IllegalArgumentException when Building Certificate with Delta Certificate Extension if Subject and Issuer are the Same

[NoahFreising]

While playing around with delta certificates, I noticed that BC throws an IllegalArgumentException when building a certificate with a delta certificate extension constructed using DeltaCertificateTool.makeDeltaCertificateExtension(). This occurs when both the Issuer and Subject of the base and delta certificate are the same. The exception is not thrown, and the certificate is constructed correctly, if only one of the Issuer and Subject is the same or if they are different. The relevant Internet-Draft appears to allow the same Subject and Issuer to be present in both the delta and base certificate, meaning that neither will be present in the constructed extension.

BC version: 1.78.1.

Stack trace:

Exception in thread "main" java.lang.IllegalArgumentException: illegal object in getInstance: org.bouncycastle.asn1.x509.SubjectPublicKeyInfo
	at org.bouncycastle.asn1.ASN1BitString.getInstance(Unknown Source)
	at org.bouncycastle.asn1.x509.DeltaCertificateDescriptor.<init>(Unknown Source)
	at org.bouncycastle.asn1.x509.DeltaCertificateDescriptor.trimTo(Unknown Source)
	at org.bouncycastle.cert.X509v3CertificateBuilder.build(Unknown Source)
	at ch.freising.pqcthesis.DeltaCertMinimalTest.main(DeltaCertMinimalTest.java:52)

Minimum example that throws an exception:

package ch.freising.pqcthesis;

import org.bouncycastle.asn1.x500.X500Name;
import org.bouncycastle.asn1.x509.Extension;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.cert.DeltaCertificateTool;
import org.bouncycastle.cert.X509CertificateHolder;
import org.bouncycastle.cert.X509v3CertificateBuilder;
import org.bouncycastle.jcajce.provider.asymmetric.rsa.KeyPairGeneratorSpi;
import org.bouncycastle.operator.ContentSigner;
import org.bouncycastle.operator.OperatorCreationException;
import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder;

import java.io.IOException;

public class DeltaCertMinimalTest {
    public static void main(String[] args) throws OperatorCreationException, IOException {
        // Generate RSA KeyPairs
        KeyPairGeneratorSpi rsaKeyGen = new KeyPairGeneratorSpi();
        rsaKeyGen.initialize(2048, new java.security.SecureRandom());
        java.security.KeyPair deltaKeyPair = rsaKeyGen.generateKeyPair();
        java.security.KeyPair baseKeyPair = rsaKeyGen.generateKeyPair();

        // Generate a self-signed Delta Certificate
        X509v3CertificateBuilder deltaCertBuilder = new X509v3CertificateBuilder(
                new X500Name("CN=Issuer"),
                java.math.BigInteger.valueOf(1L),
                new java.util.Date(System.currentTimeMillis()),
                new java.util.Date(System.currentTimeMillis() + 365L * 24 * 60 * 60 * 1000),
                new X500Name("CN=Subject"),
                SubjectPublicKeyInfo.getInstance(deltaKeyPair.getPublic().getEncoded())
        );
        ContentSigner deltaRootSigner = new JcaContentSignerBuilder("SHA256withRSA").build(deltaKeyPair.getPrivate());
        X509CertificateHolder deltaCert = deltaCertBuilder.build(deltaRootSigner);

        // Generate a self-signed Base Certificate
        X509v3CertificateBuilder baseCertBuilder = new X509v3CertificateBuilder(
                new X500Name("CN=Issuer"), // Same as Delta Certificate
                java.math.BigInteger.valueOf(2L),
                new java.util.Date(System.currentTimeMillis()),
                new java.util.Date(System.currentTimeMillis() + 365L * 24 * 60 * 60 * 1000),
                new X500Name("CN=Subject"), // Same as Delta Certificate
                SubjectPublicKeyInfo.getInstance(baseKeyPair.getPublic().getEncoded())
        );

        // Create Delta Extension
        Extension deltaCertExtension = DeltaCertificateTool.makeDeltaCertificateExtension(false, deltaCert);
        // Add Delta Extension to Base Certificate
        baseCertBuilder.addExtension(deltaCertExtension);
        // Build Base Certificate
        ContentSigner baseRootSigner = new JcaContentSignerBuilder("SHA256withRSA").build(baseKeyPair.getPrivate());
        X509CertificateHolder baseCert = baseCertBuilder.build(baseRootSigner); // <= Exception thrown here
    }
}