Brief notes on TLS/SSL keys and certificates, their file formats and converting them using OpenSSL.

tls-ssl-openssl.md

TLS/SSL & OpenSSL

Brief notes on TLS/SSL private keys and certificates, their various formats and converting them to different formats using OpenSSL.

Resources & References

• OpenSSL Commands: https://www.openssl.org/docs/man1.1.1/man1/ (current stable version - v1.1.1)
• OpenSSL Cookbook: https://www.feistyduck.com/books/openssl-cookbook/
• Public-key Cryptography Standards (PKCS): https://en.wikipedia.org/wiki/PKCS
• X.509 certificates filename extensions: https://en.wikipedia.org/wiki/X.509#Certificate_filename_extensions
• Java keytool: https://docs.oracle.com/javase/8/docs/technotes/tools/windows/keytool.html

File Formats of Keys and Certificates

• Keys and X.509 certificates are usually stored in PKCS formats.
• They can be converted to:
  • Binary files of ASN.1 notation in DER format.
  • ASCII (text) files of Base64 encoding in PEM format.

Keys

• Keys are usually stored as Base64 encoded .pem files.
• Keys can be generated and processed using following commands, depending on their type —
  • genrsa, rsa: RSA keys.
  • gendsa, dsa: DSA keys.
  • genpkey, pkey: Recommended for both RSA and DSA keys.
• In OpenSSL 3.0 genrsa command was deprecated and genpkey should be used instead.
• The genpkey command generates keys in PKCS#8 format. Encrypted keys of this format have the phrase ENCRYPTED PRIVATE KEY in both header and trailer records.
• Depending on how they were generated, keys can be converted from PEM to DER format, and vice-versa, using the rsa, dsa, and pkey commands.

Certificates

• X.509 certificates are usually stored in PKCS#7 format of extensions .p7b and .p7c.
• PKCS7 files can be converted to readable PEM files using the command:

openssl pkcs7 -in <p7b input> -print_certs -out <output name>

• Certificates in PEM format can be converted to PKCS7 format using crl2pkcs7 command.
• Certificates in PEM format can be converted to DER format, and vice-versa, using the command:

openssl x509 -inform <PEM|DER> -in <input file> -outform <DER|PEM> -out <output name>

• DER certificates filename extensions are .der, .cer, and .crt.

Certificates & Keys

• On server-side, certificate and keys can be stored together in PKCS#12 format as .p12 files.
• PKCS12 files can be split into constituent key, certificate and cert-chain, and vice-versa, using pkcs12 command.

Verifying RSA Key & Certificate Pair

1. Check key:

openssl rsa -check -noout -in <key file>

2. Verify output is: RSA key ok.
3. Get MD5 hash of the key's modulus:

openssl rsa -modulus -noout -in <key file> | openssl md5

4. Get MD5 hash of the certificate's modulus:

openssl x509 -modulus -noout -in <DER/PEM certificate file> | openssl md5

5. Compare the MD5 hashes of above two steps. If they match, the key and the certificate are a pair.

Appendix

• ASN.1 JavaScript decoder: https://lapo.it/asn1js/