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+------------------------------------------+
|            擴展結構                       |
+------------------------------------------+
| OID (Object Identifier)  - 擴展的唯一識別 |
| Critical Flag            - 是否為關鍵擴展 |
| Value                    - 擴展的實際值   |
+------------------------------------------+

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擴展處理流程：

+------------------+
| 讀取憑證擴展     |
+--------+---------+
         |
         v
+------------------+     Yes    +------------------+
| 是否為關鍵擴展？ | ---------->| 能否理解此擴展？ |
+--------+---------+            +--------+---------+
         | No                            |
         v                               v
+------------------+            +------------------+
| 選擇性處理擴展   |            | Yes: 處理擴展    |
+------------------+            | No: 拒絕憑證     |
                                +------------------+

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Key Usage Bits (9 位元):

Bit 0: digitalSignature    - 數位簽章
Bit 1: nonRepudiation      - 不可否認性（contentCommitment）
Bit 2: keyEncipherment     - 金鑰加密
Bit 3: dataEncipherment    - 資料加密
Bit 4: keyAgreement        - 金鑰協商
Bit 5: keyCertSign         - 憑證簽發
Bit 6: cRLSign             - CRL 簽發
Bit 7: encipherOnly        - 僅加密（與 keyAgreement 配合）
Bit 8: decipherOnly        - 僅解密（與 keyAgreement 配合）

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# 檢視憑證的 Key Usage
openssl x509 -in cert.crt -text -noout | grep -A1 "Key Usage"

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TLS RSA 金鑰交換：

用戶端                              伺服器
   |                                   |
   |  1. 產生隨機的 Pre-Master Secret  |
   |                                   |
   |  2. 用伺服器公鑰加密              |
   |  -------------------------------->|
   |                                   |  3. 用私鑰解密
   |                                   |
   |  4. 雙方計算 Master Secret        |
   |                                   |

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encipherOnly + keyAgreement：僅能用於加密方向的金鑰協商
decipherOnly + keyAgreement：僅能用於解密方向的金鑰協商

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+------------------------+------------------------------------------+
| 憑證類型               | 常見 Key Usage 組合                       |
+------------------------+------------------------------------------+
| TLS 伺服器（RSA）      | digitalSignature, keyEncipherment         |
| TLS 伺服器（ECDSA）    | digitalSignature                          |
| TLS 用戶端             | digitalSignature, keyAgreement            |
| CA 憑證                | keyCertSign, cRLSign                      |
| 程式碼簽章             | digitalSignature                          |
| S/MIME 加密            | keyEncipherment                           |
| S/MIME 簽章            | digitalSignature, nonRepudiation          |
+------------------------+------------------------------------------+

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Extended Key Usage 結構：

+------------------------------------------+
| Extended Key Usage                        |
+------------------------------------------+
| OID 1: serverAuth (1.3.6.1.5.5.7.3.1)    |
| OID 2: clientAuth (1.3.6.1.5.5.7.3.2)    |
| ...                                       |
+------------------------------------------+

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Key Usage 和 EKU 的搭配：

+------------------+     +------------------+
|   Key Usage      |     | Extended Key     |
|   (基本用途)     |     | Usage (特定用途) |
+------------------+     +------------------+
         |                        |
         v                        v
    低階控制                 高階控制
    定義基本操作             定義應用場景

驗證邏輯：
1. 首先檢查 Key Usage 是否允許操作
2. 接著檢查 EKU 是否符合應用需求
3. 兩者都必須通過才能使用憑證

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# TLS 伺服器憑證
extendedKeyUsage = serverAuth

# TLS 雙向驗證憑證
extendedKeyUsage = serverAuth, clientAuth

# 程式碼簽章憑證
extendedKeyUsage = codeSigning

# 電子郵件憑證（簽章和加密）
extendedKeyUsage = emailProtection

# OCSP 回應者憑證
extendedKeyUsage = OCSPSigning

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# 不建議在生產環境使用
extendedKeyUsage = anyExtendedKeyUsage

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Basic Constraints 結構：

+------------------------------------------+
| Basic Constraints                         |
+------------------------------------------+
| CA: TRUE/FALSE                           |
| pathLenConstraint: 0, 1, 2, ... (選用)   |
+------------------------------------------+

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路徑長度限制範例：

根 CA (pathLen: 2)
    |
    +-- 中繼 CA Level 1 (pathLen: 1)
    |       |
    |       +-- 中繼 CA Level 2 (pathLen: 0)
    |               |
    |               +-- 終端憑證 (CA:FALSE)
    |               +-- 終端憑證 (CA:FALSE)
    |               X-- 不能再簽發 CA 憑證
    |
    +-- 中繼 CA Level 1 (pathLen: 0)
            |
            +-- 終端憑證 (CA:FALSE)
            X-- 不能再簽發 CA 憑證

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# 根 CA（無路徑限制）
basicConstraints = critical, CA:TRUE

# 中繼 CA（最多再簽 1 層 CA）
basicConstraints = critical, CA:TRUE, pathlen:1

# 簽發 CA（只能簽終端憑證）
basicConstraints = critical, CA:TRUE, pathlen:0

# 終端實體憑證
basicConstraints = critical, CA:FALSE

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安全考量：

如果 Basic Constraints 不是 Critical：
+------------------+
| 終端憑證         |  <-- 攻擊者取得此憑證
| CA:FALSE         |
| (非 Critical)    |
+------------------+
         |
         v
+------------------+
| 惡意軟體忽略     |  <-- 不支援此擴展的軟體
| Basic Constraints|      可能接受其簽發的憑證
+------------------+
         |
         v
+------------------+
| 偽造的子憑證     |  <-- 安全漏洞！
+------------------+

解決方案：
- 將 Basic Constraints 設為 Critical
- 確保所有驗證軟體都會檢查此擴展

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Subject Alternative Name 結構：

+------------------------------------------+
| Subject Alternative Name                  |
+------------------------------------------+
| DNS: example.com                         |
| DNS: www.example.com                     |
| DNS: api.example.com                     |
| IP: 192.168.1.100                        |
| Email: admin@example.com                 |
| URI: https://example.com/auth            |
+------------------------------------------+

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傳統方式（已過時）：
+------------------+
| Subject          |
| CN=www.example.com|  <-- 舊方法：使用 CN 存放域名
+------------------+

現代方式（推薦）：
+------------------+
| Subject          |
| CN=Example Corp  |  <-- CN 存放組織名稱
+------------------+
| SAN              |
| DNS:example.com  |  <-- SAN 存放域名
| DNS:www.example.com|
+------------------+

注意：
- Chrome 58+ 已不再使用 CN 作為域名驗證
- 必須使用 SAN 擴展
- 建議 CN 使用組織名稱或主要域名

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萬用字元規則：

有效的萬用字元：
*.example.com     -> 匹配 www.example.com, api.example.com
                  -> 不匹配 example.com
                  -> 不匹配 www.sub.example.com

無效的萬用字元：
www.*.com         -> 不允許
*.*.example.com   -> 不允許
example.*         -> 不允許

建議的 SAN 設定：
DNS:example.com
DNS:*.example.com

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# 一張憑證保護多個域名
subjectAltName = @alt_names

[alt_names]
DNS.1 = example.com
DNS.2 = www.example.com
DNS.3 = example.org
DNS.4 = www.example.org
DNS.5 = api.example.com
IP.1 = 10.0.0.1
IP.2 = 10.0.0.2

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+------------------------------------------+
| TLS Server Certificate                    |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature                     |
|   - keyEncipherment (RSA only)           |
|                                          |
| Extended Key Usage:                       |
|   - serverAuth                           |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
|                                          |
| Subject Alternative Name:                 |
|   - DNS: example.com                     |
|   - DNS: www.example.com                 |
+------------------------------------------+

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+------------------------------------------+
| TLS Client Certificate                    |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature                     |
|                                          |
| Extended Key Usage:                       |
|   - clientAuth                           |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
|                                          |
| Subject Alternative Name:                 |
|   - Email: user@example.com              |
+------------------------------------------+

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+------------------------------------------+
| mTLS Certificate (Server + Client)        |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature                     |
|   - keyEncipherment                      |
|                                          |
| Extended Key Usage:                       |
|   - serverAuth                           |
|   - clientAuth                           |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
+------------------------------------------+

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+------------------------------------------+
| Code Signing Certificate                  |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature                     |
|                                          |
| Extended Key Usage:                       |
|   - codeSigning                          |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
+------------------------------------------+

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+------------------------------------------+
| S/MIME Email Certificate                  |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature (簽章)              |
|   - keyEncipherment (加密)               |
|   - nonRepudiation (不可否認)            |
|                                          |
| Extended Key Usage:                       |
|   - emailProtection                      |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
|                                          |
| Subject Alternative Name:                 |
|   - Email: user@example.com              |
+------------------------------------------+

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+------------------------------------------+
| Intermediate CA Certificate               |
+------------------------------------------+
| Key Usage:                               |
|   - keyCertSign                          |
|   - cRLSign                              |
|                                          |
| Extended Key Usage: (選用)                |
|   - 不設定或限制特定用途                  |
|                                          |
| Basic Constraints:                        |
|   - CA:TRUE                              |
|   - pathlen:0 (只能簽終端憑證)           |
+------------------------------------------+

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+------------------------------------------+
| OCSP Responder Certificate                |
+------------------------------------------+
| Key Usage:                               |
|   - digitalSignature                     |
|                                          |
| Extended Key Usage:                       |
|   - OCSPSigning                          |
|                                          |
| Basic Constraints:                        |
|   - CA:FALSE                             |
|                                          |
| OCSP No Check: (選用)                     |
|   - 存在此擴展表示不需檢查此憑證的撤銷狀態 |
+------------------------------------------+

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# OpenSSL 擴展設定檔

# 根 CA 擴展
[ v3_ca ]
basicConstraints = critical, CA:TRUE
keyUsage = critical, keyCertSign, cRLSign
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always

# 中繼 CA 擴展（只能簽終端憑證）
[ v3_intermediate_ca ]
basicConstraints = critical, CA:TRUE, pathlen:0
keyUsage = critical, keyCertSign, cRLSign
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always
authorityInfoAccess = caIssuers;URI:http://ca.example.com/root.crt
crlDistributionPoints = URI:http://crl.example.com/root.crl

# TLS 伺服器憑證擴展
[ v3_server ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always
authorityInfoAccess = OCSP;URI:http://ocsp.example.com, caIssuers;URI:http://ca.example.com/intermediate.crt
crlDistributionPoints = URI:http://crl.example.com/intermediate.crl

# TLS 用戶端憑證擴展
[ v3_client ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature
extendedKeyUsage = clientAuth
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always

# 雙向 TLS 憑證擴展
[ v3_server_client ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth, clientAuth
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always

# 程式碼簽章憑證擴展
[ v3_codesign ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature
extendedKeyUsage = codeSigning
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always

# S/MIME 電子郵件憑證擴展
[ v3_email ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature, keyEncipherment, nonRepudiation
extendedKeyUsage = emailProtection
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always

# OCSP 簽發憑證擴展
[ v3_ocsp ]
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature
extendedKeyUsage = OCSPSigning
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always, issuer:always
noCheck = ignored

# SAN 設定區塊
[ alt_names ]
DNS.1 = example.com
DNS.2 = www.example.com
DNS.3 = api.example.com
IP.1 = 10.0.0.1

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# 產生根 CA 私鑰
openssl genrsa -aes256 -out rootCA.key 4096

# 產生根 CA 憑證
openssl req -x509 -new -nodes \
    -key rootCA.key \
    -sha256 \
    -days 7300 \
    -out rootCA.crt \
    -extensions v3_ca \
    -config openssl-extensions.cnf \
    -subj "/C=TW/ST=Taiwan/L=Taipei/O=Example Corp/CN=Example Root CA"

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# 產生中繼 CA 私鑰
openssl genrsa -aes256 -out intermediateCA.key 4096

# 產生 CSR
openssl req -new \
    -key intermediateCA.key \
    -out intermediateCA.csr \
    -subj "/C=TW/ST=Taiwan/L=Taipei/O=Example Corp/CN=Example Intermediate CA"

# 使用根 CA 簽發中繼 CA 憑證
openssl x509 -req \
    -in intermediateCA.csr \
    -CA rootCA.crt \
    -CAkey rootCA.key \
    -CAcreateserial \
    -out intermediateCA.crt \
    -days 3650 \
    -sha256 \
    -extensions v3_intermediate_ca \
    -extfile openssl-extensions.cnf

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# 產生伺服器私鑰
openssl genrsa -out server.key 2048

# 產生 CSR（使用設定檔定義 SAN）
openssl req -new \
    -key server.key \
    -out server.csr \
    -subj "/C=TW/ST=Taiwan/L=Taipei/O=Example Corp/CN=www.example.com"

# 使用中繼 CA 簽發伺服器憑證
openssl x509 -req \
    -in server.csr \
    -CA intermediateCA.crt \
    -CAkey intermediateCA.key \
    -CAcreateserial \
    -out server.crt \
    -days 365 \
    -sha256 \
    -extensions v3_server \
    -extfile openssl-extensions.cnf

# 或者使用命令列指定 SAN
openssl x509 -req \
    -in server.csr \
    -CA intermediateCA.crt \
    -CAkey intermediateCA.key \
    -CAcreateserial \
    -out server.crt \
    -days 365 \
    -sha256 \
    -extfile <(cat <<EOF
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth
subjectAltName = DNS:example.com, DNS:www.example.com, DNS:api.example.com, IP:10.0.0.1
EOF
)

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# 產生用戶端私鑰
openssl genrsa -out client.key 2048

# 產生 CSR
openssl req -new \
    -key client.key \
    -out client.csr \
    -subj "/C=TW/ST=Taiwan/L=Taipei/O=Example Corp/CN=User Name/emailAddress=user@example.com"

# 簽發用戶端憑證
openssl x509 -req \
    -in client.csr \
    -CA intermediateCA.crt \
    -CAkey intermediateCA.key \
    -CAcreateserial \
    -out client.crt \
    -days 365 \
    -sha256 \
    -extensions v3_client \
    -extfile openssl-extensions.cnf

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# 產生 ECDSA 私鑰
openssl ecparam -genkey -name prime256v1 -out server-ecdsa.key

# 產生 CSR
openssl req -new \
    -key server-ecdsa.key \
    -out server-ecdsa.csr \
    -subj "/C=TW/ST=Taiwan/L=Taipei/O=Example Corp/CN=www.example.com"

# 簽發憑證（ECDSA 不需要 keyEncipherment）
openssl x509 -req \
    -in server-ecdsa.csr \
    -CA intermediateCA.crt \
    -CAkey intermediateCA.key \
    -CAcreateserial \
    -out server-ecdsa.crt \
    -days 365 \
    -sha256 \
    -extfile <(cat <<EOF
basicConstraints = critical, CA:FALSE
keyUsage = critical, digitalSignature
extendedKeyUsage = serverAuth
subjectAltName = DNS:example.com, DNS:www.example.com
EOF
)

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# 檢視完整憑證內容
openssl x509 -in cert.crt -text -noout

# 僅檢視擴展欄位
openssl x509 -in cert.crt -text -noout | grep -A20 "X509v3 extensions"

# 檢視 Key Usage
openssl x509 -in cert.crt -text -noout | grep -A1 "Key Usage"

# 檢視 Extended Key Usage
openssl x509 -in cert.crt -text -noout | grep -A2 "Extended Key Usage"

# 檢視 Basic Constraints
openssl x509 -in cert.crt -text -noout | grep -A1 "Basic Constraints"

# 檢視 Subject Alternative Name
openssl x509 -in cert.crt -text -noout | grep -A5 "Subject Alternative Name"

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# 驗證完整憑證鏈
openssl verify -CAfile rootCA.crt -untrusted intermediateCA.crt server.crt

# 驗證憑證並顯示詳細資訊
openssl verify -verbose -CAfile ca-bundle.crt server.crt

# 建立完整的 CA Bundle
cat intermediateCA.crt rootCA.crt > ca-bundle.crt
openssl verify -CAfile ca-bundle.crt server.crt

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# 連接伺服器並顯示憑證資訊
openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | \
    openssl x509 -text -noout

# 顯示完整憑證鏈
openssl s_client -connect example.com:443 -servername example.com -showcerts

# 檢查憑證到期日
echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | \
    openssl x509 -noout -dates

# 檢查 SAN
echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | \
    openssl x509 -noout -text | grep -A1 "Subject Alternative Name"

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# 驗證憑證是否可用於 SSL 伺服器
openssl verify -purpose sslserver -CAfile ca-bundle.crt server.crt

# 驗證憑證是否可用於 SSL 用戶端
openssl verify -purpose sslclient -CAfile ca-bundle.crt client.crt

# 驗證憑證是否可用於 S/MIME 簽章
openssl verify -purpose smimesign -CAfile ca-bundle.crt email.crt

# 可用的 purpose 選項
# sslclient, sslserver, nssslserver, smimesign, smimeencrypt, crlsign, any, ocsphelper, timestampsign

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#!/bin/bash
# check_cert_usage.sh - 檢查憑證用途設定

CERT=$1

if [ -z "$CERT" ]; then
    echo "Usage: $0 <certificate.crt>"
    exit 1
fi

echo "=== 憑證用途分析 ==="
echo ""

# 檢查 Basic Constraints
echo "Basic Constraints:"
openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep -A1 "Basic Constraints" | tail -1
echo ""

# 檢查 Key Usage
echo "Key Usage:"
openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep -A1 "Key Usage" | tail -1
echo ""

# 檢查 Extended Key Usage
echo "Extended Key Usage:"
openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep -A2 "Extended Key Usage" | tail -2
echo ""

# 檢查 SAN
echo "Subject Alternative Name:"
openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep -A5 "Subject Alternative Name" | tail -5
echo ""

# 判斷憑證類型
echo "=== 憑證類型判斷 ==="
IS_CA=$(openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep "CA:TRUE")
HAS_SERVER_AUTH=$(openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep "TLS Web Server Authentication")
HAS_CLIENT_AUTH=$(openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep "TLS Web Client Authentication")
HAS_CODE_SIGN=$(openssl x509 -in "$CERT" -text -noout 2>/dev/null | grep "Code Signing")

if [ -n "$IS_CA" ]; then
    echo "- CA 憑證"
fi
if [ -n "$HAS_SERVER_AUTH" ]; then
    echo "- 可用於 TLS 伺服器"
fi
if [ -n "$HAS_CLIENT_AUTH" ]; then
    echo "- 可用於 TLS 用戶端"
fi
if [ -n "$HAS_CODE_SIGN" ]; then
    echo "- 可用於程式碼簽章"
fi

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# 安裝 cfssl
go install github.com/cloudflare/cfssl/cmd/...@latest

# 檢視憑證資訊
cfssl certinfo -cert server.crt

# 以 JSON 格式輸出
cfssl certinfo -cert server.crt | jq .

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+----------------------------------------+
| 憑證健康檢查清單                        |
+----------------------------------------+

[ ] 基本驗證
    [ ] 憑證格式正確（PEM/DER）
    [ ] 憑證尚未過期
    [ ] 憑證已經生效
    [ ] 私鑰與憑證匹配

[ ] 信任鏈
    [ ] 完整的憑證鏈
    [ ] 中繼憑證正確
    [ ] 根憑證受信任
    [ ] 沒有過期的鏈上憑證

[ ] 擴展欄位
    [ ] Basic Constraints 正確設定
    [ ] Key Usage 符合用途
    [ ] Extended Key Usage 符合應用
    [ ] SAN 包含所有必要的域名
    [ ] Critical 擴展正確標記

[ ] 安全性
    [ ] 使用強加密演算法（SHA-256+）
    [ ] 足夠的金鑰長度（RSA 2048+, ECDSA 256+）
    [ ] 合理的有效期限
    [ ] 未被撤銷

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# 完整的憑證健康檢查腳本
#!/bin/bash
# cert_health_check.sh

CERT=$1
KEY=$2
CA_BUNDLE=$3

echo "=== 憑證健康檢查 ==="

# 檢查憑證有效期
echo -e "\n[1] 有效期檢查"
openssl x509 -in "$CERT" -noout -dates
NOT_AFTER=$(openssl x509 -in "$CERT" -noout -enddate | cut -d= -f2)
EXPIRE_EPOCH=$(date -d "$NOT_AFTER" +%s)
NOW_EPOCH=$(date +%s)
DAYS_LEFT=$(( (EXPIRE_EPOCH - NOW_EPOCH) / 86400 ))
echo "剩餘天數: $DAYS_LEFT"

# 檢查私鑰匹配
if [ -n "$KEY" ]; then
    echo -e "\n[2] 私鑰匹配檢查"
    CERT_MOD=$(openssl x509 -in "$CERT" -noout -modulus 2>/dev/null | md5sum)
    KEY_MOD=$(openssl rsa -in "$KEY" -noout -modulus 2>/dev/null | md5sum)
    if [ "$CERT_MOD" = "$KEY_MOD" ]; then
        echo "結果: 匹配"
    else
        echo "結果: 不匹配 (錯誤!)"
    fi
fi

# 檢查憑證鏈
if [ -n "$CA_BUNDLE" ]; then
    echo -e "\n[3] 憑證鏈驗證"
    openssl verify -CAfile "$CA_BUNDLE" "$CERT"
fi

# 檢查加密演算法
echo -e "\n[4] 加密演算法"
openssl x509 -in "$CERT" -noout -text | grep "Signature Algorithm" | head -1
openssl x509 -in "$CERT" -noout -text | grep "Public-Key"

# 檢查擴展
echo -e "\n[5] 關鍵擴展"
openssl x509 -in "$CERT" -noout -text | grep -E "(Basic Constraints|Key Usage|Extended Key Usage)" | head -5