diff --git a/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/CertifyUserIdImpl.kt b/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/CertifyUserIdImpl.kt
new file mode 100644
index 00000000..4b07237b
--- /dev/null
+++ b/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/CertifyUserIdImpl.kt
@@ -0,0 +1,86 @@
+// SPDX-FileCopyrightText: 2025 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.sop
+
+import java.io.InputStream
+import java.io.OutputStream
+import org.bouncycastle.bcpg.PacketFormat
+import org.bouncycastle.openpgp.api.OpenPGPKey
+import org.pgpainless.PGPainless
+import org.pgpainless.exception.KeyException.UnboundUserIdException
+import org.pgpainless.key.OpenPgpFingerprint
+import org.pgpainless.util.ArmoredOutputStreamFactory
+import org.pgpainless.util.Passphrase
+import sop.Ready
+import sop.operation.CertifyUserId
+
+class CertifyUserIdImpl(private val api: PGPainless) : CertifyUserId {
+
+    private var armor: Boolean = true
+    private val keys: MutableList<OpenPGPKey> = mutableListOf()
+    private var requireSelfSig = true
+    private val userIds: MutableSet<String> = mutableSetOf()
+    private var protector: MatchMakingSecretKeyRingProtector = MatchMakingSecretKeyRingProtector()
+
+    override fun certs(certs: InputStream): Ready {
+        return object : Ready() {
+            override fun writeTo(outputStream: OutputStream) {
+                val out =
+                    if (armor) {
+                        ArmoredOutputStreamFactory.get(outputStream)
+                    } else outputStream
+
+                api.readKey()
+                    .parseCertificates(certs)
+                    .onEach { cert ->
+                        if (requireSelfSig) {
+                            // Check for non-bound user-ids
+                            userIds
+                                .find { cert.getUserId(it)?.isBound != true }
+                                ?.let {
+                                    throw UnboundUserIdException(
+                                        OpenPgpFingerprint.Companion.of(cert), it, null, null)
+                                }
+                        }
+                    }
+                    .forEach { cert ->
+                        var certificate = cert
+                        keys.forEach { key ->
+                            userIds.forEach { userId ->
+                                certificate =
+                                    api.generateCertification()
+                                        .certifyUserId(userId, certificate)
+                                        .withKey(key, protector)
+                                        .build()
+                                        .certifiedCertificate
+                            }
+                        }
+
+                        out.write(certificate.getEncoded(PacketFormat.CURRENT))
+                    }
+
+                out.close()
+                if (armor) {
+                    // armored output stream does not close inner stream
+                    outputStream.close()
+                }
+            }
+        }
+    }
+
+    override fun keys(keys: InputStream): CertifyUserId = apply {
+        this.keys.addAll(api.readKey().parseKeys(keys).onEach { protector.addSecretKey(it) })
+    }
+
+    override fun noArmor(): CertifyUserId = apply { armor = false }
+
+    override fun noRequireSelfSig(): CertifyUserId = apply { requireSelfSig = false }
+
+    override fun userId(userId: String): CertifyUserId = apply { this.userIds.add(userId) }
+
+    override fun withKeyPassword(password: ByteArray): CertifyUserId = apply {
+        protector.addPassphrase(Passphrase.fromPassword(String(password)))
+    }
+}
diff --git a/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/SOPImpl.kt b/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/SOPImpl.kt
index 0c01c561..95f5d93a 100644
--- a/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/SOPImpl.kt
+++ b/pgpainless-sop/src/main/kotlin/org/pgpainless/sop/SOPImpl.kt
@@ -36,7 +36,7 @@ class SOPImpl(
 
     override fun armor(): Armor = ArmorImpl(api)
 
-    override fun certifyUserId(): CertifyUserId? = null
+    override fun certifyUserId(): CertifyUserId = CertifyUserIdImpl(api)
 
     override fun changeKeyPassword(): ChangeKeyPassword = ChangeKeyPasswordImpl(api)