Open Source for ESIM Integration - Harald Welte, sysmocom GmbH

### Overview In this presentation, Harald Welte of sysmocom GmbH discusses the technical landscape and practical implementation of eSIM technology for embedded devices using open-source software. The talk demystifies cellular smart card terminology, traces the evolution from classic 2G SIM cards to USIMs and virtualized eSIMs (eUICC), evaluates existing GSMA standards across consumer, M2M, and IoT segments, and demonstrates how open-source tooling such as `lpac` can be used to manage eSIM profiles on embedded systems.

### Key Topics - **Terminology and Architecture**: Clarifying the distinction between SIM (2G), USIM (3G/4G/5G logical application), and eSIM/eUICC (Embedded Universal Integrated Circuit Card). An eSIM profile is conceptually a cryptographically secured and signed container (analogous to a file system archive with metadata and optional Java Cardlets) downloaded onto an eUICC chip. - **eSIM Architectures (M2M vs. Consumer vs. IoT)**: The original M2M specification (SGP.01/02) used centralized remote control without an LPA, but failed to deliver smooth operator switching and required high minimum order volumes. Consumer eSIM (SGP.22) uses a decentralized pull model managed by a Local Profile Assistant (LPA) communicating with a Subscription Manager Data Preparation+ (SMDP+) server. The emerging IoT specification (SGP.31/32) bridges these models by splitting LPA duties between the on-device IPA and a centralized eSIM IoT Remote Manager (eIM), allowing standard consumer profiles to be managed centrally. - **Open Source LPA Implementation (`lpac`)**: While operating a production SMDP+ server requires onerous and costly GSMA Security Accreditation Scheme (SAS) certifications, the client-side LPA requires no accreditation. Welte details `lpac`, an open-source C-based client that interfaces with SMDP+ over HTTP and communicates with the eUICC using APDU drivers. - **Modem and Chip Interfacing**: Discusses how embedded Linux devices communicate with soldered (e.g., MFF2) or removable plastic eUICCs. Communication occurs across cellular modems using either 3GPP TS 27.007 AT commands (`AT+CCHO`, `AT+CCHC`, `AT+CGLA`) to manage logical channels and APDUs, or Qualcomm's QMI UIM subsystem. - **Testing and Verification**: Under GSMA SGP.26, developers can utilize test certificates with a publicly disclosed root CA to conduct thousands of profile download and management tests without incurring commercial operator subscription costs.

### Technical Details - **Cryptographic Binding**: Profiles downloaded from an SMDP+ are encrypted specifically for the target eUICC's public key (derived from a GSMA PKI). Once profile binding occurs, that profile cannot be installed on another eUICC. - **Notifications and State Verification**: Actions such as enabling, disabling, and deleting profiles generate cryptographically signed notifications inside the eUICC, which the LPA retrieves and uploads back to the SMDP+ to prove state changes. - **Form Factors & Hardware**: eUICCs share identical electrical and logical transmission layers with classic smart cards and can be deployed in standard removable plastic formats or surface-mount packages (e.g., standard form factor MFF2). - **Multiple Enabled Profiles (MEP)**: While version 2 eUICC specifications restrict devices to a single enabled profile at a time, version 3 introduces MEP, enabling a single eUICC to support dual concurrent network registrations over a multiplexed UART bus to the modem.

### Takeaways - Embedded developers can retrofit or design eSIM-capable devices using open-source LPA clients like `lpac` without proprietary client software vendor lock-in. - The consumer eSIM ecosystem currently offers the broadest availability at unit quantities of one, while SGP.32 IoT specifications and open-source eIM tooling will enable centralized fleet management using consumer profiles. - Developers testing eSIM integrations should leverage SGP.26 test certificate infrastructure to eliminate commercial recurring costs during development.