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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5.

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package org.bouncycastle.asn1.x509;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1OctetString;
import org.bouncycastle.asn1.ASN1TaggedObject;
import org.bouncycastle.asn1.DERObject;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.digests.SHA1Digest;

/**
 * The SubjectKeyIdentifier object.
 *  * SubjectKeyIdentifier::= OCTET STRING
 * 
 */
public class SubjectKeyIdentifier
    extends ASN1Encodable
{
    private byte[] keyidentifier;

    public static SubjectKeyIdentifier getInstance(
        ASN1TaggedObject obj,
        boolean          explicit)
    {
        return getInstance(ASN1OctetString.getInstance(obj, explicit));
    }

    public static SubjectKeyIdentifier getInstance(
        Object obj)
    {
        if (obj instanceof SubjectKeyIdentifier)
        {
            return (SubjectKeyIdentifier)obj;
        }
        
        if (obj instanceof SubjectPublicKeyInfo) 
        {
            return new SubjectKeyIdentifier((SubjectPublicKeyInfo)obj);
        }
        
        if (obj instanceof ASN1OctetString) 
        {
            return new SubjectKeyIdentifier((ASN1OctetString)obj);
        }

        if (obj instanceof X509Extension)
        {
            return getInstance(X509Extension.convertValueToObject((X509Extension)obj));
        }

        throw new IllegalArgumentException("Invalid SubjectKeyIdentifier: " + obj.getClass().getName());
    }
    
    public SubjectKeyIdentifier(
        byte[] keyid)
    {
        this.keyidentifier=keyid;
    }

    public SubjectKeyIdentifier(
        ASN1OctetString  keyid)
    {
        this.keyidentifier=keyid.getOctets();
    }

    /**
     * Calculates the keyidentifier using a SHA1 hash over the BIT STRING
     * from SubjectPublicKeyInfo as defined in RFC3280.
     *
     * @param spki the subject public key info.
     */
    public SubjectKeyIdentifier(
        SubjectPublicKeyInfo    spki)
    {
        this.keyidentifier = getDigest(spki);
    }

    public byte[] getKeyIdentifier()
    {
        return keyidentifier;
    }

    public DERObject toASN1Object()
    {
        return new DEROctetString(keyidentifier);
    }

    /**
     * Return a RFC 3280 type 1 key identifier. As in:
     *      * (1) The keyIdentifier is composed of the 160-bit SHA-1 hash of the
     * value of the BIT STRING subjectPublicKey (excluding the tag,
     * length, and number of unused bits).
     * 
     * @param keyInfo the key info object containing the subjectPublicKey field.
     * @return the key identifier.
     */
    public static SubjectKeyIdentifier createSHA1KeyIdentifier(SubjectPublicKeyInfo keyInfo)
    {
        return new SubjectKeyIdentifier(keyInfo);
    }

    /**
     * Return a RFC 3280 type 2 key identifier. As in:
     *      * (2) The keyIdentifier is composed of a four bit type field with
     * the value 0100 followed by the least significant 60 bits of the
     * SHA-1 hash of the value of the BIT STRING subjectPublicKey.
     * 
     * @param keyInfo the key info object containing the subjectPublicKey field.
     * @return the key identifier.
     */
    public static SubjectKeyIdentifier createTruncatedSHA1KeyIdentifier(SubjectPublicKeyInfo keyInfo)
    {
        byte[] dig = getDigest(keyInfo);
        byte[] id = new byte[8];

        System.arraycopy(dig, dig.length - 8, id, 0, id.length);

        id[0] &= 0x0f;
        id[0] |= 0x40;
        
        return new SubjectKeyIdentifier(id);
    }

    private static byte[] getDigest(SubjectPublicKeyInfo spki)
    {
        Digest digest = new SHA1Digest();
        byte[]  resBuf = new byte[digest.getDigestSize()];

        byte[] bytes = spki.getPublicKeyData().getBytes();
        digest.update(bytes, 0, bytes.length);
        digest.doFinal(resBuf, 0);
        return resBuf;
    }
}