FA-72471 / Check-digit algorithms / Open access

IBAN rearrangement swaps country code and check digits · case 01

Every correct IBAN is reported invalid.

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

ROOT CAUSE

The first four characters are moved as check digits then country (t[2:4] + t[:2]) instead of in order.

VERIFIED REPAIR

Move exactly t[:4] (country then check digits) to the end unchanged.

Unsuccessful approach: Moving only the country code to the end leaves the check digits in front.

Case contract

IBAN validation for the countries DE 22, GB 22, FR 27, NL 18, BE 16, NO 15, MT 31 (other countries: "malformed"). Spaces (print format) are removed and letters are upper-cased. The check digits must be 02..98. Move the first four characters to the end, replace letters with 10..35, and the IBAN is "valid" when the number is 1 mod 97.

Why this case matters

Payment initiation rejects mistyped account identifiers before submitting transfers.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(s):
    LENGTHS = {'DE': 22, 'GB': 22, 'FR': 27, 'NL': 18, 'BE': 16, 'NO': 15, 'MT': 31}
    t = s.replace(' ', '').upper()
    if len(t) < 5 or not t.isascii() or not t.isalnum():
        return 'malformed'
    if not t[:2].isalpha() or not t[2:4].isdigit() or not 2 <= int(t[2:4]) <= 98:
        return 'malformed'
    if LENGTHS.get(t[:2]) != len(t):
        return 'malformed'
    r = t[4:] + t[2:4] + t[:2]
    n = ''.join(str(int(ch, 36)) for ch in r)
    return 'valid' if int(n) % 97 == 1 else 'invalid'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["DE59811903138375291124"]', ['DE59811903138375291124'], 'valid'], ['regression ["DE50296496123032354476"]', ['DE50296496123032354476'], 'valid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid'], ['control ["GB30TWRL09177880233697"]', ['GB30TWRL09177880233697'], 'invalid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid']], [['regression ["GB12YUBO17171961366336"]', ['GB12YUBO17171961366336'], 'valid'], ['regression ["GB30TWRL09177880233695"]', ['GB30TWRL09177880233695'], 'valid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid'], ['control ["DE00050434738327048798"]', ['DE00050434738327048798'], 'malformed'], ['control ["DE01819403115748858673"]', ['DE01819403115748858673'], 'malformed'], ['control ["DE01435241396484727591"]', ['DE01435241396484727591'], 'malformed'], ['control ["DE00470841292401747110"]', ['DE00470841292401747110'], 'malformed'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid']], [['regression ["FR88HIO0QY01NRB0VCRB14QO450"]', ['FR88HIO0QY01NRB0VCRB14QO450'], 'valid'], ['regression ["FR38Q7OJKEPW4X8D3YP3O6RO9CF"]', ['FR38Q7OJKEPW4X8D3YP3O6RO9CF'], 'valid'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid'], ['control ["DE49296496123032354476"]', ['DE49296496123032354476'], 'invalid'], ['control ["DE57862288567935243525"]', ['DE57862288567935243525'], 'invalid'], ['control ["GB11YUBO17171961366336"]', ['GB11YUBO17171961366336'], 'invalid'], ['control ["GB29TWRL09177880233695"]', ['GB29TWRL09177880233695'], 'invalid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid']], [['regression ["NL34HYUK1800961826"]', ['NL34HYUK1800961826'], 'valid'], ['regression ["NL15KCEZ0926524218"]', ['NL15KCEZ0926524218'], 'valid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid'], ['control ["XX82WEST12345698765432"]', ['XX82WEST12345698765432'], 'malformed'], ['control ["US64SVBKUS6S3300958879"]', ['US64SVBKUS6S3300958879'], 'malformed'], ['control ["GB82WEST1234569876543"]', ['GB82WEST1234569876543'], 'malformed'], ['control ["GB82-WEST-1234"]', ['GB82-WEST-1234'], 'malformed'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed']], [['regression ["BE73177155519075"]', ['BE73177155519075'], 'valid'], ['regression ["BE31287393626938"]', ['BE31287393626938'], 'valid'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed'], ['control ["NO0886011117947"]', ['NO0886011117947'], 'invalid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid']]]
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 ["DE59811903138375291124"]	`invalid`	`valid`	Failed
regression ["DE50296496123032354476"]	`invalid`	`valid`	Failed
control ["DE59811903138375291127"]	`invalid`	`invalid`	Passed
control ["DE50296496123032354477"]	`invalid`	`invalid`	Passed
control ["DE58862288567935243527"]	`invalid`	`invalid`	Passed
control ["GB12YUBO17171961366337"]	`invalid`	`invalid`	Passed
control ["GB30TWRL09177880233697"]	`invalid`	`invalid`	Passed
control ["GB23HJGZ79203524065167"]	`invalid`	`invalid`	Passed

SHA-256 / fba6a947c50550c69974490869e8a4f1cae85bfd4beac2aeb467e9c2afe556b3

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(s):
    LENGTHS = {'DE': 22, 'GB': 22, 'FR': 27, 'NL': 18, 'BE': 16, 'NO': 15, 'MT': 31}
    t = s.replace(' ', '').upper()
    if len(t) < 5 or not t.isascii() or not t.isalnum():
        return 'malformed'
    if not t[:2].isalpha() or not t[2:4].isdigit() or not 2 <= int(t[2:4]) <= 98:
        return 'malformed'
    if LENGTHS.get(t[:2]) != len(t):
        return 'malformed'
    r = t[2:] + t[:2]
    n = ''.join(str(int(ch, 36)) for ch in r)
    return 'valid' if int(n) % 97 == 1 else 'invalid'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["DE59811903138375291124"]', ['DE59811903138375291124'], 'valid'], ['regression ["DE50296496123032354476"]', ['DE50296496123032354476'], 'valid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid'], ['control ["GB30TWRL09177880233697"]', ['GB30TWRL09177880233697'], 'invalid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid']], [['regression ["GB12YUBO17171961366336"]', ['GB12YUBO17171961366336'], 'valid'], ['regression ["GB30TWRL09177880233695"]', ['GB30TWRL09177880233695'], 'valid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid'], ['control ["DE00050434738327048798"]', ['DE00050434738327048798'], 'malformed'], ['control ["DE01819403115748858673"]', ['DE01819403115748858673'], 'malformed'], ['control ["DE01435241396484727591"]', ['DE01435241396484727591'], 'malformed'], ['control ["DE00470841292401747110"]', ['DE00470841292401747110'], 'malformed'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid']], [['regression ["FR88HIO0QY01NRB0VCRB14QO450"]', ['FR88HIO0QY01NRB0VCRB14QO450'], 'valid'], ['regression ["FR38Q7OJKEPW4X8D3YP3O6RO9CF"]', ['FR38Q7OJKEPW4X8D3YP3O6RO9CF'], 'valid'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid'], ['control ["DE49296496123032354476"]', ['DE49296496123032354476'], 'invalid'], ['control ["DE57862288567935243525"]', ['DE57862288567935243525'], 'invalid'], ['control ["GB11YUBO17171961366336"]', ['GB11YUBO17171961366336'], 'invalid'], ['control ["GB29TWRL09177880233695"]', ['GB29TWRL09177880233695'], 'invalid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid']], [['regression ["NL34HYUK1800961826"]', ['NL34HYUK1800961826'], 'valid'], ['regression ["NL15KCEZ0926524218"]', ['NL15KCEZ0926524218'], 'valid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid'], ['control ["XX82WEST12345698765432"]', ['XX82WEST12345698765432'], 'malformed'], ['control ["US64SVBKUS6S3300958879"]', ['US64SVBKUS6S3300958879'], 'malformed'], ['control ["GB82WEST1234569876543"]', ['GB82WEST1234569876543'], 'malformed'], ['control ["GB82-WEST-1234"]', ['GB82-WEST-1234'], 'malformed'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed']], [['regression ["BE73177155519075"]', ['BE73177155519075'], 'valid'], ['regression ["BE31287393626938"]', ['BE31287393626938'], 'valid'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed'], ['control ["NO0886011117947"]', ['NO0886011117947'], 'invalid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid']]]
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 ["DE59811903138375291124"]	`invalid`	`valid`	Failed
regression ["DE50296496123032354476"]	`invalid`	`valid`	Failed
control ["DE59811903138375291127"]	`invalid`	`invalid`	Passed
control ["DE50296496123032354477"]	`invalid`	`invalid`	Passed
control ["DE58862288567935243527"]	`invalid`	`invalid`	Passed
control ["GB12YUBO17171961366337"]	`invalid`	`invalid`	Passed
control ["GB30TWRL09177880233697"]	`invalid`	`invalid`	Passed
control ["GB23HJGZ79203524065167"]	`invalid`	`invalid`	Passed

SHA-256 / a901fa5a6e127745d95fc6928065ef83b413dc5f9ab9ff607627d3b3b8d8a9d0

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(s):
    LENGTHS = {'DE': 22, 'GB': 22, 'FR': 27, 'NL': 18, 'BE': 16, 'NO': 15, 'MT': 31}
    t = s.replace(' ', '').upper()
    if len(t) < 5 or not t.isascii() or not t.isalnum():
        return 'malformed'
    if not t[:2].isalpha() or not t[2:4].isdigit() or not 2 <= int(t[2:4]) <= 98:
        return 'malformed'
    if LENGTHS.get(t[:2]) != len(t):
        return 'malformed'
    r = t[4:] + t[:4]
    n = ''.join(str(int(ch, 36)) for ch in r)
    return 'valid' if int(n) % 97 == 1 else 'invalid'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[['regression ["DE59811903138375291124"]', ['DE59811903138375291124'], 'valid'], ['regression ["DE50296496123032354476"]', ['DE50296496123032354476'], 'valid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid'], ['control ["GB30TWRL09177880233697"]', ['GB30TWRL09177880233697'], 'invalid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid']], [['regression ["GB12YUBO17171961366336"]', ['GB12YUBO17171961366336'], 'valid'], ['regression ["GB30TWRL09177880233695"]', ['GB30TWRL09177880233695'], 'valid'], ['control ["GB23HJGZ79203524065167"]', ['GB23HJGZ79203524065167'], 'invalid'], ['control ["DE00050434738327048798"]', ['DE00050434738327048798'], 'malformed'], ['control ["DE01819403115748858673"]', ['DE01819403115748858673'], 'malformed'], ['control ["DE01435241396484727591"]', ['DE01435241396484727591'], 'malformed'], ['control ["DE00470841292401747110"]', ['DE00470841292401747110'], 'malformed'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid']], [['regression ["FR88HIO0QY01NRB0VCRB14QO450"]', ['FR88HIO0QY01NRB0VCRB14QO450'], 'valid'], ['regression ["FR38Q7OJKEPW4X8D3YP3O6RO9CF"]', ['FR38Q7OJKEPW4X8D3YP3O6RO9CF'], 'valid'], ['control ["DE58811903138375291124"]', ['DE58811903138375291124'], 'invalid'], ['control ["DE49296496123032354476"]', ['DE49296496123032354476'], 'invalid'], ['control ["DE57862288567935243525"]', ['DE57862288567935243525'], 'invalid'], ['control ["GB11YUBO17171961366336"]', ['GB11YUBO17171961366336'], 'invalid'], ['control ["GB29TWRL09177880233695"]', ['GB29TWRL09177880233695'], 'invalid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid']], [['regression ["NL34HYUK1800961826"]', ['NL34HYUK1800961826'], 'valid'], ['regression ["NL15KCEZ0926524218"]', ['NL15KCEZ0926524218'], 'valid'], ['control ["GB22HJGZ79203524065161"]', ['GB22HJGZ79203524065161'], 'invalid'], ['control ["XX82WEST12345698765432"]', ['XX82WEST12345698765432'], 'malformed'], ['control ["US64SVBKUS6S3300958879"]', ['US64SVBKUS6S3300958879'], 'malformed'], ['control ["GB82WEST1234569876543"]', ['GB82WEST1234569876543'], 'malformed'], ['control ["GB82-WEST-1234"]', ['GB82-WEST-1234'], 'malformed'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed']], [['regression ["BE73177155519075"]', ['BE73177155519075'], 'valid'], ['regression ["BE31287393626938"]', ['BE31287393626938'], 'valid'], ['control ["DE00370400440532013000"]', ['DE00370400440532013000'], 'malformed'], ['control ["NO0886011117947"]', ['NO0886011117947'], 'invalid'], ['control ["DE59811903138375291127"]', ['DE59811903138375291127'], 'invalid'], ['control ["DE50296496123032354477"]', ['DE50296496123032354477'], 'invalid'], ['control ["DE58862288567935243527"]', ['DE58862288567935243527'], 'invalid'], ['control ["GB12YUBO17171961366337"]', ['GB12YUBO17171961366337'], 'invalid']]]
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 ["DE59811903138375291124"]	`valid`	`valid`	Passed
regression ["DE50296496123032354476"]	`valid`	`valid`	Passed
control ["DE59811903138375291127"]	`invalid`	`invalid`	Passed
control ["DE50296496123032354477"]	`invalid`	`invalid`	Passed
control ["DE58862288567935243527"]	`invalid`	`invalid`	Passed
control ["GB12YUBO17171961366337"]	`invalid`	`invalid`	Passed
control ["GB30TWRL09177880233697"]	`invalid`	`invalid`	Passed
control ["GB23HJGZ79203524065167"]	`invalid`	`invalid`	Passed

SHA-256 / 63c2f132429d135b9be76df4a696f50b8f76cb575fca959799424fd6c235bc36

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 2026-09-29T14:48:38.807807+00:00.

Case digest / 2014f2c5f6ff5c671e36e79be26a90f6843f4bf4e4006fed5f1184b4cbcd439d