What is the difference between x509.MarshalPKIXPublicKey and x509.MarshalPKCS1PublicKey in Go?

Understanding the Difference Between x509.MarshalPKIXPublicKey and x509.MarshalPKCS1PublicKey

In the realm of cryptography, the concepts of DER-encoded PKIX format and PKCS#1 can be encountered. These are two distinct encodings used for representing public keys, specifically in the context of RSA encryption. To elucidate the difference between the functions x509.MarshalPKIXPublicKey and x509.MarshalPKCS1PublicKey in Go, it's crucial to understand these underlying concepts.

Distinguished Encoding Rules and Abstract Syntax Notation One

Distinguished Encoding Rules (DER) defines a method for encoding data structures created using Abstract Syntax Notation One (ASN.1) into a compact and platform-independent binary format. ASN.1, in turn, provides a standardized language for describing data types and structures used in various protocols and applications.

PKCS#1 refers to a set of standards developed by RSA Security and standardized by the IETF. PKCS#1, Appendix A defines an ASN.1 structure for representing an RSA public key called RSAPublicKey. This structure can be DER-encoded using the DER encoding rules.

MarshalPKIXPublicKey

This function in Go's crypto/x509 package serializes a public key into the DER-encoded PKIX format. PKIX, or Public Key Infrastructure X.509, is a variant of the X.509 standard that primarily defines an ASN.1 structure for representing public-key certificates.

In the context of RSA, the DER-encoded PKIX format includes the following components:

• AlgorithmIdentifier: Identifies the algorithm used for the public key, which is RSA in this case.
• BIT STRING: Contains the DER-encoded RSAPublicKey structure.

MarshalPKCS1PublicKey

This function converts an RSA public key into its PKCS#1 form, which is ASN.1 DER-encoded. As mentioned earlier, RSAPublicKey is the ASN.1 structure used to represent an RSA public key in PKCS#1. Hence, x509.MarshalPKCS1PublicKey serializes the RSA public key into the DER-encoded RSAPublicKey structure.

Summary

In essence, MarshalPKIXPublicKey generates a binary representation of an RSA public key following the PKIX format, while MarshalPKCS1PublicKey produces a binary representation of the public key in PKCS#1 format. PKIX format encompasses more context than just the key itself, including information on the certifying authority and the associated algorithm, making it suitable for use in digital certificates.

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