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

Understanding MarshalPKIXPublicKey() vs MarshalPKCS1PublicKey() in Golang

Introduction

MarshalPKIXPublicKey() and MarshalPKCS1PublicKey() are two functions defined in the x509 package of the Go standard library. While they both deal with public key serialization, their respective usage differs based on the specific requirements of the underlying protocols or applications.

MarshalPKIXPublicKey()

Purpose: Serializes a public key into DER-encoded PKIX format.

DER-encoded PKIX format:

• DER (Distinguished Encoding Rules) is a scheme for encoding ASN.1 (Abstract Syntax Notation One) data structures into byte sequences.
• PKIX (Public Key Infrastructure X.509) is a variant of the X.509 standard that specifies the format of digital certificates.

In the context of MarshalPKIXPublicKey(), the public key is serialized into a SubjectPublicKeyInfo structure, which is then DER-encoded. The SubjectPublicKeyInfo structure includes information about the algorithm used and the public key value itself.

MarshalPKCS1PublicKey()

Purpose: Converts an RSA public key into PKCS#1, ASN.1 DER form.

PKCS#1:

• PKCS#1 is a set of standards for cryptographic operations, including one for defining a public key format (RSA public key in this case).
• The ASN.1 encoding specified in PKCS#1 is different from the DER encoding used by PKIX.

MarshalPKCS1PublicKey() serializes an RSA public key into an ASN.1 structure defined by PKCS#1. This structure includes information about the RSA algorithm parameters and the key modulus and exponent. The resulting data is then DER-encoded.

Key Differences

Feature	MarshalPKIXPublicKey()	MarshalPKCS1PublicKey()
Input	Any type of public key	RSA public key specifically
Output format	DER-encoded PKIX SubjectPublicKeyInfo	DER-encoded PKCS#1 ASN.1 structure
Algorithm independence	Yes	No (only RSA public keys)

Applicability

MarshalPKIXPublicKey() is used when you need to serialize a public key in a general-purpose format that can be used by a wide range of protocols and applications that rely on X.509 certificates.

MarshalPKCS1PublicKey() is specifically used for serializing RSA public keys in a format that conforms to the PKCS#1 standard. This is commonly used in contexts such as digitally signing data or verifying signatures created using RSA keys.

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