Initial implementation of WKD address generation

wkd-java/src/main/java/pgp/wkd/UserIdToWKDAddress.java

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+// SPDX-FileCopyrightText: 2022 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package pgp.wkd;
+
+import java.net.URI;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.apache.commons.codec.binary.ZBase32;
+
+public class UserIdToWKDAddress {
+
+    // RegEx for Email Addresses
+    // https://www.baeldung.com/java-email-validation-regex#regular-expression-by-rfc-5322-for-email-validation
+    // Modified by adding capture groups '()' for local and domain part
+    private static final Pattern PATTERN_EMAIL = Pattern.compile("^([a-zA-Z0-9_!#$%&'*+/=?`{|}~^.-]+)@([a-zA-Z0-9.-]+)$");
+    private static final Pattern PATTERN_USER_ID = Pattern.compile("^.*\\<([a-zA-Z0-9_!#$%&'*+/=?`{|}~^.-]+@[a-zA-Z0-9.-]+)\\>.*");
+
+    /**
+     * Extract the email address from a user-id.
+     * The user-id is expected to correspond to a RFC2822 name-addr.
+     * The email address is expected to be framed by angle brackets.
+     *
+     * @see <a href="https://datatracker.ietf.org/doc/html/rfc2822#section-3.4">RFC2822 - Internet Message Format §3.4: Address Specification</a>
+     * @param userId user-id name-addr
+     * @return WKD URI
+     *
+     * @throws IllegalArgumentException in case the user-id does not match the expected format
+     */
+    public URI userIdToUri(String userId) {
+        String lowerCase = userId.toLowerCase();
+        Matcher matcher = PATTERN_USER_ID.matcher(lowerCase);
+        if (!matcher.matches()) {
+            throw new IllegalArgumentException("User-ID does not follow excepted pattern \"Firstname Lastname <email.address> [Optional Comment]\"");
+        }
+        String email = matcher.group(1);
+        return mailToUri(email);
+    }
+
+    /**
+     * Translate an email address (localpart@domainpart) to a WKD URI.
+     *
+     * @param email email address
+     * @return WKD URI
+     * @throws IllegalArgumentException in case of a malformed email address
+     */
+    public URI mailToUri(String email) {
+        String lowerCase = email.toLowerCase();
+        Matcher matcher = PATTERN_EMAIL.matcher(lowerCase);
+        if (!matcher.matches()) {
+            throw new IllegalArgumentException("Invalid email address.");
+        }
+
+        String localPart = matcher.group(1);
+        String domainPart = matcher.group(2);
+
+        MessageDigest sha1;
+        try {
+            sha1 = MessageDigest.getInstance("SHA1");
+        } catch (NoSuchAlgorithmException e) {
+            throw new AssertionError("SHA-1 is available on all JVMs.", e);
+        }
+        sha1.update(localPart.getBytes(StandardCharsets.UTF_8));
+        byte[] digest = sha1.digest();
+
+        String base32KeyHandle = new ZBase32().encodeAsString(digest);
+
+        return URI.create("https://" + domainPart + "/.well-known/openpgpkey/hu/" + base32KeyHandle);
+    }
+}