diff --git a/src/main/java/app/aoki/cinpo/gp/scp/Scp02Protocol.java b/src/main/java/app/aoki/cinpo/gp/scp/Scp02Protocol.java
index 6a224d2..fcb2c93 100644
--- a/src/main/java/app/aoki/cinpo/gp/scp/Scp02Protocol.java
+++ b/src/main/java/app/aoki/cinpo/gp/scp/Scp02Protocol.java
@@ -81,6 +81,20 @@ final class Scp02Protocol implements InternalSecureChannelProtocol {
         validateStaticConfiguration();
     }
 
+    /**
+     * Performs SCP02 authentication and initializes secure-messaging state.
+     *
+     * <p><strong>Precondition:</strong> the card's Security Domain identified by
+     * {@link SecureChannelProfile#securityDomainAid()} must already be selected before calling
+     * this method. Callers are responsible for issuing the plain SELECT command.
+     *
+     * <p>The flow starts by discovering implementation options, then performs explicit
+     * (INITIALIZE UPDATE + EXTERNAL AUTHENTICATE) or implicit initiation depending on the card
+     * configuration.
+     *
+     * @param channel the raw APDU transport channel
+     * @throws IllegalStateException if SCP02 setup or cryptographic checks fail
+     */
     @Override
     public void authenticate(ApduChannel channel) {
         try {
diff --git a/src/main/java/app/aoki/cinpo/gp/scp/Scp03Protocol.java b/src/main/java/app/aoki/cinpo/gp/scp/Scp03Protocol.java
index 4e8476a..830b14b 100644
--- a/src/main/java/app/aoki/cinpo/gp/scp/Scp03Protocol.java
+++ b/src/main/java/app/aoki/cinpo/gp/scp/Scp03Protocol.java
@@ -124,10 +124,12 @@ final class Scp03Protocol implements InternalSecureChannelProtocol {
     /**
      * Performs SCP03 mutual authentication with the card (Figure 5-1, [Amd D] §5.2).
      *
+     * <p><strong>Precondition:</strong> the card's Security Domain identified by
+     * {@link SecureChannelProfile#securityDomainAid()} must already be selected before calling
+     * this method. Callers are responsible for issuing the plain SELECT command.
+     *
      * <p>The authentication flow consists of the following steps:
      * <ol>
-     *   <li><b>SELECT</b> – selects the Security Domain identified by
-     *       {@link SecureChannelProfile#securityDomainAid()}.</li>
      *   <li><b>INITIALIZE UPDATE</b> ([Amd D] §7.1.1) – sends the 8-byte host challenge to
      *       the card. The card generates its own card challenge, derives session keys, and
      *       returns its card cryptogram along with the SCP identifier and the "i" parameter.</li>
diff --git a/src/main/java/app/aoki/cinpo/gp/scp/SecureChannelSession.java b/src/main/java/app/aoki/cinpo/gp/scp/SecureChannelSession.java
index 6c0e350..eb5a519 100644
--- a/src/main/java/app/aoki/cinpo/gp/scp/SecureChannelSession.java
+++ b/src/main/java/app/aoki/cinpo/gp/scp/SecureChannelSession.java
@@ -82,10 +82,12 @@ public static SecureChannelSession create(ApduChannel channel, SecureChannelProf
     /**
      * Performs explicit Secure Channel initiation ([GPCS] §10.2.1).
      *
+     * <p><strong>Precondition:</strong> the card's Security Domain identified by
+     * {@link SecureChannelProfile#securityDomainAid()} must already be selected before calling
+     * this method. Callers are responsible for issuing the plain SELECT command.
+     *
      * <p>The following sequence is executed:
      * <ol>
-     *   <li><b>SELECT</b> – selects the Security Domain identified by
-     *       {@link SecureChannelProfile#securityDomainAid()}.</li>
      *   <li><b>INITIALIZE UPDATE</b> ([Amd D] §7.1.1) – transmits the 8-byte host challenge
      *       to the card and receives key diversification data, the card challenge, and the
      *       card cryptogram.  Session keys are derived from the static key set.</li>