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 cd758c3..88567c2 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
@@ -68,36 +68,28 @@ class DownloadWizardDetailsFragment : DownloadWizardActivity.DownloadWizardStepF
     }
 
     private fun updateInputCompleteness() {
-        validate()
-        val errors = arrayOf(
-            smdp.editText!!.error,
-            imei.editText!!.error,
-        )
-        inputComplete = errors.all { it == null }
+        inputComplete = runCatching(::validate).isSuccess
         refreshButtons()
     }
 
     private fun validate() {
-        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(Patterns.DOMAIN_NAME.matcher(smdp.editText!!.text).matches()) {
+            "Invalid SM-DP+ address"
         }
-        imei.editText!!.error = imei.editText!!.text?.let {
-            if (it.isEmpty() || luhnValid(it)) return@let null
-            getString(R.string.download_wizard_error_invalid_imei_format)
+        check(imei.editText!!.text.let { it.isEmpty() || isValidIMEI(it) }) {
+            "Invalid IMEI"
         }
     }
 }
 
-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'
+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()
     }
-    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
+
+    val sum = input.dropLast(1).map(Char::digitToInt).sumOf(::sumOfDigits)
+    return (sum * 9) % 10 == input.last().digitToInt()
+}
diff --git a/app-common/src/main/res/values/strings.xml b/app-common/src/main/res/values/strings.xml
index 94f7f7a..a45ce1f 100644
--- a/app-common/src/main/res/values/strings.xml
+++ b/app-common/src/main/res/values/strings.xml
@@ -98,8 +98,6 @@
     <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>