Lei.Net 1.0.0

Lei.Net

Correct, zero-dependency validation and parsing of Legal Entity Identifiers (LEI, ISO 17442) for .NET, with ISO 7064 MOD 97-10 check-digit computation and self-checking reference test vectors.

No NuGet package existed for the LEI when this was written. Lei.Net fills that gap for the .NET identifier family alongside SecurityIds.Net, Bic.Net, and Nuban.Net.

• Zero runtime dependencies. Targets net8.0, AOT-compatible, trimming-safe.
• Culture-independent (ToUpperInvariant, ordinal comparisons), allocation-light iterative MOD 97.
• Fully XML-documented public API.

Install

dotnet add package Lei.Net

Quick start

using Lei;

// Fast boolean check
bool ok = LeiValidator.IsValid("529900T8BM49AURSDO55"); // true

// Structured result with parsed parts and a typed failure reason
var result = LeiValidator.Validate("529900T8BM49AURSDO55");
if (result.IsValid)
{
    Console.WriteLine(result.LouPrefix);   // "5299"
    Console.WriteLine(result.EntityPart);  // "T8BM49AURSDO"
    Console.WriteLine(result.CheckDigits); // "55"
}
else
{
    Console.WriteLine(result.Error); // NullOrEmpty | WrongLength | InvalidCharacter | ReservedDigitsNotZero | BadCheckDigits
}

// Compute the correct check digits for the first 18 characters
string check = LeiValidator.ComputeCheckDigits("529900T8BM49AURSDO"); // "55"

Input is trimmed and upper-cased invariantly; null, empty, and whitespace-only values are rejected.

What an LEI is

A Legal Entity Identifier is exactly 20 uppercase alphanumeric characters:

5299 00 T8BM49AURSDO 55
 |   |       |        |
 |   |       |        +-- positions 19-20: 2 check digits
 |   |       +----------- positions  7-18: 12-char entity-specific part
 |   +------------------- positions  5-6:  reserved, always "00"
 +----------------------- positions  1-4:  Local Operating Unit (LOU) prefix

Validate enforces all four structural rules: length 20, allowed character set (0-9, A-Z), reserved "00" at positions 5-6, and correct check digits.

The MOD 97-10 check (ISO 7064)

The two trailing digits protect against typos. To verify them:

1. Take the full 20-character LEI.
2. Convert each letter to a two-digit number: A = 10, B = 11, ... Z = 35. Digits stay as themselves.
3. Read the concatenation as one large base-10 integer.
4. The LEI is valid when that integer mod 97 equals 1.

To generate the check digits from the first 18 characters: append "00", take the number mod 97, and the check digits are 98 - remainder, zero-padded to two digits.

Lei.Net computes the remainder iteratively, one character at a time, so it needs no big-integer arithmetic and allocates nothing on the hot path.

The test suite is self-checking: for every reference LEI it asserts that ComputeCheckDigits(first18) reproduces the identifier's own last two characters. That equality is the objective correctness oracle.

Who needs this

• MiFID II transaction reporting and EMIR trade reporting, where the LEI identifies counterparties and issuers.
• KYC / onboarding pipelines validating a legal entity before it reaches downstream systems.
• Any service that ingests, stores, or forwards LEIs and wants to reject malformed values at the boundary.

License

MIT (c) Israel Iyonsi. See LICENSE.