Certificates

From Network Security Wiki


X.509 Certificate

  • In cryptography, X.509 is a standard defining the format of public key certificates.
  • X.509 certificates are used in many protocols like TLS/SSL, which is the basis for HTTPS.
  • They are also used in offline applications like Electronic Signatures.
  • It contains a public key and an identity - hostname, organization or individual.
  • It is either signed by a Certificate Authority or Self-Signed.
  • When a certificate is signed by a trusted certificate authority or validated by other means, someone holding that certificate can rely on the public key it contains.
  • X.509 also defines certificate revocation lists, which are a means to distribute information about certificates that have been deemed invalid by a signing authority, as well as a certification path validation algorithm, which allows for certificates to be signed by intermediate CA certificates, which are, in turn, signed by other certificates, eventually reaching a trust anchor.
Working of Certificates
  • In the X.509 system, an organization that wants a signed certificate requests one via a Certificate Signing Request (CSR).
  • To do this, it first generates a key pair, keeping the private key secret and using it to sign the CSR.
  • This contains information identifying the applicant and the applicant's public key that is used to verify the signature of the CSR - and the Distinguished Name (DN) that the certificate is for.
  • The CSR may be accompanied by other credentials or proofs of identity required by the certificate authority.
  • The Certification Authority issues a certificate binding a public key to a particular distinguished name.
  • An organization's trusted root certificates can be distributed to all employees so that they can use the company PKI system.
  • Browsers such as Internet Explorer, Firefox, Opera, Safari and Chrome come with a predetermined set of root certificates pre-installed.
  • SSL certificates from major certificate authorities will work instantly.
Structure of an X.509 v3 Digital certificate
  • Certificate
    • Version Number
    • Serial Number
    • Signature Algorithm ID
    • Issuer Name
    • Validity period
      • Not Before
      • Not After
    • Subject name
    • Subject Public Key Info
      • Public Key Algorithm
      • Subject Public Key
    • Issuer Unique Identifier (optional)
    • Subject Unique Identifier (optional)
    • Extensions (optional)
  • Certificate Signature Algorithm
  • Certificate Signature


  • The serial number must be unique for each certificate issued by a specific CA.

OpenSSL

Source: sslshopper.com

Generate Certificates

  • Generate a new private key and Certificate Signing Request
openssl req -out CSR.csr -new -newkey rsa:2048 -nodes -keyout privateKey.key
  • Generate a self-signed certificate
openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:2048 -keyout privateKey.key -out certificate.crt
  • Generate a certificate signing request (CSR) for an existing private key
openssl req -out CSR.csr -key privateKey.key -new
  • Generate a certificate signing request based on an existing certificate
 openssl x509 -x509toreq -in certificate.crt -out CSR.csr -signkey privateKey.key
  • Remove a passphrase from a private key
openssl rsa -in privateKey.pem -out newPrivateKey.pem

Verifying Certificates

  • Check a Certificate Signing Request (CSR)
openssl req -text -noout -verify -in CSR.csr
  • Check a private key
openssl rsa -in privateKey.key -check
  • Check a certificate
openssl x509 -in certificate.crt -text -noout
  • Check a PKCS#12 file (.pfx or .p12)
openssl pkcs12 -info -in keyStore.p12

Debugging

  • Check an MD5 hash of the public key to ensure that it matches with what is in a CSR or private key
openssl x509 -noout -modulus -in certificate.crt | openssl md5
openssl rsa -noout -modulus -in privateKey.key | openssl md5
openssl req -noout -modulus -in CSR.csr | openssl md5
  • Check an SSL connection. All the certificates (including Intermediates) should be displayed
openssl s_client -connect www.paypal.com:443

Converting Format

  • Convert a DER file (.crt .cer .der) to PEM
openssl x509 -inform der -in certificate.cer -out certificate.pem
  • Convert a PEM file to DER
openssl x509 -outform der -in certificate.pem -out certificate.der
  • Convert a PKCS#12 file (.pfx .p12) containing a private key and certificates to PEM
openssl pkcs12 -in keyStore.pfx -out keyStore.pem -nodes

You can add -nocerts to only output the private key or add -nokeys to only output the certificates.

  • Convert a PEM certificate file and a private key to PKCS#12 (.pfx .p12)
openssl pkcs12 -export -out certificate.pfx -inkey privateKey.key -in certificate.crt -certfile CACert.crt


Troubleshooting



References





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