When to Use `x509.MarshalPKIXPublicKey()` vs. `x509.MarshalPKCS1PublicKey()` in Go?

Understanding x509.MarshalPKIXPublicKey() and MarshalPKCS1PublicKey()

In Go, the x509 package provides functions for encoding public keys into DER-encoded formats. Two such functions are MarshalPKIXPublicKey() and MarshalPKCS1PublicKey(), which serve specific purposes for different scenarios.

DER-encoded PKIX Format

DER (Distinguished Encoding Rules) is a method of encoding ASN.1 (Abstract Syntax Notation One) data into sequences of bytes. PKIX (Public Key Infrastructure X.509) defines a framework for verifying public keys and issuing digital certificates. A public key encoded in DER-encoded PKIX format conforms to the structure defined in PKIX standards, which includes an algorithm identifier and the DER-encoded algorithm-specific public key value.

x509.MarshalPKIXPublicKey() Function

The MarshalPKIXPublicKey() function converts an ASN.1 SubjectPublicKeyInfo structure containing the public key into DER-encoded PKIX format. This structure consists of an AlgorithmIdentifier specifying the public key algorithm and a BIT STRING containing the DER-encoded public key value. In the case of RSA public keys, this value is the RSAPublicKey structure defined inPKCS1.

x509.MarshalPKCS1PublicKey() Function

The MarshalPKCS1PublicKey() function specifically converts RSA public keys into DER-encoded PKCS#1 format. PKCS#1 (Public Key Cryptography Standard #1) is another standard that defines a structure for representing RSA public keys. It consists of an AlgorithmIdentifier for RSA and a BIT STRING containing the DER-encoded RSA public key value.

Key Differences

• MarshalPKIXPublicKey() encodes the public key in DER-encoded PKIX format, which includes an algorithm identifier and can handle multiple public key algorithms.
• MarshalPKCS1PublicKey() specifically encodes RSA public keys in DER-encoded PKCS#1 format.
• PKIX format is intended for public key certificates, while PKCS#1 format is commonly used for public key encryption and signing.

In summary, MarshalPKIXPublicKey() provides a generic way to encode public keys in DER-encoded PKIX format, while MarshalPKCS1PublicKey() specifically handles RSA public keys and encodes them in PKCS#1 format. The appropriate function to use depends on the specific requirements of the application.

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