PEM, DER, PKCS

In the context of OpenSSL and cryptography, PEM, DER, and PKCS refer to different formats and standards for storing and exchanging cryptographic keys, certificates, and other data.

PEM (Privacy-Enhanced Mail)

PEM is a text-based format for encoding binary data, such as cryptographic keys and certificates. PEM files are Base64 encoded and enclosed between -----BEGIN and -----END lines. PEM format is widely used because it is easy to read and transfer through text-based protocols like email.

PEM File Example:

-----BEGIN CERTIFICATE-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuKQ/JffsbEOK+0FypUtw
...
-----END CERTIFICATE-----

Common PEM File Extensions:

• .pem

• .crt (for certificates)

• .cer (for certificates)

• .key (for private keys)

DER (Distinguished Encoding Rules)

DER is a binary format for encoding data structures described by ASN.1. It is a more compact representation compared to PEM and is commonly used in Java environments.

Converting PEM to DER:

To convert a PEM-formatted certificate to DER format, use the following OpenSSL command:

openssl x509 -in certificate.pem -outform der -out certificate.der

Converting DER to PEM:

To convert a DER-formatted certificate to PEM format, use the following command:

openssl x509 -in certificate.der -inform der -out certificate.pem -outform pem

Common DER File Extensions:

• .der

• .cer (for certificates)

• .crt (for certificates)

PKCS (Public Key Cryptography Standards)

PKCS refers to a group of standards published by RSA Laboratories for public key cryptography. Several PKCS standards are relevant to OpenSSL:

PKCS#1 (RSA Cryptography Standard): Defines the format for RSA public and private keys.

PKCS#7 (Cryptographic Message Syntax Standard): Used to sign and/or encrypt messages under a PKI. Commonly used for certificate chains.

PKCS#8 (Private-Key Information Syntax Standard): Defines the format for private keys, supporting multiple algorithms.

PKCS#10 (Certification Request Standard): Defines the format for certificate signing requests (CSRs).

PKCS#12 (Personal Information Exchange Syntax Standard): Defines a format for storing multiple cryptographic objects, such as certificates and private keys, in a single file. Often used for transferring private keys and certificates together.

Creating a PKCS#12 File:

To create a PKCS#12 file containing a private key and a certificate:

openssl pkcs12 -export -out keystore.p12 -inkey private.key -in certificate.crt -certfile ca-chain.crt

Extracting from a PKCS#12 File:

To extract the private key and certificate from a PKCS#12 file:

openssl pkcs12 -in keystore.p12 -nocerts -out private.key
openssl pkcs12 -in keystore.p12 -nokeys -out certificate.crt

Common PKCS File Extensions:

• .p12 or .pfx (for PKCS#12 files)

Use Cases

PEM Format:

• Ideal for use in web servers (e.g., Apache, Nginx) and other software that can easily handle text-based files.

• Suitable for transmitting certificates and keys via email or other text-based protocols.

DER Format:

• Preferred in environments where compact binary encoding is required, such as Java applications and certain hardware implementations.

PKCS Standards:

• PKCS#1: Used for storing RSA keys.

• PKCS#7: Used for handling certificate chains and signed data.

• PKCS#8: Used for storing private keys in a standardized format.

• PKCS#10: Used for creating certificate signing requests.

• PKCS#12: Used for securely transporting private keys and certificates together, often in client-server applications.

Conclusion

Understanding the differences between PEM, DER, and PKCS formats is crucial for managing cryptographic keys and certificates effectively. Each format has its specific use cases and benefits, making them suitable for different environments and purposes. By leveraging OpenSSL's capabilities to convert and manage these formats, administrators can ensure secure and efficient handling of cryptographic data.