Implements extracting (CN) subject which is the issued-to person, used to show "Signed by: <from-address>"

.gitignore

@@ -5,3 +5,5 @@
 .project
 .classpath
 .checkstyle
+/.idea
+/*.iml

src/main/java/net/markenwerk/utils/mail/smime/SmimeUtil.java

@@ -60,7 +60,10 @@
 import org.bouncycastle.asn1.smime.SMIMECapability;
 import org.bouncycastle.asn1.smime.SMIMECapabilityVector;
 import org.bouncycastle.asn1.smime.SMIMEEncryptionKeyPreferenceAttribute;
+import org.bouncycastle.asn1.x500.RDN;
 import org.bouncycastle.asn1.x500.X500Name;
+import org.bouncycastle.asn1.x500.style.BCStyle;
+import org.bouncycastle.asn1.x500.style.IETFUtils;
 import org.bouncycastle.cert.X509CertificateHolder;
 import org.bouncycastle.cert.jcajce.JcaCertStore;
 import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
@@ -491,6 +494,61 @@ private static boolean checkSignature(SMIMESigned smimeSigned) throws MessagingE
 			throw handledException(e);
 		}
 	}
+
+	/**
+	 * @param mimeMultipart
+	 *            The {@link MimeMultipart} to be checked.
+	 * @return The subject / address to which the certificate was issued to. Email clients may use this to show
+	 * 			{@code "Signed by: <subject / address>"}
+	 */
+	public static String getSignedByAddress(MimeMultipart mimeMultipart) {
+		try {
+			return getSignedByAddress(new SMIMESigned(mimeMultipart));
+		} catch (Exception e) {
+			throw handledException(e);
+		}
+	}
+
+	/**
+	 * @param mimePart
+	 *            The {@link MimePart} to be checked.
+	 * @return The subject / address to which the certificate was issued to. Email clients may use this to show
+	 * 			{@code "Signed by: <subject / address>"}
+	 */
+	public static String getSignedByAddress(MimePart mimePart) {
+		try {
+			if (mimePart.isMimeType("multipart/signed")) {
+				return getSignedByAddress(new SMIMESigned((MimeMultipart) mimePart.getContent()));
+			} else if (mimePart.isMimeType("application/pkcs7-mime") || mimePart.isMimeType("application/x-pkcs7-mime")) {
+				return getSignedByAddress(new SMIMESigned(mimePart));
+			} else {
+				throw new SmimeException("Message not signed");
+			}
+		} catch (Exception e) {
+			throw handledException(e);
+		}
+	}
+
+	/**
+	 * Returns the subject / address to which the certificate was issued to. Email clients may use this to show
+	 * {@code "Signed by: <subject / address>"}
+	 */
+	private static String getSignedByAddress(SMIMESigned smimeSigned) {
+		try {
+			@SuppressWarnings("rawtypes")
+			Store certificates = smimeSigned.getCertificates();
+
+			SignerInformation signerInformation = smimeSigned.getSignerInfos().getSigners().iterator().next();
+			X509Certificate certificate = getCertificate(certificates, signerInformation.getSID());
+			SignerInformationVerifier verifier = getVerifier(certificate);
+			X500Name x500name = verifier.getAssociatedCertificate().getSubject();
+			RDN cn = x500name.getRDNs(BCStyle.CN)[0];
+			return IETFUtils.valueToString(cn.getFirst().getValue());
+
+		} catch (Exception e) {
+			throw handledException(e);
+		}
+	}
 
 	private static X509Certificate getCertificate(@SuppressWarnings("rawtypes") Store certificates, SignerId signerId)
 			throws CertificateException {