From 3d374fe0f14bfbdbdf6fd09b37af0fcffa5ce58d Mon Sep 17 00:00:00 2001
From: David Koblas <david@koblas.com>
Date: Wed, 11 Nov 2026 08:06:59 -0500
Subject: [PATCH] fix(kr/rrn): skip the check digit for alien registration
 numbers
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Numbers with century/gender digit 5-8 are alien registration numbers
(외국인등록번호). Their check digit formula isn't published: the
Enforcement Decree of the Immigration Act (Article 40-3) leaves the
scheme to the Minister of Justice. python-stdnum applies the RRN formula,
other validators add 2 to it, and neither cites a source, so valid
numbers could be rejected. Check only the length, digits and birth date,
as for numbers issued since 2020.
---
 src/kr/rrn.spec.ts | 20 ++++++++++++++++++++
 src/kr/rrn.ts      | 26 ++++++++++++++++++++++----
 2 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/src/kr/rrn.spec.ts b/src/kr/rrn.spec.ts
index e5cda65d..76b877f7 100644
--- a/src/kr/rrn.spec.ts
+++ b/src/kr/rrn.spec.ts
@@ -65,4 +65,24 @@ describe('kr/rrn', () => {
 
     expect(result.error).toBeInstanceOf(InvalidChecksum);
   });
+
+  // Alien registration numbers (7th digit 5-8): no published check digit
+  // formula, so only the length, digits and birth date are checked
+  test.each([
+    '850126-6401473', // passes the RRN formula
+    '850126-6401475', // passes the "RRN + 2" formula used by some validators
+    '850126-5991470', // office code above 96
+    '050126-7401470',
+    '050126-8401471',
+  ])('validate:%s', value => {
+    const result = validate(value);
+
+    expect(result.isValid && result.isIndividual).toEqual(true);
+  });
+
+  test.each(['851326-6401473', '991231-7401470'])('validate:%s', value => {
+    const result = validate(value);
+
+    expect(result.error).toBeInstanceOf(InvalidComponent);
+  });
 });
diff --git a/src/kr/rrn.ts b/src/kr/rrn.ts
index 6be9bd97..b4cc337b 100644
--- a/src/kr/rrn.ts
+++ b/src/kr/rrn.ts
@@ -3,8 +3,8 @@
  *
  * The RRN (resident registration number, 주민등록번호) is a 13-digit number
  * issued to all residents of the Republic of Korea. Foreigners residing in the
- * Republic of Korea receive an alien registration number (ARN) which follows
- * the same encoding pattern.
+ * Republic of Korea receive an alien registration number (ARN) which has the
+ * same layout, but no published check digit (see below).
  *
  * The first six digits code the date of birth. The seventh digit encodes the
  * century and gender. The next four digits encode the place of birth for
@@ -21,11 +21,22 @@
  * Birth dates in the future are rejected; python-stdnum only checks that the
  * date exists. See https://github.com/koblas/stdnum-js/issues/91.
  *
+ * For an alien registration number (century/gender digit 5 to 8) only the
+ * length, digits and birth date are checked. The Enforcement Decree of the
+ * Immigration Act (Article 40-3) defines it as 13 digits showing the date of
+ * birth, sex and registering office, and leaves the rest of the scheme to the
+ * Minister of Justice, who hasn't published it. The check digit formula
+ * isn't public: python-stdnum applies the RRN formula, other validators add
+ * 2 to it, and neither cites a source. This differs from python-stdnum, see
+ * https://github.com/koblas/stdnum-js/issues/91.
+ *
  * Source:
  *   http://www.law.go.kr/lsSc.do?tabMenuId=tab18&p1=&subMenu=1&nwYn=1&section=&tabNo=&query=개인정보+보호법
  *   https://en.wikipedia.org/wiki/Resident_registration_number
  *   https://techscience.org/a/2015092901/
  *   https://www.fnnews.com/news/202006291754239525
+ *   https://www.law.go.kr/LSW//lumLsLinkPop.do?lspttninfSeq=82633&chrClsCd=010202
+ *   https://overseas.mofa.go.kr/hn-ko/brd/m_6364/view.do?seq=918639
  *
  * PERSON
  */
@@ -52,6 +63,9 @@ const centuryPrefix: Record<string, number> = {
   9: 18,
 };
 
+// Century/gender digits of alien registration numbers
+const ARN_CENTURY = new Set(['5', '6', '7', '8']);
+
 const impl: Validator = {
   name: 'South Korean Resident Registration Number',
   localName: '주민등록번호',
@@ -92,8 +106,12 @@ const impl: Validator = {
       return { isValid: false, error: new exceptions.InvalidComponent() };
     }
 
-    // Numbers issued since October 2020 end in six random digits
-    if (parseInt(birthDate.slice(0, 4), 10) >= 2020) {
+    // Numbers issued since October 2020 end in six random digits, and the
+    // check digit of alien registration numbers isn't published
+    if (
+      parseInt(birthDate.slice(0, 4), 10) >= 2020 ||
+      ARN_CENTURY.has(century)
+    ) {
       return {
         isValid: true,
         compact: value,