diff --git a/src/auth.rs b/src/auth.rs
index f9e62b4..5ed344e 100644
--- a/src/auth.rs
+++ b/src/auth.rs
@@ -3,7 +3,13 @@
 use crate::config::AuthOptions;
 use crate::constants::{ATTR_NONCE, ATTR_REALM, ATTR_USERNAME};
 use crate::models::stun::StunMessage;
-use crate::stun::{find_message_integrity, validate_message_integrity, validate_message_integrity_len_preserved};
+use crate::stun::{
+    compute_message_integrity_adjusted,
+    compute_message_integrity_len_preserved as compute_mi_len_preserved,
+    find_message_integrity,
+    validate_message_integrity,
+    validate_message_integrity_len_preserved,
+};
 use crate::traits::CredentialStore;
 use async_trait::async_trait;
 use base64::Engine;
@@ -204,12 +210,24 @@ impl<S: CredentialStore + Clone> AuthManager<S> {
             return AuthStatus::Granted { username, key };
         }
 
-        // No acceptance without MI validation.
+        // No acceptance without MI validation. Emit detailed diagnostics.
         let mi_attr = find_message_integrity(msg).map(|a| hex::encode(&a.value));
-        let mi_long = hex::encode(&crate::stun::compute_message_integrity(&key, msg.raw.as_slice()));
-        let mi_short = hex::encode(&crate::stun::compute_message_integrity(short_key, msg.raw.as_slice()));
-        warn!("auth reject: bad credentials username={} realm={} peer={} a1_md5={} mi_attr={:?} mi_long(fullmsg)={} mi_short(fullmsg)={}",
-            username, realm, peer, hex::encode(&key), mi_attr, mi_long, mi_short);
+        let mi_long_adj = compute_message_integrity_adjusted(msg, &key).map(hex::encode);
+        let mi_long_len = compute_mi_len_preserved(msg, &key).map(hex::encode);
+        let mi_short_adj = compute_message_integrity_adjusted(msg, short_key).map(hex::encode);
+        let mi_short_len = compute_mi_len_preserved(msg, short_key).map(hex::encode);
+        warn!(
+            "auth reject: bad credentials username={} realm={} peer={} a1_md5={} mi_attr={:?} mi_long_adj={:?} mi_long_len={:?} mi_short_adj={:?} mi_short_len={:?}",
+            username,
+            realm,
+            peer,
+            hex::encode(&key),
+            mi_attr,
+            mi_long_adj,
+            mi_long_len,
+            mi_short_adj,
+            mi_short_len
+        );
         AuthStatus::Reject {
             code: 401,
             reason: "Bad Credentials",
diff --git a/src/stun.rs b/src/stun.rs
index 2406196..030bb8e 100644
--- a/src/stun.rs
+++ b/src/stun.rs
@@ -386,6 +386,51 @@ pub fn validate_message_integrity(msg: &StunMessage, key: &[u8]) -> bool {
     false
 }
 
+/// Compute the expected MESSAGE-INTEGRITY value following the RFC-adjusted length rule.
+pub fn compute_message_integrity_adjusted(msg: &StunMessage, key: &[u8]) -> Option<Vec<u8>> {
+    let mi = find_message_integrity(msg)?;
+    if mi.value.len() != HMAC_SHA1_LEN {
+        return None;
+    }
+    let mi_end = mi.offset + 4 + HMAC_SHA1_LEN;
+    if mi_end > msg.raw.len() {
+        return None;
+    }
+
+    let mut signed = msg.raw[..mi_end].to_vec();
+    let len = (mi_end - 20) as u16;
+    let len_bytes = len.to_be_bytes();
+    signed[2] = len_bytes[0];
+    signed[3] = len_bytes[1];
+
+    let value_start = mi.offset + 4;
+    for b in &mut signed[value_start..value_start + HMAC_SHA1_LEN] {
+        *b = 0;
+    }
+
+    Some(crate::stun::compute_message_integrity(key, &signed))
+}
+
+/// Compute MESSAGE-INTEGRITY keeping the original header length (interop len-preserved mode).
+pub fn compute_message_integrity_len_preserved(msg: &StunMessage, key: &[u8]) -> Option<Vec<u8>> {
+    let mi = find_message_integrity(msg)?;
+    if mi.value.len() != HMAC_SHA1_LEN {
+        return None;
+    }
+    let mi_end = mi.offset + 4 + HMAC_SHA1_LEN;
+    if mi_end > msg.raw.len() {
+        return None;
+    }
+
+    let mut signed = msg.raw[..mi_end].to_vec();
+    let value_start = mi.offset + 4;
+    for b in &mut signed[value_start..value_start + HMAC_SHA1_LEN] {
+        *b = 0;
+    }
+
+    Some(crate::stun::compute_message_integrity(key, &signed))
+}
+
 /// Fallback validator: compute MESSAGE-INTEGRITY without adjusting the STUN header length.
 /// Some clients incorrectly leave the header length unchanged when appending FINGERPRINT;
 /// this matches that behaviour for interop.