &Z3, &delta, &p);
+    paze_bn_t four; paze_bn_set_u32(&four, 4);
+    paze_bn_t beta4; paze_bn_modmul(&beta4, &beta, &four, &p);
+    paze_bn_t tmp; fe_sub(&tmp, &beta4, &X3, &p);
+    fe_mul(&Y3, &alpha, &tmp, &p);
+    paze_bn_t gamma2; fe_sqr(&gamma2, &gamma, &p);
+    paze_bn_t gamma2_8; paze_bn_modmul(&gamma2_8, &gamma2, &eight, &p);
+    fe_sub(&Y3, &Y3, &gamma2_8, &p);
+    paze_bn_copy(&R->X, &X3); paze_bn_copy(&R->Y, &Y3); paze_bn_copy(&R->Z, &Z3);
+    return PAZE_OK;
+}
+
+paze_status_t paze_p521_add(paze_p521_point_t *R,
+                            const paze_p521_point_t *P,
+                            const paze_p521_point_t *Q) {
+    if (paze_p521_is_inf(P) && paze_p521_is_inf(Q)) { paze_p521_set_inf(R); return PAZE_OK; }
+    if (paze_p521_is_inf(P)) { paze_p521_copy(R, Q); return PAZE_OK; }
+    if (paze_p521_is_inf(Q)) { paze_p521_copy(R, P); return PAZE_OK; }
+    paze_bn_t p; paze_p521_load_p(&p);
+    paze_bn_t Z1Z1, Z2Z2, U1, U2, S1, S2, H, rr;
+    fe_sqr(&Z1Z1, &P->Z, &p); fe_sqr(&Z2Z2, &Q->Z, &p);
+    fe_mul(&U1, &P->X, &Z2Z2, &p); fe_mul(&U2, &Q->X, &Z1Z1, &p);
+    paze_bn_t t1;
+    fe_mul(&t1, &Q->Z, &Z2Z2, &p); fe_mul(&S1, &P->Y, &t1, &p);
+    fe_mul(&t1, &P->Z, &Z1Z1, &p); fe_mul(&S2, &Q->Y, &t1, &p);
+    fe_sub(&H, &U2, &U1, &p); fe_sub(&rr, &S2, &S1, &p);
+    if (paze_bn_is_zero(&H)) {
+        if (paze_bn_is_zero(&rr)) return paze_p521_double(R, P);
+        paze_p521_set_inf(R); return PAZE_OK;
+    }
+    paze_bn_t H2, H3; fe_sqr(&H2, &H, &p); fe_mul(&H3, &H2, &H, &p);
+    paze_bn_t X3; fe_sqr(&X3, &rr, &p); fe_sub(&X3, &X3, &H3, &p);
+    paze_bn_t U1H2; fe_mul(&U1H2, &U1, &H2, &p);
+    paze_bn_t U1H2_2; paze_bn_t two; paze_bn_set_u32(&two, 2);
+    paze_bn_modmul(&U1H2_2, &U1H2, &two, &p); fe_sub(&X3, &X3, &U1H2_2, &p);
+    paze_bn_t Y3, tmp2; fe_sub(&tmp2, &U1H2, &X3, &p);
+    fe_mul(&Y3, &rr, &tmp2, &p);
+    paze_bn_t S1H3; fe_mul(&S1H3, &S1, &H3, &p); fe_sub(&Y3, &Y3, &S1H3, &p);
+    paze_bn_t Z3; fe_mul(&Z3, &P->Z, &Q->Z, &p); fe_mul(&Z3, &Z3, &H, &p);
+    paze_bn_copy(&R->X, &X3); paze_bn_copy(&R->Y, &Y3); paze_bn_copy(&R->Z, &Z3);
+    return PAZE_OK;
+}
+
+/* 标量乘:R = k*P。P-521 阶 n 为 521 位;k_be 为大端 66 字节。
+ * 数值位 t(0=最低位)对应的大端字节下标 = 65 - (t/8),字节内位 = t&7。
+ * 从最高有效位(t=520)向最低位(t=0) double-and-add。 */
+paze_status_t paze_p521_scalar_mult(paze_p521_point_t *R,
+                                    const uint8_t k_be[66],
+                                    const paze_p521_point_t *P) {
+    paze_p521_point_t base, acc, addtmp;
+    if (P) paze_p521_copy(&base, P);
+    else   paze_p521_load_generator(&base);
+    paze_p521_set_inf(&acc);
+    for (int t = 520; t >= 0; t--) {
+        int byte = 65 - (t / 8);
+        int bit  = t & 7;
+        paze_p521_double(&acc, &acc);
+        paze_p521_add(&addtmp, &acc, &base);
+        uint8_t b = (k_be[byte] >> bit) & 1;
+        if (b) paze_p521_copy(&acc, &addtmp);
+    }
+    paze_p521_copy(R, &acc);
+    return PAZE_OK;
+}
+
+static void fe_inv(paze_bn_t *r, const paze_bn_t *a, const paze_bn_t *p) {
+    paze_bn_t e; paze_bn_set_u32(&e, 2); paze_bn_sub(&e, p, &e);   /* p - 2 */
+    paze_bn_modexp(r, a, &e, p);
+}
+
+paze_status_t paze_p521_to_affine(const paze_p521_point_t *P, uint8_t *out, size_t len) {
+    if (!P || !out) return PAZE_ERR_INVALID;
+    if (paze_p521_is_inf(P)) return PAZE_ERR_INVALID;
+    if (len < 133) return PAZE_ERR_SHORT;
+    paze_bn_t p; paze_p521_load_p(&p);
+    paze_bn_t zinv, zinv2, zinv3, x, y;
+    fe_inv(&zinv, &P->Z, &p);
+    fe_sqr(&zinv2, &zinv, &p);
+    fe_mul(&zinv3, &zinv2, &zinv, &p);
+    fe_mul(&x, &P->X, &zinv2, &p);
+    fe_mul(&y, &P->Y, &zinv3, &p);
+    out[0] = 0x04;
+    paze_bn_to_bytes(&x, out + 1, 66);
+    paze_bn_to_bytes(&y, out + 67, 66);
+    return PAZE_OK;
+}
+
+paze_status_t paze_p521_from_affine(paze_p521_point_t *P, const uint8_t *in, size_t len) {
+    if (!P || !in) return PAZE_ERR_INVALID;
+    if (len < 133) return PAZE_ERR_SHORT;
+    if (in[0] != 0x04) return PAZE_ERR_NOTSUP;
+    paze_bn_t p; paze_p521_load_p(&p);
+    paze_bn_from_bytes(&P->X, in + 1, 66);
+    paze_bn_from_bytes(&P->Y, in + 67, 66);
+    paze_bn_set_u32(&P->Z, 1);
+    /* y^2 == x^3 - 3x + b (mod p) */
+    paze_bn_t lhs, rhs, x3, three, b, t;
+    paze_bn_from_bytes(&b, P521_B_BE, 66);
+    fe_sqr(&lhs, &P->Y, &p);            /* y^2 */
+    fe_sqr(&x3, &P->X, &p); fe_mul(&rhs, &x3, &P->X, &p);   /* x^3 */
+    paze_bn_set_u32(&three, 3);
+    fe_mul(&t, &three, &P->X, &p); fe_sub(&rhs, &rhs, &t, &p);   /* x^3-3x */
+    fe_add(&rhs, &rhs, &b, &p);
+    if (paze_bn_cmp(&lhs, &rhs) != 0) return PAZE_ERR_VERIFY;
+    return PAZE_OK;
+}
+
+paze_status_t paze_p521_get_x_mod_n(paze_bn_t *x, const paze_p521_point_t *P) {
+    if (paze_p521_is_inf(P)) return PAZE_ERR_INVALID;
+    paze_bn_t p, n; paze_p521_load_p(&p); paze_p521_load_n(&n);
+    paze_bn_t zinv, zinv2, xa;
+    fe_inv(&zinv, &P->Z, &p);
+    fe_sqr(&zinv2, &zinv, &p);
+    fe_mul(&xa, &P->X, &zinv2, &p);
+    paze_bn_mod(x, &xa, &n);
+    return PAZE_OK;
+}
diff --git a/src/crypto/x509.c b/src/crypto/x509.c
index 099a9cc..670d747 100644
--- a/src/crypto/x509.c
+++ b/src/crypto/x509.c
@@ -21,6 +21,7 @@
  */
 #include "paze/x509.h"
 #include "paze/p384.h"
+#include "paze/p521.h"
 #include "paze/asn1.h"
 #include "paze/encoding.h"
 #include "paze/hash.h"
@@ -42,8 +43,10 @@ static const uint8_t OID_SHA512_RSA[]  = {0x2A,0x86,0x48,0x86,0xF7,0x0D,0x01,0x0
 static const uint8_t OID_EC_PUBKEY[]   = {0x2A,0x86,0x48,0xCE,0x3D,0x02,0x01};
 static const uint8_t OID_ECDSA_SHA256[]= {0x2A,0x86,0x48,0xCE,0x3D,0x04,0x03,0x02};
 static const uint8_t OID_ECDSA_SHA384[]= {0x2A,0x86,0x48,0xCE,0x3D,0x04,0x03,0x03};
+static const uint8_t OID_ECDSA_SHA512[]= {0x2A,0x86,0x48,0xCE,0x3D,0x04,0x03,0x04};
 static const uint8_t OID_P256[]        = {0x2A,0x86,0x48,0xCE,0x3D,0x03,0x01,0x07};
 static const uint8_t OID_P384[]        = {0x2B,0x81,0x04,0x00,0x22};
+static const uint8_t OID_P521[]        = {0x2B,0x81,0x04,0x00,0x23};
 static const uint8_t OID_SAN[]         = {0x55,0x1D,0x11};
 
 static int oid_eq(const uint8_t *a, size_t alen, const uint8_t *b, size_t blen) {
@@ -57,6 +60,7 @@ static paze_x509_sig_alg_t sig_alg_from_oid(const uint8_t *o, size_t n) {
     if (oid_eq(o,n,OID_SHA512_RSA,9))  return PAZE_X509_SIG_SHA512_RSA;
     if (oid_eq(o,n,OID_ECDSA_SHA256,8))return PAZE_X509_SIG_ECDSA_SHA256;
     if (oid_eq(o,n,OID_ECDSA_SHA384,8))return PAZE_X509_SIG_ECDSA_SHA384;
+    if (oid_eq(o,n,OID_ECDSA_SHA512,8))return PAZE_X509_SIG_ECDSA_SHA512;
     return PAZE_X509_SIG_UNKNOWN;
 }
 
@@ -198,6 +202,15 @@ static paze_status_t parse_spki(paze_asn1_reader_t *r, paze_x509_cert_t *cert) {
             memcpy(cert->ecdsa_p384_pub, pk, 97);
             return PAZE_OK;
         }
+        if (oid_eq(b2, l2, OID_P521, sizeof(OID_P521))) {
+            if (pklen != 133 || pk[0] != 0x04) return PAZE_ERR_FORMAT;
+            paze_p521_point_t Q;
+            paze_status_t st = paze_p521_from_affine(&Q, pk, 133);
+            if (st != PAZE_OK) return st;
+            cert->pk_kind = PAZE_X509_PK_ECDSA_P521;
+            memcpy(cert->ecdsa_p521_pub, pk, 133);
+            return PAZE_OK;
+        }
         return PAZE_ERR_NOTSUP;   /* 未知 EC 曲线 */
     }
     return PAZE_ERR_NOTSUP;
@@ -420,6 +433,9 @@ paze_status_t paze_x509_verify_signature(const paze_x509_cert_t *cert,
             if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P384)
                 return paze_ecdsa_p384_verify(issuer->ecdsa_p384_pub, PAZE_RSA_HASH_SHA256,
                                               hash, 32, cert->signature, cert->signature_len);
+            if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P521)
+                return paze_ecdsa_p521_verify(issuer->ecdsa_p521_pub, PAZE_RSA_HASH_SHA256,
+                                              hash, 32, cert->signature, cert->signature_len);
             return PAZE_ERR_VERIFY;
         case PAZE_X509_SIG_ECDSA_SHA384:
             if (paze_sha384(cert->tbs_der, cert->tbs_der_len, hash) != PAZE_OK) return PAZE_ERR;
@@ -429,6 +445,21 @@ paze_status_t paze_x509_verify_signature(const paze_x509_cert_t *cert,
             if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P256)
                 return paze_ecdsa_verify(&issuer->ecdsa_pub, PAZE_RSA_HASH_SHA384,
                                          hash, 48, cert->signature, cert->signature_len);
+            if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P521)
+                return paze_ecdsa_p521_verify(issuer->ecdsa_p521_pub, PAZE_RSA_HASH_SHA384,
+                                              hash, 48, cert->signature, cert->signature_len);
+            return PAZE_ERR_VERIFY;
+        case PAZE_X509_SIG_ECDSA_SHA512:
+            if (paze_sha512(cert->tbs_der, cert->tbs_der_len, hash) != PAZE_OK) return PAZE_ERR;
+            if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P521)
+                return paze_ecdsa_p521_verify(issuer->ecdsa_p521_pub, PAZE_RSA_HASH_SHA512,
+                                              hash, 64, cert->signature, cert->signature_len);
+            if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P384)
+                return paze_ecdsa_p384_verify(issuer->ecdsa_p384_pub, PAZE_RSA_HASH_SHA512,
+                                              hash, 64, cert->signature, cert->signature_len);
+            if (issuer->pk_kind == PAZE_X509_PK_ECDSA_P256)
+                return paze_ecdsa_verify(&issuer->ecdsa_pub, PAZE_RSA_HASH_SHA512,
+                                         hash, 64, cert->signature, cert->signature_len);
             return PAZE_ERR_VERIFY;
         default:
             return PAZE_ERR_NOTSUP;
@@ -505,6 +536,9 @@ static int pubkey_eq(const paze_x509_cert_t *a, const paze_x509_cert_t *b) {
     if (a->pk_kind == PAZE_X509_PK_ECDSA_P384) {
         return paze_ct_equal(a->ecdsa_p384_pub, b->ecdsa_p384_pub, 97) == 0;
     }
+    if (a->pk_kind == PAZE_X509_PK_ECDSA_P521) {
+        return paze_ct_equal(a->ecdsa_p521_pub, b->ecdsa_p521_pub, 133) == 0;
+    }
     return 0;
 }
 
@@ -738,6 +772,38 @@ paze_status_t paze_x509_build_spki384(uint8_t *out, size_t *outlen,
     return st;
 }
 
+/* 构造 secp521r1 的 SubjectPublicKeyInfo DER */
+paze_status_t paze_x509_build_spki521(uint8_t *out, size_t *outlen,
+                                      const uint8_t pub_affine[133]) {
+    if (!out || !outlen || !pub_affine) return PAZE_ERR_INVALID;
+    paze_asn1_writer_t alg, bs, spki;
+    paze_asn1_w_init(&alg, NULL, 0);
+    paze_asn1_w_init(&bs, NULL, 0);
+    paze_asn1_w_init(&spki, NULL, 0);
+    paze_status_t st;
+    st = paze_asn1_w_oid(&alg, OID_EC_PUBKEY, sizeof(OID_EC_PUBKEY));
+    if (st == PAZE_OK) st = paze_asn1_w_oid(&alg, OID_P521, sizeof(OID_P521));
+    if (st == PAZE_OK) st = paze_asn1_w_byte(&bs, 0x00);
+    if (st == PAZE_OK) st = paze_asn1_w_data(&bs, pub_affine, 133);
+    {
+        paze_asn1_writer_t inner;
+        paze_asn1_w_init(&inner, NULL, 0);
+        if (st == PAZE_OK) st = w_seq(&inner, alg.buf, alg.len);
+        if (st == PAZE_OK) st = paze_asn1_w_tag_len(&inner, ASN1_BITSTRING, bs.len);
+        if (st == PAZE_OK) st = paze_asn1_w_data(&inner, bs.buf, bs.len);
+        if (st == PAZE_OK) st = w_seq(&spki, inner.buf, inner.len);
+        paze_asn1_w_free(&inner);
+    }
+    if (st == PAZE_OK) {
+        if (*outlen < spki.len) { *outlen = spki.len; st = PAZE_ERR_SHORT; }
+        else { memcpy(out, spki.buf, spki.len); *outlen = spki.len; }
+    }
+    paze_asn1_w_free(&alg);
+    paze_asn1_w_free(&bs);
+    paze_asn1_w_free(&spki);
+    return st;
+}
+
 paze_status_t paze_x509_sign_cert(uint8_t *out, size_t *outlen,
                                   const uint8_t *subject_spki, size_t spki_len,
                                   const uint8_t *subject_dn, size_t subj_len,
@@ -756,6 +822,8 @@ paze_status_t paze_x509_sign_cert(uint8_t *out, size_t *outlen,
         sig_oid = OID_ECDSA_SHA256; sig_oid_len = sizeof(OID_ECDSA_SHA256); with_null = 0;
     } else if (issuer_priv->kind == PAZE_X509_PK_ECDSA_P384) {
         sig_oid = OID_ECDSA_SHA384; sig_oid_len = sizeof(OID_ECDSA_SHA384); with_null = 0;
+    } else if (issuer_priv->kind == PAZE_X509_PK_ECDSA_P521) {
+        sig_oid = OID_ECDSA_SHA512; sig_oid_len = sizeof(OID_ECDSA_SHA512); with_null = 0;
     } else return PAZE_ERR_INVALID;
 
     char nb[32], na[32];
@@ -816,11 +884,16 @@ paze_status_t paze_x509_sign_cert(uint8_t *out, size_t *outlen,
         paze_ecdsa_priv_t ek;
         paze_ecdsa_priv_set_d(&ek, issuer_priv->ecdsa_d);
         st = paze_ecdsa_sign(&ek, PAZE_RSA_HASH_SHA256, hsh, 32, sig, &sig_len);
-    } else {   /* P-384 */
+    } else if (issuer_priv->kind == PAZE_X509_PK_ECDSA_P384) {
         uint8_t hsh[48];
         paze_sha384(tbsfull.buf, tbsfull.len, hsh);
         st = paze_ecdsa_p384_sign(issuer_priv->ecdsa_d384, PAZE_RSA_HASH_SHA384,
                                   hsh, 48, sig, &sig_len);
+    } else {   /* P-521 */
+        uint8_t hsh[64];
+        paze_sha512(tbsfull.buf, tbsfull.len, hsh);
+        st = paze_ecdsa_p521_sign(issuer_priv->ecdsa_d521, PAZE_RSA_HASH_SHA512,
+                                  hsh, 64, sig, &sig_len);
     }
     if (st != PAZE_OK) { paze_asn1_w_free(&tbsfull); return st; }
 
@@ -869,6 +942,15 @@ paze_status_t paze_x509_make_self_signed(uint8_t *out, size_t *outlen,
             st = paze_p384_to_affine(&Q, qa, sizeof(qa));
             if (st == PAZE_OK) st = paze_x509_build_spki384(spki, &sl, qa);
         }
+    } else if (priv->kind == PAZE_X509_PK_ECDSA_P521) {
+        /* 公钥 = d521 * G */
+        paze_p521_point_t Q;
+        st = paze_p521_scalar_mult(&Q, priv->ecdsa_d521, NULL);
+        if (st == PAZE_OK) {
+            uint8_t qa[133];
+            st = paze_p521_to_affine(&Q, qa, sizeof(qa));
+            if (st == PAZE_OK) st = paze_x509_build_spki521(spki, &sl, qa);
+        }
     } else return PAZE_ERR_INVALID;
     if (st != PAZE_OK) return st;
 
diff --git a/src/ssl/tls_conn.c b/src/ssl/tls_conn.c
index db94ee4..3e3c07c 100644
--- a/src/ssl/tls_conn.c
+++ b/src/ssl/tls_conn.c
@@ -70,7 +70,7 @@ paze_status_t paze_tls_config_enable_early_data(paze_tls_config_t *cfg, int enab
 paze_status_t paze_tls_config_set_kex_group(paze_tls_config_t *cfg, uint16_t group) {
     if (!cfg) return PAZE_ERR_INVALID;
     if (group != PAZE_TLS_GROUP_X25519 && group != PAZE_TLS_GROUP_SECP256R1 &&
-        group != PAZE_TLS_GROUP_SECP384R1)
+        group != PAZE_TLS_GROUP_SECP384R1 && group != PAZE_TLS_GROUP_SECP521R1)
         return PAZE_ERR_INVALID;
     cfg->kex_group = group;
     return PAZE_OK;
@@ -179,6 +179,10 @@ paze_status_t paze_tls_config_set_cert_chain(paze_tls_config_t *cfg,
             cfg->privkey.kind = PAZE_X509_PK_ECDSA_P384;
             return PAZE_OK;
         }
+        if (paze_ecdsa_p521_priv_from_pem(cfg->privkey.ecdsa_d521, privkey_pem) == PAZE_OK) {
+            cfg->privkey.kind = PAZE_X509_PK_ECDSA_P521;
+            return PAZE_OK;
+        }
         return PAZE_ERR_NOTSUP;   /* 不支持的 EC 曲线 / 格式 */
     }
     if (strstr(privkey_pem, "PRIVATE KEY")) {
diff --git a/src/ssl/tls_extensions.c b/src/ssl/tls_extensions.c
index 15ad5b2..5ec640c 100644
--- a/src/ssl/tls_extensions.c
+++ b/src/ssl/tls_extensions.c
@@ -72,30 +72,33 @@ int paze_tls_ext_parse_supported_versions(const uint8_t *data, size_t len,
  * ============================================================ */
 size_t paze_tls_ext_write_supported_groups(uint8_t *buf) {
     uint8_t *p = buf + 4;
-    p[0] = 0; p[1] = 6;    /* list length = 6 (3 groups × 2 bytes) */
+    p[0] = 0; p[1] = 8;    /* list length = 8 (4 groups × 2 bytes) */
     paze_store16_be(p + 2, PAZE_TLS_GROUP_X25519);
     paze_store16_be(p + 4, PAZE_TLS_GROUP_SECP256R1);
     paze_store16_be(p + 6, PAZE_TLS_GROUP_SECP384R1);
-    size_t body = 8;
+    paze_store16_be(p + 8, PAZE_TLS_GROUP_SECP521R1);
+    size_t body = 10;
     return put_ext_header(buf, PAZE_TLS_EXT_SUPPORTED_GROUPS, body) + body;
 }
 
 /* 解析 supported_groups:
- *   group = 若 X25519 在列表中则优先 X25519,否则 P-256,否则 P-384,否则 0 */
+ *   group = X25519 > P-256 > P-384 > P-521,否则 0 */
 uint16_t paze_tls_ext_parse_supported_groups(const uint8_t *data, size_t len) {
     if (len < 2) return 0;
     size_t ll = paze_load16_be(data);
     if (len != 2 + ll || (ll & 1)) return 0;
-    int have_x25519 = 0, have_p256 = 0, have_p384 = 0;
+    int have_x25519 = 0, have_p256 = 0, have_p384 = 0, have_p521 = 0;
     for (size_t i = 0; i < ll; i += 2) {
         uint16_t g = paze_load16_be(data + 2 + i);
         if (g == PAZE_TLS_GROUP_X25519)      have_x25519 = 1;
         if (g == PAZE_TLS_GROUP_SECP256R1)   have_p256 = 1;
         if (g == PAZE_TLS_GROUP_SECP384R1)   have_p384 = 1;
+        if (g == PAZE_TLS_GROUP_SECP521R1)   have_p521 = 1;
     }
     if (have_x25519) return PAZE_TLS_GROUP_X25519;
     if (have_p256)   return PAZE_TLS_GROUP_SECP256R1;
     if (have_p384)   return PAZE_TLS_GROUP_SECP384R1;
+    if (have_p521)   return PAZE_TLS_GROUP_SECP521R1;
     return 0;
 }
 
@@ -111,7 +114,8 @@ size_t paze_tls_ext_write_signature_algorithms(uint8_t *buf) {
         PAZE_TLS_SIG_RSA_PSS_RSAE_SHA384,
         PAZE_TLS_SIG_RSA_PKCS1_SHA256,
         PAZE_TLS_SIG_RSA_PKCS1_SHA384,
-        PAZE_TLS_SIG_ECDSA_SECP384R1_SHA384
+        PAZE_TLS_SIG_ECDSA_SECP384R1_SHA384,
+        PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512
     };
     int n = sizeof(algs)/sizeof(algs[0]);
     uint16_t ll = (uint16_t)(n * 2);
@@ -156,6 +160,13 @@ size_t paze_tls_ext_write_key_share_client(uint8_t *buf, const paze_tls_kex_t *k
         paze_store16_be(p, 97);                     p += 2;
         memcpy(p, kex->p384_pub, 97);               p += 97;
     }
+    if (only_group == PAZE_TLS_GROUP_SECP521R1 ||
+        (only_group == 0 && kex->kind == PAZE_KEX_P521)) {
+        /* P-521 share(仅 kind==P521 或 HRR 选定组时) */
+        paze_store16_be(p, PAZE_TLS_GROUP_SECP521R1); p += 2;
+        paze_store16_be(p, 133);                    p += 2;
+        memcpy(p, kex->p521_pub, 133);              p += 133;
+    }
     if (p == s + 2) return 0;   /* 无 share 可写 */
     bodylen = (size_t)(p - s);
     paze_store16_be(s, (uint16_t)(bodylen - 2));   /* client_shares 长度 */
diff --git a/src/ssl/tls_handshake12.c b/src/ssl/tls_handshake12.c
index 0443edb..50e6b89 100644
--- a/src/ssl/tls_handshake12.c
+++ b/src/ssl/tls_handshake12.c
@@ -356,6 +356,7 @@ static paze_status_t tls12_client_build_cke(paze_tls_t *tls,
         if (tls->neg.group == PAZE_TLS_GROUP_X25519) { pub = tls->kex.x25519_pub; plen = 32; }
         else if (tls->neg.group == PAZE_TLS_GROUP_SECP256R1) { pub = tls->kex.p256_pub; plen = 65; }
         else if (tls->neg.group == PAZE_TLS_GROUP_SECP384R1) { pub = tls->kex.p384_pub; plen = 97; }
+        else if (tls->neg.group == PAZE_TLS_GROUP_SECP521R1) { pub = tls->kex.p521_pub; plen = 133; }
         else { paze_tls_set_err(tls, "1.2 group %04x not supported", tls->neg.group); return PAZE_ERR_NOTSUP; }
         if (1 + plen > 128) return PAZE_ERR_OVERFLOW;
         out[0] = (uint8_t)plen;   /* RFC 5246 §7.4.7:ECDHE point 用 1 字节长度 */
@@ -608,6 +609,7 @@ static paze_status_t tls12_server_build_ske(paze_tls_t *tls,
     if (nc == PAZE_TLS_GROUP_X25519) { pub = tls->kex.x25519_pub; plen = 32; }
     else if (nc == PAZE_TLS_GROUP_SECP256R1) { pub = tls->kex.p256_pub; plen = 65; }
     else if (nc == PAZE_TLS_GROUP_SECP384R1) { pub = tls->kex.p384_pub; plen = 97; }
+    else if (nc == PAZE_TLS_GROUP_SECP521R1) { pub = tls->kex.p521_pub; plen = 133; }
     else return PAZE_ERR_NOTSUP;
 
     out[0] = 0x03;
diff --git a/src/ssl/tls_handshake13.c b/src/ssl/tls_handshake13.c
index d48da46..b99cdef 100644
--- a/src/ssl/tls_handshake13.c
+++ b/src/ssl/tls_handshake13.c
@@ -10,6 +10,7 @@
  */
 #include "tls_internal.h"
 #include "paze/p384.h"
+#include "paze/p521.h"
 #include <time.h>
 #include <stdio.h>
 
@@ -27,29 +28,27 @@ uint8_t *paze_tls_hs_put_header(uint8_t *msg, uint8_t type, size_t body_len) {
 
 static paze_status_t paze_tls_kex_gen_p256(paze_tls_kex_t *kex);
 static paze_status_t paze_tls_kex_gen_p384(paze_tls_kex_t *kex);
+static paze_status_t paze_tls_kex_gen_p521(paze_tls_kex_t *kex);
 
 paze_status_t paze_tls_kex_generate(paze_tls_kex_t *kex, uint16_t group_hint) {
     paze_memzero(kex, sizeof(*kex));
     int prefer_p256 = (group_hint == PAZE_TLS_GROUP_SECP256R1);
     int prefer_p384 = (group_hint == PAZE_TLS_GROUP_SECP384R1);
+    int prefer_p521 = (group_hint == PAZE_TLS_GROUP_SECP521R1);
     int only_x = (group_hint == PAZE_TLS_GROUP_X25519);
-    if (prefer_p384) kex->kind = PAZE_KEX_P384;
+    if (prefer_p521) kex->kind = PAZE_KEX_P521;
+    else if (prefer_p384) kex->kind = PAZE_KEX_P384;
     else if (prefer_p256) kex->kind = PAZE_KEX_P256;
     else kex->kind = PAZE_KEX_X25519;
 
     if (only_x) {
-        /* 仅生成 X25519 密钥对(其余清零) */
         if (paze_random(kex->x25519_priv, 32) != PAZE_OK) return PAZE_ERR_PLATFORM;
         if (paze_x25519_public(kex->x25519_pub, kex->x25519_priv) != 0) return PAZE_ERR;
         return PAZE_OK;
     }
-    if (prefer_p384) {
-        return paze_tls_kex_gen_p384(kex);
-    }
-    if (prefer_p256) {
-        /* 仅生成 P-256 密钥对 */
-        return paze_tls_kex_gen_p256(kex);
-    }
+    if (prefer_p521) return paze_tls_kex_gen_p521(kex);
+    if (prefer_p384) return paze_tls_kex_gen_p384(kex);
+    if (prefer_p256) return paze_tls_kex_gen_p256(kex);
 
     /* 默认:X25519 优先(仅发 X25519 share;P-256 密钥对也生成备用) */
     if (paze_random(kex->x25519_priv, 32) != PAZE_OK) return PAZE_ERR_PLATFORM;
@@ -89,6 +88,22 @@ static paze_status_t paze_tls_kex_gen_p384(paze_tls_kex_t *kex) {
     return PAZE_OK;
 }
 
+/* 生成 P-521 密钥对(priv 归一化到 [1, n-1],pub = 0x04||X||Y) */
+static paze_status_t paze_tls_kex_gen_p521(paze_tls_kex_t *kex) {
+    if (paze_random(kex->p521_priv, 66) != PAZE_OK) return PAZE_ERR_PLATFORM;
+    paze_bn_t d, n, tmp;
+    paze_p521_load_n(&n);
+    paze_bn_from_bytes(&d, kex->p521_priv, 66);
+    paze_bn_mod(&d, &d, &n);
+    paze_bn_set_u32(&tmp, 1);
+    paze_bn_add(&d, &d, &tmp);
+    paze_bn_to_bytes(&d, kex->p521_priv, 66);
+    paze_p521_point_t Q;
+    paze_p521_scalar_mult(&Q, kex->p521_priv, NULL);
+    if (paze_p521_to_affine(&Q, kex->p521_pub, 133) != PAZE_OK) return PAZE_ERR;
+    return PAZE_OK;
+}
+
 paze_status_t paze_tls_kex_ecdh(paze_tls_kex_t *mine, uint16_t server_group,
                                 const uint8_t *peer_pub, size_t peer_pub_len,
                                 uint8_t *shared_out, size_t *shared_len) {
@@ -132,6 +147,24 @@ paze_status_t paze_tls_kex_ecdh(paze_tls_kex_t *mine, uint16_t server_group,
         *shared_len = 48;
         return PAZE_OK;
     }
+    if (server_group == PAZE_TLS_GROUP_SECP521R1) {
+        if (peer_pub_len != 133 || peer_pub[0] != 0x04) return PAZE_ERR_FORMAT;
+        paze_p521_point_t Q, R;
+        if (paze_p521_from_affine(&Q, peer_pub, 133) != PAZE_OK) return PAZE_ERR_FORMAT;
+        if (paze_p521_scalar_mult(&R, mine->p521_priv, &Q) != PAZE_OK) return PAZE_ERR;
+        /* TLS 1.3 ECDHE 共享密钥 = 共享点 x 坐标(SEC1,固定 66 字节) */
+        paze_bn_t zinv, zinv2, x;
+        {
+            paze_bn_t p; paze_p521_load_p(&p);
+            paze_bn_t e; paze_bn_set_u32(&e, 2); paze_bn_sub(&e, &p, &e);
+            paze_bn_modexp(&zinv, &R.Z, &e, &p);
+            paze_bn_modmul(&zinv2, &zinv, &zinv, &p);
+            paze_bn_modmul(&x, &R.X, &zinv2, &p);
+        }
+        paze_bn_to_bytes(&x, shared_out, 66);
+        *shared_len = 66;
+        return PAZE_OK;
+    }
     return PAZE_ERR_NOTSUP;
 }
 
@@ -388,6 +421,14 @@ static paze_status_t sign_cv(paze_tls_t *tls, uint16_t *sig_alg,
         *sig_alg = PAZE_TLS_SIG_ECDSA_SECP384R1_SHA384;
         return PAZE_OK;
     }
+    if (cfg->privkey.kind == PAZE_X509_PK_ECDSA_P521) {
+        uint8_t h[64]; paze_sha512(tbs, tbs_len, h);
+        paze_status_t st = paze_ecdsa_p521_sign(cfg->privkey.ecdsa_d521,
+                                                PAZE_RSA_HASH_SHA512, h, 64, sig, sig_len);
+        if (st != PAZE_OK) return st;
+        *sig_alg = PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512;
+        return PAZE_OK;
+    }
     return PAZE_ERR_NOTSUP;
 }
 
@@ -406,11 +447,13 @@ static paze_status_t verify_cv(paze_tls_t *tls, int is_server_peer,
         case PAZE_TLS_SIG_RSA_PKCS1_SHA384:        rh = PAZE_RSA_HASH_SHA384; break;
         case PAZE_TLS_SIG_ECDSA_SECP256R1_SHA256:  rh = PAZE_RSA_HASH_SHA256; break;
         case PAZE_TLS_SIG_ECDSA_SECP384R1_SHA384:  rh = PAZE_RSA_HASH_SHA384; break;
+        case PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512:  rh = PAZE_RSA_HASH_SHA512; break;
         default: return PAZE_ERR_NOTSUP;
     }
     uint8_t h[64]; size_t hl = 0;
     if (rh == PAZE_RSA_HASH_SHA256)       { paze_sha256(tbs, tbs_len, h); hl = 32; }
     else if (rh == PAZE_RSA_HASH_SHA384)  { paze_sha384(tbs, tbs_len, h); hl = 48; }
+    else if (rh == PAZE_RSA_HASH_SHA512)  { paze_sha512(tbs, tbs_len, h); hl = 64; }
     else return PAZE_ERR_NOTSUP;
 
     if (leaf->pk_kind == PAZE_X509_PK_RSA) {
@@ -432,6 +475,12 @@ static paze_status_t verify_cv(paze_tls_t *tls, int is_server_peer,
         return paze_ecdsa_p384_verify(leaf->ecdsa_p384_pub, PAZE_RSA_HASH_SHA384,
                                       h, hl, sig, sig_len);
     }
+    if (leaf->pk_kind == PAZE_X509_PK_ECDSA_P521) {
+        if (sig_alg != PAZE_TLS_SIG_ECDSA_SECP521R1_SHA512)
+            return PAZE_ERR_VERIFY;
+        return paze_ecdsa_p521_verify(leaf->ecdsa_p521_pub, PAZE_RSA_HASH_SHA512,
+                                      h, hl, sig, sig_len);
+    }
     return PAZE_ERR_VERIFY;
 }
 
@@ -717,7 +766,7 @@ static paze_status_t client_handle_hrr(paze_tls_t *tls,
         return PAZE_ERR_PROTOCOL;
     }
     if (hx.g != PAZE_TLS_GROUP_X25519 && hx.g != PAZE_TLS_GROUP_SECP256R1 &&
-        hx.g != PAZE_TLS_GROUP_SECP384R1) {
+        hx.g != PAZE_TLS_GROUP_SECP384R1 && hx.g != PAZE_TLS_GROUP_SECP521R1) {
         paze_tls_set_err(tls, "HRR selected unsupported group %d", hx.g);
         return PAZE_ERR_NOTSUP;
     }
@@ -850,7 +899,7 @@ paze_status_t paze_tls13_client_handshake(paze_tls_t *tls) {
         }
         tls->neg.group = group;
 
-        uint8_t shared[64]; size_t shared_len = 0;
+        uint8_t shared[68]; size_t shared_len = 0;
         if (found_ks) {
             st = paze_tls_kex_ecdh(&tls->kex, group, ks_pub, ks_publen, shared, &shared_len);
             if (st != PAZE_OK) return st;
@@ -1291,7 +1340,7 @@ paze_status_t paze_tls13_server_handshake(paze_tls_t *tls) {
         ks_lookup_ctx_t ksl = { select_group, &ks_pub, &ks_publen, &have_share };
         paze_tls_extensions_foreach(exts, 2 + ext_total, ks_lookup_cb, &ksl);
 
-        uint8_t shared[64]; size_t shared_len = 0;
+        uint8_t shared[68]; size_t shared_len = 0;
 
         /* 0-RTT 密钥派生:CH 带 early_data 且 PSK 被接受 → 派生 early_read
          * (AEAD = 客户端 CH 第一个套件的 AEAD,RFC 8446 §4.2.10)。
@@ -1412,8 +1461,10 @@ paze_status_t paze_tls13_server_handshake(paze_tls_t *tls) {
                 n = paze_tls_ext_write_key_share_server(sp, PAZE_TLS_GROUP_X25519, tls->kex.x25519_pub, 32);
             else if (select_group == PAZE_TLS_GROUP_SECP256R1)
                 n = paze_tls_ext_write_key_share_server(sp, PAZE_TLS_GROUP_SECP256R1, tls->kex.p256_pub, 65);
-            else
+            else if (select_group == PAZE_TLS_GROUP_SECP384R1)
                 n = paze_tls_ext_write_key_share_server(sp, PAZE_TLS_GROUP_SECP384R1, tls->kex.p384_pub, 97);
+            else
+                n = paze_tls_ext_write_key_share_server(sp, PAZE_TLS_GROUP_SECP521R1, tls->kex.p521_pub, 133);
             sp += n;
         }
         if (tls->psk_mode) {
diff --git a/src/ssl/tls_internal.h b/src/ssl/tls_internal.h
index 80978fd..9480b99 100644
--- a/src/ssl/tls_internal.h
+++ b/src/ssl/tls_internal.h
@@ -76,7 +76,8 @@ typedef enum {
     PAZE_KEX_NONE = 0,
     PAZE_KEX_X25519,
     PAZE_KEX_P256,
-    PAZE_KEX_P384
+    PAZE_KEX_P384,
+    PAZE_KEX_P521
 } paze_tls_kex_kind_t;
 
 typedef struct {
@@ -90,6 +91,9 @@ typedef struct {
     /* P-384 */
     uint8_t p384_priv[48];
     uint8_t p384_pub[97];     /* 0x04||X||Y */
+    /* P-521 */
+    uint8_t p521_priv[66];
+    uint8_t p521_pub[133];    /* 0x04||X||Y */
 } paze_tls_kex_t;
 
 /* ============================================================