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OpenSSL gives you the command-line workflow for creating private keys, generating PKCS#10 certificate signing requests (CSRs), producing test certificates, inspecting X.509 data, checking certificate validity and packaging credentials for servers. A safe production workflow is: create and protect a key, generate a CSR with the correct Subject Alternative Names (SANs), obtain a certificate from a trusted certificate authority (CA), verify the certificate and chain, convert it to the format your software requires, then deploy it without exposing the key.
This guide uses commands that work across common OpenSSL 1.1.1 and 3.x installations, while calling out defaults that changed in OpenSSL 3.2 and 3.3. Run openssl version -a first and record the result with your command options so a renewal can be reproduced.
What OpenSSL handles in a certificate workflow
The OpenSSL project describes itself as “a cryptography toolkit implementing the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) network protocols and related cryptography standards required for them.” In day-to-day certificate work, that means one toolkit can create keys, build PKCS#10 requests, create test certificates, parse X.509 fields, verify signatures and chains, and convert between common encodings.
A CSR contains a public key and identifying information, signed with the corresponding private key. It does not contain the private key. When a CA signs the request, the CA places that public key and approved subject information into an X.509 certificate. A self-signed certificate skips the CA and is signed by its own key, which is useful for controlled testing but is not automatically trusted by public browsers.
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- Generate a new private key or select an existing one.
- Create a CSR with the names and other subject information the certificate must cover.
- Submit the CSR to a CA, or self-sign only for a lab, internal test, or private trust domain.
- Inspect the resulting certificate, including dates, issuer, SANs, key usage and public key.
- Verify the chain and hostname with appropriate trust material.
- Convert and deploy the certificate, intermediates and private key in the format required by the server or client.
Check the OpenSSL release before relying on defaults
OpenSSL options and defaults are release-sensitive. Capture the version and build details with:
openssl version -aSince OpenSSL 3.2, certificates generated by req are X.509 version 3 unless you explicitly request -x509v1; key-identifier extensions are included by default. Since OpenSSL 3.3, a failed req -verify check exits with status 1, which matters in scripts that test the command result. The spelling -noenc is the current option for leaving a generated private key unencrypted; the older -nodes spelling is deprecated in OpenSSL 3.0. Check the local man page when moving between 1.1.1, 3.x and later releases.
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Generate a private key and CSR
Create a key and CSR in one command
This is the shortest interactive workflow for an RSA key:
openssl req -newkey rsa:2048 -keyout private.key -out request.csrOpenSSL creates the private key and then asks for the CSR subject fields. By default, the generated key is protected with a passphrase. Keep that encryption when the service can securely supply the password at startup. If an appliance or unattended service cannot read an encrypted key, add -noenc deliberately and compensate with strict file permissions, host protection and a key-management process:
openssl req -newkey rsa:2048 -noenc -keyout private.key -out request.csrAn unencrypted key is a deployment choice, not a harmless convenience: anyone who obtains the file can use the identity represented by the certificate.
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Create a CSR from an existing key
When a key already exists, generate a new request without replacing it:
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Put modern hostnames in Subject Alternative Name
Modern TLS hostname validation uses SAN entries. Do not rely on a Common Name alone for every client. For repeatable requests, place the subject and SAN values in a configuration file rather than answering prompts:
[req]
distinguished_name = req_distinguished_name
req_extensions = req_ext
prompt = no
[req_distinguished_name]
C = US
ST = State
L = City
O = Example Operations
CN = www.example.com
[req_ext]
subjectAltName = @alt_names
[alt_names]
DNS.1 = www.example.com
DNS.2 = example.comSave it as openssl.cnf, then run:
openssl req -new -key private.key -out request.csr -config openssl.cnfInclude every DNS name clients will actually use. A wildcard, an IP address and an internal name have different validation implications; list an IP under an IP SAN rather than pretending it is a DNS name. Confirm the final SAN list in the issued certificate instead of assuming the CA copied every requested value.
Use RSA or elliptic-curve keys intentionally
RSA is widely interoperable and the examples above use a 2048-bit RSA key. Elliptic-curve keys can provide comparable security with smaller keys, but support and operational procedures vary by server, library and policy. Generate the type your target software and CA support, then keep the same type documented for renewal.
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| Choice | Operational trade-off |
|---|---|
| RSA | Broad compatibility; the example uses rsa:2048. |
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Inspect and verify a CSR
Before submitting a request, inspect exactly what it contains and verify its self-signature:
openssl req -in request.csr -text -verify -nooutThe text output lets you check the subject, public-key algorithm, requested extensions and SANs. The -verify operation checks that the CSR was signed by the private key corresponding to its public key; it does not prove that a CA will issue the names requested. In OpenSSL 3.3 and later, a failed check returns exit status 1, so automation can stop instead of submitting a bad request.
Create a self-signed certificate for testing
For a lab or an isolated private trust domain, create a key and self-signed X.509 certificate in one step:
openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pemUse -noenc only when the test service cannot handle a passphrase-protected key. Set an explicit lifetime when your test should model a known expiry window:
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Read an X.509 certificate with openssl x509
Display all encoded fields
openssl x509 -in cert.pem -text -nooutUse this output to inspect the subject, issuer, serial number, public-key algorithm, validity period, extensions, key-usage and extended-key-usage values, SANs and fingerprints. It shows what is encoded; it does not by itself establish that a client trusts the issuer or that a hostname matches.
Check dates, issuer and subject quickly
openssl x509 -in cert.pem -dates -issuer -subject -nooutThis compact check catches a surprisingly common deployment error: installing a certificate whose validity has not started, has expired, or belongs to a different host or organization.
Test whether expiry is approaching
openssl x509 -in cert.pem -checkend 2592000 -nooutThe argument is seconds. 2592000 is 30 days, so the command exits successfully when the certificate remains valid beyond that interval and fails when it expires sooner. Use the exit status in renewal monitoring rather than relying on a human reading the date.
Inspect intended purposes
openssl x509 -in cert.pem -purpose -nooutPurpose output helps reveal certificates that are validly signed but lack the key-usage or extended-key-usage needed for a server, client or CA role.
Verify a chain and a hostname, not just the file syntax
Parsing a certificate proves that OpenSSL can decode it. Trust verification requires a trusted CA store and, when appropriate, intermediate certificates:
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openssl verify -CAfile root-ca.pem -untrusted intermediate.pem cert.pemThe leaf certificate is checked against the trusted root while the intermediate is supplied as an untrusted chain-building certificate. A successful result means the signature chain and relevant certificate constraints pass for that verification context; it does not automatically test every application-specific policy.
Where the installed OpenSSL version supports hostname verification, add the expected name:
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Convert PEM, DER, PKCS#12 and PKCS#7 files
PEM is Base64 text with delimiters such as -----BEGIN CERTIFICATE-----. DER is the binary encoding of the same ASN.1 data. PKCS#12 (usually .p12 or .pfx) is a password-protected container that can hold a private key, leaf certificate and chain. PKCS#7 (often .p7b) carries certificates and, in some cases, a chain, but not a private key.
| Task | Command | Result |
|---|---|---|
| PEM certificate to DER | openssl x509 -in cert.pem -outform DER -out cert.der |
Binary DER certificate |
| DER certificate to PEM | openssl x509 -inform DER -in cert.der -outform PEM -out cert.pem |
PEM certificate |
| Create a PKCS#12 bundle | openssl pkcs12 -export -inkey private.key -in cert.pem -certfile chain.pem -out bundle.p12 |
Password-protected key, leaf and optional chain |
| Extract certificates from PKCS#12 | openssl pkcs12 -in bundle.p12 -clcerts -nokeys -out cert.pem |
Leaf certificate in PEM form |
| Create PKCS#7 certificate bundle | openssl crl2pkcs7 -nocrl -certfile cert.pem -out certs.p7b |
PKCS#7 bundle without a revocation list |
| Extract PEM certificates from PKCS#7 | openssl pkcs7 -in certs.p7b -print_certs -out certs.pem |
Certificates printed as PEM |
For a DER-encoded PKCS#7 file, add -inform DER to the pkcs7 command; to write DER, add -outform DER. PKCS#12 export prompts for a container password. Protect that password and the extracted key as separate secrets, and avoid leaving temporary unencrypted files in shared directories.
Quick Recap
Diagnose common TLS certificate failures
| Symptom | Likely cause | Fix |
|---|---|---|
| Hostname mismatch | The requested name is absent from SANs or the client connected with a different name. | Issue a certificate containing the exact DNS or IP SAN and verify with the name clients use. |
| “Unable to get local issuer” or similar chain error | An intermediate is missing, in the wrong order, or the client lacks the correct trust anchor. | Supply the complete leaf-to-intermediate chain and install or reference the intended root CA. |
| Not yet valid or expired | The certificate dates do not cover the current time, or a system clock is wrong. | Correct the clock and replace the certificate before its validity window closes. |
| Private key does not match certificate | A different key was paired with the certificate during deployment. | Compare the public key or modulus derived from both files and select the matching pair. |
| Unsupported format or “bad end line” | The application expects DER, PEM, PKCS#12 or PKCS#7 different from the supplied file. | Convert explicitly, preserve the PEM delimiters, and confirm the target software’s import setting. |
| Certificate rejected for purpose | Key usage or extended key usage does not authorize the server, client or CA role. | Inspect -purpose and request a certificate with the required usages. |
| Self-signed or unknown authority warning | The certificate is not anchored in a CA trusted by the client. | Use a publicly trusted certificate for public services, or distribute the private CA trust anchor to every controlled client. |
| Service cannot start after key installation | The key is encrypted and the service has no password mechanism, or file permissions block its account. | Configure secure password delivery, or use an unencrypted key only with compensating host controls and restrictive permissions. |
Deploy and maintain certificates safely
- Keep private keys out of tickets, chat, source control and general-purpose backups. Treat a leaked key as a compromised identity.
- Restrict the key file to the account or secret-management process that needs it. Remove temporary extraction files after import.
- Deploy the leaf certificate with the required intermediate chain. Do not assume clients can download missing intermediates.
- Before restarting a service, inspect the certificate dates, issuer, SANs, key usage, public key and fingerprint.
- Test both chain building and hostname verification against the same trust roots your clients use.
- Record the OpenSSL version, configuration file, key algorithm, command options and certificate identifiers. That record makes renewals and incident response reproducible.
- Monitor expiry with
-checkendand begin replacement early enough to account for CA validation, approval and rollout time.
Choose the workflow that fits the environment
| Decision | Use this when | Important caution |
|---|---|---|
| Self-signed | Lab testing, temporary development or a controlled private trust domain | Every client must explicitly trust it; public browsers will not normally do so. |
| CA-issued | Public HTTPS or any service whose clients already trust the issuing hierarchy | Validate the returned chain, SANs and usage before deployment. |
| Encrypted private key | A service can retrieve a password securely at startup | Password loss can prevent an automated restart; document recovery. |
| Unencrypted private key | An appliance or service has no password support | Use strict permissions, host hardening and secret-handling controls. |
| Interactive CSR | One-off work where a human can review every prompt | Prompt mistakes and omitted SANs are easy to miss. |
| Configuration-driven CSR | Renewals, automation and repeatable infrastructure | Review the configuration and generated CSR so stale names are not carried forward. |
| PEM | Unix-like servers and software that accepts text certificates and keys | Keep the BEGIN/END delimiters intact and protect key files. |
| DER, PKCS#12 or PKCS#7 | Platforms that require binary certificates or a packaged import format | PKCS#7 does not carry a private key; PKCS#12 does and must be password-protected. |
A final pre-deployment checklist
- Run
openssl version -aand save the version with the change record. - Confirm the private key is the intended algorithm, is stored securely and is not present in the CSR submission.
- Run
openssl req -in request.csr -text -verify -noout; check the subject and every SAN. - Inspect the issued certificate with
openssl x509 -in cert.pem -text -noout. - Check
-dates, issuer, key usage, extended key usage, fingerprint and public key. - Run chain verification with the correct root and intermediate files, plus hostname verification where supported.
- Convert only after verification, then inspect the converted output and import it into the target service.
- Restart or reload the service using a controlled change, and test from a real client using the production hostname.
- Set an expiry alert and retain the exact commands and configuration needed for the next renewal.
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