FA-72381 / Check-digit algorithms / Open access

ISIN country prefix accepts a digit · case 01

Identifiers without an alphabetic country code are checksummed as if they were ISINs.

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The alphabetic test on the two-character country code was dropped.

VERIFIED REPAIR

Require both leading characters to be uppercase letters.

Unsuccessful approach: Checking only the first character still admits a digit in the second country position.

Case contract

Input a 12-character ISIN: two uppercase ASCII letters (country), nine uppercase alphanumerics, one check digit; anything else is "malformed". Letters expand to two digits (A=10 .. Z=35), then Luhn doubling starts at the rightmost digit of the expanded 11-character payload. Return [computed check digit, whether it matches].

Why this case matters

Securities settlement systems reject ISINs whose check digit does not match before routing trades.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(s):
    if len(s) != 12 or not s.isascii() or not s[:2].isupper():
        return 'malformed'
    if not s[2:11].isalnum() or s[2:11] != s[2:11].upper() or not s[11].isdigit():
        return 'malformed'
    digits = ''
    for ch in s[:11]:
        digits += ch if ch.isdigit() else str(ord(ch) - 55)
    total = 0
    for i, ch in enumerate(reversed(digits)):
        d = int(ch)
        if i % 2 == 0:
            d *= 2
            if d > 9:
                d -= 9
        total += d
    check = (10 - total % 10) % 10
    return [check, check == int(s[11])]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US4V5PGS4IX8"]', ['US4V5PGS4IX8'], [0, False]], ['control ["GBOV8F5J6090"]', ['GBOV8F5J6090'], [7, False]], ['control ["DEC85GG2TSS1"]', ['DEC85GG2TSS1'], [4, False]], ['control ["JPFWKDL124I1"]', ['JPFWKDL124I1'], [1, True]], ['control ["CHXVXSVIT016"]', ['CHXVXSVIT016'], [1, False]], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]], ['control ["XS0K4JDSXBN1"]', ['XS0K4JDSXBN1'], [4, False]], ['control ["NL5BVKR8BB60"]', ['NL5BVKR8BB60'], [3, False]], ['control ["AUKWRNHD2XR3"]', ['AUKWRNHD2XR3'], [8, False]], ['control ["CA3CKMM2IAN8"]', ['CA3CKMM2IAN8'], [0, False]], ['control ["US6754047769"]', ['US6754047769'], [8, False]]], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US6754047769"]', ['US6754047769'], [8, False]], ['control ["IE9141151847"]', ['IE9141151847'], [0, False]], ['control ["LU7763909076"]', ['LU7763909076'], [8, False]], ['control ["KY3791880781"]', ['KY3791880781'], [4, False]], ['control ["BM9170095145"]', ['BM9170095145'], [7, False]], ['control ["US0378331005"]', ['US0378331005'], [5, True]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["US0378331005"]', ['US0378331005'], [5, True]], ['control ["US5949181045"]', ['US5949181045'], [5, True]], ['control ["GB0002634946"]', ['GB0002634946'], [6, True]], ['control ["AU0000XVGZA3"]', ['AU0000XVGZA3'], [3, True]], ['control ["DE000BAY0017"]', ['DE000BAY0017'], [7, True]], ['control ["US037833100"]', ['US037833100'], 'malformed']], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US037833100"]', ['US037833100'], 'malformed'], ['control ["us0378331005"]', ['us0378331005'], 'malformed'], ['control ["US03783310a5"]', ['US03783310a5'], 'malformed'], ['control ["12Q037833100"]', ['12Q037833100'], 'malformed'], ['control ["XS2021832634"]', ['XS2021832634'], [4, True]], ['control ["US38259P5089"]', ['US38259P5089'], [9, True]]]]
for label, args, expected in fixtures[N - 1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
regression ["1S0378331005"]	`[0, False]`	`malformed`	Failed
regression ["U10378331005"]	`[9, False]`	`malformed`	Failed
control ["US4V5PGS4IX8"]	`[0, False]`	`[0, False]`	Passed
control ["GBOV8F5J6090"]	`[7, False]`	`[7, False]`	Passed
control ["DEC85GG2TSS1"]	`[4, False]`	`[4, False]`	Passed
control ["JPFWKDL124I1"]	`[1, True]`	`[1, True]`	Passed
control ["CHXVXSVIT016"]	`[1, False]`	`[1, False]`	Passed
control ["FRLWNIYHPT37"]	`[5, False]`	`[5, False]`	Passed

SHA-256 / 06ea00a8ea8828b11c5c36aa44b1a10bed5dff1082d7b3382049049b94d9f6b6

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(s):
    if len(s) != 12 or not s.isascii() or not s[0].isalpha() or not s[0].isupper():
        return 'malformed'
    if not s[2:11].isalnum() or s[2:11] != s[2:11].upper() or not s[11].isdigit():
        return 'malformed'
    digits = ''
    for ch in s[:11]:
        digits += ch if ch.isdigit() else str(ord(ch) - 55)
    total = 0
    for i, ch in enumerate(reversed(digits)):
        d = int(ch)
        if i % 2 == 0:
            d *= 2
            if d > 9:
                d -= 9
        total += d
    check = (10 - total % 10) % 10
    return [check, check == int(s[11])]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US4V5PGS4IX8"]', ['US4V5PGS4IX8'], [0, False]], ['control ["GBOV8F5J6090"]', ['GBOV8F5J6090'], [7, False]], ['control ["DEC85GG2TSS1"]', ['DEC85GG2TSS1'], [4, False]], ['control ["JPFWKDL124I1"]', ['JPFWKDL124I1'], [1, True]], ['control ["CHXVXSVIT016"]', ['CHXVXSVIT016'], [1, False]], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]], ['control ["XS0K4JDSXBN1"]', ['XS0K4JDSXBN1'], [4, False]], ['control ["NL5BVKR8BB60"]', ['NL5BVKR8BB60'], [3, False]], ['control ["AUKWRNHD2XR3"]', ['AUKWRNHD2XR3'], [8, False]], ['control ["CA3CKMM2IAN8"]', ['CA3CKMM2IAN8'], [0, False]], ['control ["US6754047769"]', ['US6754047769'], [8, False]]], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US6754047769"]', ['US6754047769'], [8, False]], ['control ["IE9141151847"]', ['IE9141151847'], [0, False]], ['control ["LU7763909076"]', ['LU7763909076'], [8, False]], ['control ["KY3791880781"]', ['KY3791880781'], [4, False]], ['control ["BM9170095145"]', ['BM9170095145'], [7, False]], ['control ["US0378331005"]', ['US0378331005'], [5, True]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["US0378331005"]', ['US0378331005'], [5, True]], ['control ["US5949181045"]', ['US5949181045'], [5, True]], ['control ["GB0002634946"]', ['GB0002634946'], [6, True]], ['control ["AU0000XVGZA3"]', ['AU0000XVGZA3'], [3, True]], ['control ["DE000BAY0017"]', ['DE000BAY0017'], [7, True]], ['control ["US037833100"]', ['US037833100'], 'malformed']], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US037833100"]', ['US037833100'], 'malformed'], ['control ["us0378331005"]', ['us0378331005'], 'malformed'], ['control ["US03783310a5"]', ['US03783310a5'], 'malformed'], ['control ["12Q037833100"]', ['12Q037833100'], 'malformed'], ['control ["XS2021832634"]', ['XS2021832634'], [4, True]], ['control ["US38259P5089"]', ['US38259P5089'], [9, True]]]]
for label, args, expected in fixtures[N - 1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
regression ["1S0378331005"]	`malformed`	`malformed`	Passed
regression ["U10378331005"]	`[9, False]`	`malformed`	Failed
control ["US4V5PGS4IX8"]	`[0, False]`	`[0, False]`	Passed
control ["GBOV8F5J6090"]	`[7, False]`	`[7, False]`	Passed
control ["DEC85GG2TSS1"]	`[4, False]`	`[4, False]`	Passed
control ["JPFWKDL124I1"]	`[1, True]`	`[1, True]`	Passed
control ["CHXVXSVIT016"]	`[1, False]`	`[1, False]`	Passed
control ["FRLWNIYHPT37"]	`[5, False]`	`[5, False]`	Passed

SHA-256 / 90e3b2fb9245afc7fe34d7339428e8813c8e1e9f64b85d140cd6dc33f9389495

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(s):
    if len(s) != 12 or not s.isascii() or not s[:2].isalpha() or not s[:2].isupper():
        return 'malformed'
    if not s[2:11].isalnum() or s[2:11] != s[2:11].upper() or not s[11].isdigit():
        return 'malformed'
    digits = ''
    for ch in s[:11]:
        digits += ch if ch.isdigit() else str(ord(ch) - 55)
    total = 0
    for i, ch in enumerate(reversed(digits)):
        d = int(ch)
        if i % 2 == 0:
            d *= 2
            if d > 9:
                d -= 9
        total += d
    check = (10 - total % 10) % 10
    return [check, check == int(s[11])]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US4V5PGS4IX8"]', ['US4V5PGS4IX8'], [0, False]], ['control ["GBOV8F5J6090"]', ['GBOV8F5J6090'], [7, False]], ['control ["DEC85GG2TSS1"]', ['DEC85GG2TSS1'], [4, False]], ['control ["JPFWKDL124I1"]', ['JPFWKDL124I1'], [1, True]], ['control ["CHXVXSVIT016"]', ['CHXVXSVIT016'], [1, False]], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["FRLWNIYHPT37"]', ['FRLWNIYHPT37'], [5, False]], ['control ["XS0K4JDSXBN1"]', ['XS0K4JDSXBN1'], [4, False]], ['control ["NL5BVKR8BB60"]', ['NL5BVKR8BB60'], [3, False]], ['control ["AUKWRNHD2XR3"]', ['AUKWRNHD2XR3'], [8, False]], ['control ["CA3CKMM2IAN8"]', ['CA3CKMM2IAN8'], [0, False]], ['control ["US6754047769"]', ['US6754047769'], [8, False]]], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US6754047769"]', ['US6754047769'], [8, False]], ['control ["IE9141151847"]', ['IE9141151847'], [0, False]], ['control ["LU7763909076"]', ['LU7763909076'], [8, False]], ['control ["KY3791880781"]', ['KY3791880781'], [4, False]], ['control ["BM9170095145"]', ['BM9170095145'], [7, False]], ['control ["US0378331005"]', ['US0378331005'], [5, True]]], [['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['control ["US0378331005"]', ['US0378331005'], [5, True]], ['control ["US5949181045"]', ['US5949181045'], [5, True]], ['control ["GB0002634946"]', ['GB0002634946'], [6, True]], ['control ["AU0000XVGZA3"]', ['AU0000XVGZA3'], [3, True]], ['control ["DE000BAY0017"]', ['DE000BAY0017'], [7, True]], ['control ["US037833100"]', ['US037833100'], 'malformed']], [['regression ["1S0378331005"]', ['1S0378331005'], 'malformed'], ['regression ["U10378331005"]', ['U10378331005'], 'malformed'], ['control ["US037833100"]', ['US037833100'], 'malformed'], ['control ["us0378331005"]', ['us0378331005'], 'malformed'], ['control ["US03783310a5"]', ['US03783310a5'], 'malformed'], ['control ["12Q037833100"]', ['12Q037833100'], 'malformed'], ['control ["XS2021832634"]', ['XS2021832634'], [4, True]], ['control ["US38259P5089"]', ['US38259P5089'], [9, True]]]]
for label, args, expected in fixtures[N - 1]:
    check(label, solve(*args), expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
regression ["1S0378331005"]	`malformed`	`malformed`	Passed
regression ["U10378331005"]	`malformed`	`malformed`	Passed
control ["US4V5PGS4IX8"]	`[0, False]`	`[0, False]`	Passed
control ["GBOV8F5J6090"]	`[7, False]`	`[7, False]`	Passed
control ["DEC85GG2TSS1"]	`[4, False]`	`[4, False]`	Passed
control ["JPFWKDL124I1"]	`[1, True]`	`[1, True]`	Passed
control ["CHXVXSVIT016"]	`[1, False]`	`[1, False]`	Passed
control ["FRLWNIYHPT37"]	`[5, False]`	`[5, False]`	Passed

SHA-256 / 7b76537145d32af6397188797d940b0cbc18d9277d981a99b247ce09a3b07fb1

Verification & scope

A deterministic, bounded teaching model of the named scheme under the stated contract; not a certified validator. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:48:37.932315+00:00.

Case digest / 48151244a94227f1867412e5ee389501b7955ca9c562bf8c5ee8a6de10fd55f8