====== OpenSSL Command-Line Cheat Sheet ======

**Author**: Swift Griggs 
**Date**: October 9, 2025  

This cheat sheet provides OpenSSL commands for inspecting, verifying, and managing SSL/TLS certificates and CA stores, with a focus on troubleshooting certificate verification issues (e.g., `unable to get local issuer certificate`) and configuring custom CA certificates on Oracle Linux (OEL) systems.

===== 1. Inspecting Certificates =====

Commands to view certificate details, useful for checking subjects, issuers, validity, and extensions.

==== 1.1 View Subject and Issuer ====

Display the subject and issuer of a certificate file.

<code bash>
openssl x509 -in cert.pem -noout -subject -issuer
</code>

  * **Example output**: `subject=C=US, O=TestCo, OU=PKI, CN=TestCo Issuing CA`

==== 1.2 View Full Certificate Details ====

Show all certificate details, including extensions (e.g., Basic Constraints, Key Usage).

<code bash>
openssl x509 -in cert.pem -noout -text
</code>

  * Use to check if a certificate is a CA (`CA:TRUE`) or self-signed (`subject = issuer`).

==== 1.3 Check Certificate Validity Dates ====

Display the certificate's validity period.

<code bash>
openssl x509 -in cert.pem -noout -dates
</code>

  * **Example output**: `notBefore=Jun 5 19:02:35 2018 GMT`

==== 1.4 Get Certificate Fingerprint ====

Show the certificate's fingerprint for comparison.

<code bash>
openssl x509 -in cert.pem -noout -fingerprint
</code>

  * Useful for verifying if two certificates are identical.

==== 1.5 Convert Certificate Format (DER to PEM) ====

Convert a DER certificate to PEM format.

<code bash>
openssl x509 -inform der -in cert.der -out cert.pem
</code>

===== 2. Verifying Certificates =====

Commands to verify certificates against a CA store or specific CA file, including chain validation.

==== 2.1 Verify Certificate Against System CA Store ====

Verify a certificate using the system’s CA store directory.

<code bash>
openssl verify -CApath /etc/pki/tls/certs/ -verbose -show_chain cert.pem
</code>

  * `-CApath`: Directory with hashed CA certificates.
  * `-show_chain`: Displays the certificate chain.
  * **Success**: `cert.pem: OK`
  * **Failure**: `error 20 at 0 depth lookup: unable to get local issuer certificate`

==== 2.2 Verify Certificate with Specific CA File ====

Verify a certificate using a specific CA bundle file.

<code bash>
openssl verify -CAfile /etc/pki/tls/certs/ca-bundle.crt -verbose -show_chain cert.pem
</code>

  * Use when the CA store is a single file.

==== 2.3 Verify with Intermediate CA ====

Verify a certificate with an untrusted intermediate CA.

<code bash>
openssl verify -CApath /etc/pki/tls/certs/ -untrusted intermediate.pem -verbose -show_chain cert.pem
</code>

  * `-untrusted`: Specifies intermediate CA(s) not in the trusted store.

==== 2.4 Verify with CRL Checking ====

Verify a certificate with Certificate Revocation List (CRL) checking.

<code bash>
openssl verify -CApath /etc/pki/tls/certs/ -crl_check -CRLfile crl.pem cert.pem
</code>

  * Download CRL: `curl -o crl.pem <CRL_URL>`

===== 3. Managing CA Stores =====

Commands to locate and manage the system’s CA trust store, especially for adding custom CAs.

==== 3.1 Find OpenSSL Configuration and CA Store ====

Show OpenSSL’s configuration directory and CA store location.

<code bash>
openssl version -a
</code>

  * Look for `OPENSSLDIR` (e.g., `/etc/pki/tls`) and `SYSTEM_CIPHERS_FILE`.

==== 3.2 List CA Store Contents ====

List certificates in the CA store directory.

<code bash>
ls -l /etc/pki/tls/certs/ | grep -i testco
ls -l /etc/ssl/certs/ | grep -i testco
</code>

  * **Example**: `ls -l /etc/pki/tls/certs/ | grep -i testco`

==== 3.3 Search for CA Certificate Files ====

Find certificate files containing a specific string (e.g., “TestCo”).

<code bash>
sudo find /etc -type f -name "*.crt" -exec grep -l "TestCo" {} \;
sudo find /etc -type f -name "*.pem" -exec grep -l "TestCo" {} \;
</code>

==== 3.4 Check CA Bundle Contents ====

Inspect a CA bundle file for a specific CA.

<code bash>
openssl crl2pkcs7 -nocrl -certfile /etc/pki/tls/certs/ca-bundle.crt | openssl pkcs7 -print_certs | grep -i testco
</code>

  * **Example**: `grep -i testco`

==== 3.5 Add Custom CA to Trust Store ====

Add a custom CA certificate to the system’s trust store (OEL10).

<code bash>
sudo cp custom_ca.crt /etc/pki/ca-trust/source/anchors/
sudo update-ca-trust
</code>

  * Ensure `ca-certificates` package is installed: `sudo dnf install ca-certificates`

==== 3.6 Manually Hash CA Store ====

Hash certificates in the CA store directory for OpenSSL.

<code bash>
sudo cp custom_ca.crt /etc/pki/tls/certs/
sudo c_rehash /etc/pki/tls/certs/
</code>

  * Creates hashed symlinks (e.g., `6ed01128.0`).

===== 4. Troubleshooting Verification Issues =====

Commands to diagnose why certificate verification fails (e.g., `SSLCertVerificationError`).

==== 4.1 Trace OpenSSL File Access ====

Identify which CA files OpenSSL reads during verification.

<code bash>
strace -e open,openat openssl verify -CApath /etc/pki/tls/certs/ -verbose cert.pem 2>&1 | grep -i cert
</code>

  * Look for `openat` calls to files like `/etc/pki/tls/certs/ca-bundle.crt`.

==== 4.2 Check Environment Variables ====

Verify if custom CA paths are set.

<code bash>
echo $SSL_CERT_FILE
echo $SSL_CERT_DIR
</code>

  * If set, these override the default CA store.

==== 4.3 Test Server Certificate Chain ====

Extract and inspect a server’s certificate chain (e.g., for proxy issues).

<code bash>
openssl s_client -connect <server>:443 -showcerts </dev/null > server_certs.pem
openssl x509 -in server_certs.pem -noout -text | grep -i testco
</code>

  * **Example**: `openssl s_client -connect pki.testco.com:443 -showcerts`

==== 4.4 Check OpenSSL Configuration ====

View the OpenSSL configuration file.

<code bash>
sudo cat /etc/pki/tls/openssl.cnf
</code>

  * Check for CA paths (e.g., `dir = /etc/pki/CA`).

==== 4.5 Check Crypto Policies ====

Verify FIPS or crypto policy settings (OEL10).

<code bash>
cat /etc/crypto-policies/back-ends/opensslcnf.config
</code>

  * Adjust policy if needed: `sudo update-crypto-policies --set DEFAULT`

===== 5. Application-Specific Fixes =====

Commands to address application issues (e.g., `pip` SSL errors).

==== 5.1 Fix `pip` SSL Verification ====

Use a custom CA file or proxy for `pip`.

<code bash>
pip install --cert /etc/pki/tls/certs/ca-bundle.crt some_package
export HTTPS_PROXY="http://proxy.example.com:port"
pip install --proxy http://proxy.example.com:port some_package
</code>

==== 5.2 Temporary Trust for Testing ====

Temporarily trust a CA file without modifying the system.

<code bash>
export SSL_CERT_FILE=custom_ca.crt
openssl verify -CAfile custom_ca.crt cert.pem
</code>

===== Notes =====

  * **OEL10 Specifics**: Uses `p11-kit` and `ca-certificates` for CA trust management. Always use `sudo update-ca-trust` to update the trust store.
  * **FIPS Mode**: Ensure certificates use FIPS-compliant algorithms (e.g., SHA-256, RSA ≥ 2048 bits).
  * **CRL Checking**: If CRLs are required, download from URLs in certificate extensions (e.g., `http://pki.testco.com/cdp/TestCo%20Root%20CA.crl`).
  * **Permissions**: Run CA store updates as root (`sudo`) to avoid permission errors.