OpenSSL Cheat Sheet for Self-Signed Certificates

OpenSSL version note: Commands below target OpenSSL 3.x. Some flags differ in 1.x — noted inline where relevant.

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Create Self-Signed Certificates with OpenSSL

Root CA

Create Root CA Key (With Strong Encryption)

Use -aes256 to protect the key with a passphrase:

openssl genpkey -algorithm RSA -out root.key -aes256 -pkeyopt rsa_keygen_bits:4096

Or with Ed25519 (modern, recommended where compatibility allows):

openssl genpkey -algorithm Ed25519 -aes256 -out root.key

Create Root CA Certificate

Generate a self-signed Root CA certificate, valid for 10 years:

openssl req -new -x509 -sha256 -days 3650 -key root.key -extensions v3_ca -out root.crt

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Intermediate CA

Create Intermediate CA Key

openssl genpkey -algorithm RSA -out ca.key -aes256 -pkeyopt rsa_keygen_bits:4096

Create Intermediate CA Request

openssl req -new -key ca.key -out ca.csr

Sign Intermediate CA Request with Root CA

Use an extension file (v3_intermediate_ca.ext) for proper CA settings:

openssl x509 -req -in ca.csr -CA root.crt -CAkey root.key \
  -CAcreateserial -extfile v3_intermediate_ca.ext \
  -days 3650 -sha256 -out ca.crt

Create CA Chain File

Combine the Intermediate CA and Root CA certificates:

cat ca.crt root.crt > ca_chain.crt

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Server Certificates

Create Server Certificate Key

openssl genpkey -algorithm RSA -out server.key -aes256 -pkeyopt rsa_keygen_bits:4096

Create Server Certificate Request

Ensure the request includes a Subject Alternative Name (SAN):

openssl req -new -key server.key -out server.csr -config openssl.cnf

Sign Server Certificate with Intermediate CA

Use an extension file (v3_server_cert.ext) for SAN and proper key usage:

openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key \
  -CAcreateserial -extfile v3_server_cert.ext \
  -days 730 -sha256 -out server.crt

Public vs internal PKI: Publicly trusted certificates issued by commercial CAs are capped at 398 days (enforced by browsers since 2020). The 730-day limit here is fine for internal/private PKI only.

Generate Diffie-Hellman Parameters

For TLS 1.2 cipher suites that use DHE key exchange:

openssl dhparam -out dhparam.pem 4096

TLS 1.3 note: TLS 1.3 uses X25519 for key exchange by default and does not use custom DH parameters. dhparam is only relevant if you are still supporting TLS 1.2 DHE cipher suites.

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Client Certificates

Create Client Certificate Key

openssl genpkey -algorithm RSA -out client.key -aes256 -pkeyopt rsa_keygen_bits:4096

Create Client Certificate Request

openssl req -new -key client.key -out client.csr -config openssl.cnf

Sign Client Certificate

Use a custom extension file (v3_client_cert.ext):

openssl x509 -req -in client.csr -CA ca.crt -CAkey ca.key \
  -CAcreateserial -extfile v3_client_cert.ext \
  -days 730 -sha256 -out client.crt

Convert Client Certificate to PKCS12 Format

openssl pkcs12 -export -certfile ca_chain.crt -in client.crt -inkey client.key -out client.p12

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Verify Certificates

Verify Certificate Against CA

openssl verify -verbose -CAfile ca_chain.crt server.crt

Verify Certificate Matches Private Key

openssl x509 -noout -modulus -in server.crt | openssl sha256
openssl pkey -noout -modulus -in server.key | openssl sha256

Changed from MD5 — MD5 is a broken hash algorithm. Both commands must produce the same SHA-256 output for the key and certificate to match.

For Ed25519/EC keys, use the public key fingerprint instead (modulus doesn't apply):

openssl pkey -in server.key -pubout | openssl sha256
openssl x509 -in server.crt -pubkey -noout | openssl sha256

Check SSL Protocol Support

openssl s_client -connect HOST:443 -tls1_3
openssl s_client -connect HOST:443 -tls1_2

TLS 1.0 and TLS 1.1 are deprecated by RFC 8996. A server that only accepts those versions is a security finding.

Check SSL Cipher Support

openssl s_client -connect HOST:443 -cipher ${cipher}

Test SNI (Server Name Indication)

openssl s_client -connect HOST:443 -servername ${sni_host}

Test Client Certificate Authentication

openssl s_client -connect HOST:443 -cert client.crt -key client.key -state

Dropped -debug — it outputs raw bytes to the terminal and is rarely useful outside of deep protocol debugging.

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Example Configuration Files

v3_intermediate_ca.ext

basicConstraints = CA:TRUE, pathlen:0
keyUsage = keyCertSign, cRLSign
authorityKeyIdentifier = keyid:always,issuer:always
subjectKeyIdentifier = hash

v3_server_cert.ext

basicConstraints = CA:FALSE
keyUsage = digitalSignature, keyEncipherment
extendedKeyUsage = serverAuth
subjectAltName = @alt_names

[alt_names]
DNS.1 = example.com
DNS.2 = *.example.com

EC/Ed25519 key usage note: If using an EC or Ed25519 server key, replace keyEncipherment with keyAgreement in the keyUsage line — keyEncipherment is RSA-specific.

v3_client_cert.ext

basicConstraints = CA:FALSE
keyUsage = digitalSignature
extendedKeyUsage = clientAuth

openssl.cnf

[req]
distinguished_name = req_distinguished_name
x509_extensions = v3_ca
req_extensions = v3_req
default_md = sha256
prompt = no

[req_distinguished_name]
C = US
ST = New York
L = New York
O = Example Organization
OU = IT Department
CN = example.com

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

[v3_req]
basicConstraints = CA:FALSE
keyUsage = digitalSignature, keyEncipherment
subjectAltName = @alt_names

[alt_names]
DNS.1 = example.com
DNS.2 = *.example.com

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Recommendations

1. Always use SHA-256 or higher for hashing. SHA-1 and MD5 are broken.
2. Protect all private keys with strong passphrases using -aes256.
3. Prefer modern key algorithms — Ed25519 for new systems, P-384 for ECDSA, RSA-4096 as a fallback for broad compatibility. Avoid RSA below 3072-bit for any new key.
4. Always include SANs in server certificates. CN-only certificates are rejected by all modern clients.
5. Target TLS 1.3 where possible. Disable TLS 1.0 and 1.1 — both are deprecated by RFC 8996.
6. Keep OpenSSL updated to the latest stable 3.x release.
7. Disable weak protocols and ciphers — SSLv2, SSLv3, RC4, DES, 3DES, MD5, and SHA-1 signatures have no place in current deployments.

For further details, refer to the OpenSSL Documentation.