subjectPublicKeyInfo

SubjectPublicKeyInfo is a data structure used in X.509 certificates and related PKI standards to convey the subject’s public key. It is defined in RFC 5280 as the portion of a certificate that allows others to obtain the public key corresponding to the subject’s private key, enabling signature verification and encryption operations.

The structure is a DER-encoded SEQUENCE with two components: algorithm and subjectPublicKey. The algorithm component is an AlgorithmIdentifier, which carries an object identifier (OID) for the public-key algorithm and optional parameters that describe any algorithm-specific details. The subjectPublicKey component is a BIT STRING that contains the actual public key data encoded according to the chosen algorithm.

For RSA keys, the algorithm identifier uses the OID rsaEncryption with NULL parameters, and subjectPublicKey contains the DER encoding of the RSAPublicKey structure, which comprises the modulus and public exponent. For elliptic-curve keys (ECDSA), the algorithm identifier uses the OID id-ecPublicKey with parameters identifying the named curve, while subjectPublicKey holds the EC point encoding (usually in uncompressed form) per SEC1 standards.

In practice, SubjectPublicKeyInfo appears in certificates, certificate signing requests, and other PKI artifacts to bind a subject to a public key. The SPKI data is typically encoded in DER and may be represented in PEM format as a PUBLIC KEY block. Understanding subjectPublicKeyInfo is central to parsing and validating digital certificates and public keys in TLS, S/MIME, and related protocols. See X.509, RFC 5280, and PKIX for details.