PKCS8

PKCS#8 refers to the eighth Public-Key Cryptography Standards specification, published by RSA Security. It defines a standard syntax for storing and transmitting private key information in a portable, technology-neutral form. The primary goal is to provide a single, consistent container for private keys that can be used across different algorithms and software.

The core of PKCS#8 consists of two related structures: PrivateKeyInfo and EncryptedPrivateKeyInfo. PrivateKeyInfo encodes an unencrypted private key and includes three main fields: version (typically 0), privateKeyAlgorithm (an AlgorithmIdentifier that names the public-key algorithm and its parameters), and privateKey (an OCTET STRING containing the key data encoded for the algorithm). The privateKey data is usually a PKCS#1 RSAPrivateKey for RSA keys or another algorithm-specific encoding. PrivateKeyInfo may also include optional attributes.

EncryptedPrivateKeyInfo provides a counterpart for encrypted keys. It contains an encryptionAlgorithm (AlgorithmIdentifier describing the password-based or other encryption method) and encryptedData (the resulting ciphertext). PKCS#8 supports password-based encryption schemes such as PBES2, which enable secure storage of private keys in a file or key store.

PKCS#8 is commonly used in PEM formats, where an unencrypted key is labeled as BEGIN PRIVATE KEY (PKCS#8) and an encrypted key as BEGIN ENCRYPTED PRIVATE KEY. The standard has undergone revisions, with PKCS#8 v1 establishing the PrivateKeyInfo framework and later updates (notably in RFC 5208 and RFC 5958) extending encryption options and algorithm support.

Usage: PKCS#8 provides portability and interoperability for private keys across platforms, tools, and languages, and is widely supported by libraries such as OpenSSL, Java, and others.