X.509 Section 9.8.2.
This public-key certificate extension, when present, shall contain the subject’s alternative public key information

subjectAltPublicKeyInfo EXTENSION ::= {
     SYNTAX SubjectAltPublicKeyInfo
     IDENTIFIED BY id-ce-subjectAltPublicKeyInfo }

SubjectAltPublicKeyInfo ::= SEQUENCE {
    algorithm AlgorithmIdentifier{{SupportedAlgorithms}},
    subjectAltPublicKey BIT STRING }

The SubjectAltPublicKeyInfo data type has the following components:

• the algorithm subcomponent, which shall hold the algorithm that this public key is an instance of
• the subjectAltPublicKey subcomponent, which shall hold the alternative public key

This extension may be flagged as critical or as non-critical.
NOTE – It is recommended that it be flagged as non-critical. Flagging it as critical would require relying parties to understand this extension and the alternative public-key algorithm.