FA-72476 / Check-digit algorithms / Open access

IBAN letters convert from one instead of ten · case 01

IBANs containing bank-code letters are rejected.

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

ROOT CAUSE

Letters are replaced by ord(ch) - 64 (A=1) rather than their base-36 value A=10.

THE FAILURE

Letters are replaced by ord(ch) - 64 (A=1) rather than their base-36 value A=10.

Unsuccessful approach: Keeping only the last decimal digit of each letter value still corrupts the number.

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[:4]
    n = ''.join((str(ord(ch) - 64) if ch.isalpha() else ch) 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 / e950f2d8645469a9d8cf6d47aa26d0c67a1cc5d4c2aacc823201db9fddcc8cba

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[4:] + t[:4]
    n = ''.join(str(int(ch, 36) % 10) 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 / 368517ba3d5fdbbbbff57feefb39f32061814516872582bc0e2c5a78d4545280

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 8 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

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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 2026-09-29T14:48:38.877222+00:00.

Case digest / 004ea4932bb647014af87fc7f172368ada047f53e48ea0d2f7718c76eae9e587