diff --git a/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt b/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
index 88567c2..cd758c3 100644
--- a/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
+++ b/app-common/src/main/java/im/angry/openeuicc/ui/wizard/DownloadWizardDetailsFragment.kt
@@ -68,28 +68,36 @@ class DownloadWizardDetailsFragment : DownloadWizardActivity.DownloadWizardStepF
     }
 
     private fun updateInputCompleteness() {
-        inputComplete = runCatching(::validate).isSuccess
+        validate()
+        val errors = arrayOf(
+            smdp.editText!!.error,
+            imei.editText!!.error,
+        )
+        inputComplete = errors.all { it == null }
         refreshButtons()
     }
 
     private fun validate() {
-        check(Patterns.DOMAIN_NAME.matcher(smdp.editText!!.text).matches()) {
-            "Invalid SM-DP+ address"
+        smdp.editText!!.error = smdp.editText!!.text?.let {
+            if (Patterns.DOMAIN_NAME.matcher(smdp.editText!!.text).matches()) return@let null
+            getString(R.string.download_wizard_error_invalid_address_format)
         }
-        check(imei.editText!!.text.let { it.isEmpty() || isValidIMEI(it) }) {
-            "Invalid IMEI"
+        imei.editText!!.error = imei.editText!!.text?.let {
+            if (it.isEmpty() || luhnValid(it)) return@let null
+            getString(R.string.download_wizard_error_invalid_imei_format)
         }
     }
 }
 
-private fun isValidIMEI(input: CharSequence): Boolean {
-    if (input.length != 15 || !input.all(Char::isDigit)) return false
-
-    fun sumOfDigits(input: Int): Int {
-        if (input % 2 == 0) return input
-        return (input * 2).toString().map(Char::digitToInt).sum()
+private fun luhnValid(number: CharSequence, mod: Int = 10): Boolean {
+    if (!number.all(Char::isDigit)) return false
+    var checksum = 0
+    for (i in number.length - 1 downTo 0 step 2) {
+        checksum += number[i] - '0'
     }
-
-    val sum = input.dropLast(1).map(Char::digitToInt).sumOf(::sumOfDigits)
-    return (sum * 9) % 10 == input.last().digitToInt()
-}
+    for (i in number.length - 2 downTo 0 step 2) {
+        val n: Int = (number[i] - '0') * 2
+        checksum += if (n > 9) n - 9 else n
+    }
+    return checksum % mod == 0
+}
\ No newline at end of file
diff --git a/app-common/src/main/res/values/strings.xml b/app-common/src/main/res/values/strings.xml
index a45ce1f..94f7f7a 100644
--- a/app-common/src/main/res/values/strings.xml
+++ b/app-common/src/main/res/values/strings.xml
@@ -98,6 +98,8 @@
     <string name="download_wizard_diagnostics_last_apdu_exception">Last APDU exception:</string>
     <string name="download_wizard_diagnostics_save">Save</string>
     <string name="download_wizard_diagnostics_file_template">Diagnostics at %s</string>
+    <string name="download_wizard_error_invalid_address_format">Invalid SM-DP+ address</string>
+    <string name="download_wizard_error_invalid_imei_format">Invalid IMEI format</string>
 
     <string name="logs_saved_message">Logs have been saved to the selected path. Would you like to share the log through another app?</string>