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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 to JDK 1.8. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.

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package org.bouncycastle.cert.path.validations;

import java.math.BigInteger;

import org.bouncycastle.asn1.x509.BasicConstraints;
import org.bouncycastle.asn1.x509.Extension;
import org.bouncycastle.cert.X509CertificateHolder;
import org.bouncycastle.cert.path.CertPathValidation;
import org.bouncycastle.cert.path.CertPathValidationContext;
import org.bouncycastle.cert.path.CertPathValidationException;
import org.bouncycastle.util.BigIntegers;
import org.bouncycastle.util.Integers;
import org.bouncycastle.util.Memoable;


public class BasicConstraintsValidation
    implements CertPathValidation
{

    private boolean previousCertWasCA = true;
    private Integer maxPathLength = null;
    private boolean isMandatory = true;

    public BasicConstraintsValidation()
    {
        this(true);
    }

    public BasicConstraintsValidation(boolean isMandatory)
    {
        this.isMandatory = isMandatory;
    }

    public void validate(CertPathValidationContext context, X509CertificateHolder certificate)
        throws CertPathValidationException
    {

        context.addHandledExtension(Extension.basicConstraints);

        // verify that the issuing certificate is in fact a CA
        if (!previousCertWasCA)
        {
            throw new CertPathValidationException("Basic constraints violated: issuer is not a CA");
        }

        // RFC 5280 § 6.1.4 (k)
        // If this certificate is a CA, remember that for processing in the next step
        BasicConstraints bc = BasicConstraints.fromExtensions(certificate.getExtensions());
        this.previousCertWasCA = (bc != null && bc.isCA()) || (bc == null && !this.isMandatory);

        // if the certificate is not self-issued (see § 4.2.1.9 and § 6.1.4 (l) of RFC 5280),
        // it "uses up" one path length unit.
        // NOTE: self-issued != self-signed. We only need to compare subject DN and issuer DN here.
        if (maxPathLength != null && !certificate.getSubject().equals(certificate.getIssuer()))
        {
            if (maxPathLength.intValue() < 0)
            {
                throw new CertPathValidationException("Basic constraints violated: path length exceeded");
            }
            maxPathLength = Integers.valueOf(maxPathLength.intValue() - 1);
        }

        // § 6.1.4 (m)
        // Update maxPathLength if appropriate
        if (bc != null)
        {
            BigInteger bigPathLen = bc.getPathLenConstraint();
            if (bigPathLen != null)
            {
                // use intValueExact to prevent issues with weird certificates that include ridiculous path lengths
                int newPathLength = BigIntegers.intValueExact(bigPathLen);
                maxPathLength = maxPathLength == null ? Integers.valueOf(newPathLength) : Integers.valueOf(Math.min(newPathLength, maxPathLength.intValue()));
            }
        }

    }

    public Memoable copy()
    {
        BasicConstraintsValidation result = new BasicConstraintsValidation();
        result.isMandatory = this.isMandatory;
        result.previousCertWasCA = this.previousCertWasCA;
        result.maxPathLength = this.maxPathLength;
        return result;
    }

    public void reset(Memoable other)
    {
        BasicConstraintsValidation otherBCV = (BasicConstraintsValidation)other;
        this.isMandatory = otherBCV.isMandatory;
        this.previousCertWasCA = otherBCV.previousCertWasCA;
        this.maxPathLength = otherBCV.maxPathLength;
    }
}