iso8583-fps

A small, dependency-free ISO 8583 codec in Python, with a UK Faster Payments (FPS) message profile — in both ASCII and BCD/binary encodings.

ISO 8583 is the classic card / interbank message format (4-digit MTI, a primary + optional secondary bitmap flagging which data elements are present, and typed data elements framed as fixed / LLVAR / LLLVAR). UK Faster Payments is ISO 8583:1993-based, yet there's almost no small, readable, open implementation to point people at. This is that.

• Real codec, two encodings. pack() / unpack() are proven inverses over a data-element registry — an ASCII form (hex bitmap, ASCII digits) and a BCD/binary form (raw-byte bitmap, BCD-packed numerics) — the way switches actually send it.
• Zero dependencies. Pure standard library. One file.
• FPS profile. fps_request() maps a payment onto an MTI 0200; fps_response() builds an MTI 0210 with a response code. Works on a plain dict — no framework.

from iso8583_fps import fps_request, unpack, FIELDS

msg = fps_request({
    "reference": "INV-9",
    "amount": "123.45",
    "currency": "GBP",
    "debtor":   {"sort_code": "123456", "account_number": "00087654"},
    "creditor": {"name": "Bob", "sort_code": "654321", "account_number": "00012345"},
    "remittance": "invoice 9",
})
print(msg)                       # 0200B0200000... (ASCII)

mti, fields = unpack(msg)
print(mti)                       # 0200
for de, val in sorted(fields.items()):
    print(de, FIELDS[de][3], "=", val)   # 4 Amount = 000000012345, 49 Currency = 826, ...

# BCD/binary encoding — how a switch actually sends it:
raw = fps_request({...}, binary=True)     # -> bytes
mti, fields = unpack(raw, binary=True)     # round-trips

Install

Copy the single file, or:

pip install iso8583-fps        # once published to PyPI — see PUBLISHING.md

Data elements (FPS profile)

Honest boundary

This is a structurally-real, FPS-flavoured profile — not the bit-exact, licensed Pay.UK / Vocalink data-element map (that specification is member-only and not public). The MTIs, processing codes and DE assignments here are reasonable conventions. Swapping in the licensed assignments is a data change to the FIELDS registry, not a rewrite. The generic pack()/unpack() are standard ISO 8583 and are correct as-is.

Why this exists

Built as part of KibiPay — a payments-interoperability platform that maps UK bank rails, Mojaloop mobile money, and Solana settlement onto one canonical model (an ISO 20022 superset), so a payment can arrive as ISO 8583 on FPS and leave as ISO 20022 pacs.008 on another rail. Free interactive tools (an ISO 8583 decoder, an ISO 20022 viewer, an MT↔MX mapper) live at kibipay.com.

License

MIT — see LICENSE.