SecurityIds.Net 0.1.0

SecurityIds.NET

Validate and identify financial security identifiers in .NET: ISIN, CUSIP, SEDOL and FIGI, with check-digit verification. Zero external dependencies.

Every security traded anywhere in the world carries one or more of these codes, and every one of them is protected by a check digit meant to catch typos and transcription errors before they turn into a trade on the wrong instrument. Getting the check-digit algorithm wrong is easy: ISIN, CUSIP, SEDOL and FIGI each use a different scheme, with different character-to-value mappings and different weighting rules. There is no maintained, dependency-free .NET package that implements all four correctly and ties them together with a single detection entry point. SecurityIds.NET is that package: strict, verified against published worked examples, and dependency-free.

Where you need it:

• Validating instrument identifiers on data entry, before they reach a booking or settlement system
• Detecting which identifier scheme a free-text reference string belongs to
• Deriving a US ISIN from a CUSIP for reference data pipelines that only carry one of the two
• Post-trade and reference-data reconciliation, where a single transposed character otherwise fails silently

Install

dotnet add package SecurityIds.Net

Quickstart

using SecurityIds;

SecurityIdentifiers.IsValidIsin("US0378331005");   // true, Apple Inc
SecurityIdentifiers.IsValidCusip("037833100");     // true, Apple Inc
SecurityIdentifiers.IsValidSedol("0263494");       // true
SecurityIdentifiers.IsValidFigi("BBG000BLNNH6");   // true

SecurityIdentifiers.IsValidIsin("US0378331006");   // false, corrupted check digit

Detecting an unknown identifier

using SecurityIds;

SecurityIdentifierType type = SecurityIdentifiers.TryIdentify("US0378331005");
// SecurityIdentifierType.Isin

SecurityIdentifierType none = SecurityIdentifiers.TryIdentify("not-an-identifier");
// SecurityIdentifierType.None

TryIdentify distinguishes a 12-character ISIN from a 12-character FIGI using the FIGI's fixed third character (G) together with each scheme's own check digit, so a single call is enough for free-text reference data that mixes identifier types.

Deriving an ISIN from a CUSIP

using SecurityIds;

string isin = SecurityIdentifiers.IsinFromCusip("037833100", "US");
// US0378331005

IsinFromCusip validates the CUSIP's own check digit first, then prefixes the two-letter country code and computes the ISIN check digit over the combined 11 characters.

Computing a check digit directly

using SecurityIds;

int isinCheckDigit  = SecurityIdentifiers.ComputeIsinCheckDigit("US037833100");   // 5
int cusipCheckDigit = SecurityIdentifiers.ComputeCusipCheckDigit("45920010");     // 1
int sedolCheckDigit = SecurityIdentifiers.ComputeSedolCheckDigit("026349");       // 4
int figiCheckDigit  = SecurityIdentifiers.ComputeFigiCheckDigit("BBG000BLNNH");   // 6

The four schemes

The ISIN check digit position (the 12th character) must itself be a digit 0-9; a letter in that position is rejected even if it happens to expand to a Luhn-sum-preserving digit pair. The FIGI character set excludes the vowels A, E, I, O, U from every position after the fixed G, matching the OMG FIGI specification, which also reserves certain two-letter prefixes to avoid colliding with an ISIN country code.

All four IsValid* methods are case-sensitive and require the exact character set for their scheme; they return false rather than throwing for null, wrong length, or an invalid character. The Compute*CheckDigit methods throw ArgumentNullException or ArgumentException on malformed input, since a caller asking for a check digit is expected to already have a well-formed base value.

Verified against published examples

Each scheme's implementation is checked, by hand and in the test suite, against a worked example with a known, independently verifiable check digit:

• ISIN: US0378331005 (Apple Inc, digits-only NSIN) and US38259P7069 (Alphabet Inc Class C, NSIN contains a letter)
• CUSIP: 459200101 (IBM) and 037833100 (Apple Inc), plus 68389X105 (Oracle Corp, issue number contains a letter)
• SEDOL: 0263494 (digits-only base) and B0YBKL9 (base contains letters)
• FIGI: BBG000BLNNH6 and BBG000B9XRY4 (Apple Inc)

The FIGI check-digit algorithm is documented less consistently across secondary sources than the other three. This implementation follows the description published by the Object Management Group (per-character doubling by position, followed by a decimal digit sum, modulus 10), and its output matches both worked FIGI examples above digit for digit, from two independent issuers. Corroborate independently against the OMG FIGI specification before relying on it for anything where a mismatch would be costly, such as auto-generating identifiers rather than merely validating ones you already have.

Dependencies and AOT

Zero runtime dependencies. The library is pure managed code operating on string and char, with no reflection, no unmanaged calls and no third-party packages, so it is fully compatible with Native AOT and trimming.

License

MIT. See LICENSE.