About certificate

證書spec, X509, 相似規定了一個目錄結構。其中重要內容包括

• issuer: who isued this certificate
• subject: the ID of this certificate 
• public key: 
• validate period
• sign: the sha of this certificate, encritpted with the issuer's private key. (This is the mechanism how to verify the certificate)
• in extention, other there is a link to get certificate of issuer. 

DN (distingushed name)

• include C(country), ST(state), O(organization), OU(部門，能夠多個), CN(common name)
• both Issuer and Subject are DN. 

Certificate formate

• PEM, base64 encoded DER file, easy to be edited
• DER, CER, CRT.  same, DER, Distinguished Encoding Rules. openssl -inform der -in a.cert -text -noout
• P12. Windows specific, contails both public key and private key. So the file itself should be encriypted.
• p7b, p7c. CRL (certificate revocation list) 經常使用於證書吊銷文件，不包括key
• JKS. Java Key storage(Java 專利）利用 keytool 管理

應用

1. Safari and macOS, managed by "keychain access". The each keychain is stored in separated directory. login means the current login user.
2. Java, keytool 管理，has different location from OS(e.g. /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/security/cacerts), so even safari downloaded a Root CA for a website, Java may still not work.  
   1. keytool -list -keystore cacerts
3. Python: 
   1. public certs stored in certifi module ([py_home]/site-packages/certifi/cacert.pem), then all python modules relying on certifi (e.g. requests) could load certs for ssl verification 
   2. However, pip is a standalone package that contains its own requests/certifi module and public cert storage.  One solution is to wrap original certifi.where() and pip._vendor.requests.certs.where() method to force return path ‘/etc/pki/tls/cert.pem’. Make sure all certs are store in it.
4. CN name: https://security.stackexchange.com/questions/40026/openssl-x509-whats-the-significance-of-cn-common-name
5. curl, use curl -v to see with cacert it is using, maybe /etc/ssl/cacert.pem, makeby $HOME/anaconda/ssl/cacert.pem. 
   1. 手動指定使用某個證書來驗證網站 curl --cacert mycertificate.cer -v https://www.google.com

References:

• cert format,
  • https://serverfault.com/questions/9708/what-is-a-pem-file-and-how-does-it-differ-from-other-openssl-generated-key-file
  • https://www.cnblogs.com/guogangj/p/4118605.html
• https://en.wikipedia.org/wiki/X.509  （X.509 內容說明) , 
  • 　PKIX (Public Key Infrastructure X.509)
  • 　OCSP (Online Certificate Status Protocol)
• verify certificate: https://stackoverflow.com/questions/188266/how-are-ssl-certificates-verified
• Certificate Chain: 
  • https://ssl.comodo.com/articles/understanding-an-ssl-certificate-chain.php
  • https://support.dnsimple.com/articles/what-is-ssl-certificate-chain/

Root certificate

• Intermediate certifcate
  • client certificate. In SSL, webserver might need to veifiy the certificate of the client. Usually it doesn't.