Porting a Network Cryptographic Service to RCM2000

1. PORTING A NETWORK CRYPTOGRAPHIC SERVICE TO THE RMC2000 : A CASE STUDY IN EMBEDDED SOFTWARE DEVELOPMENT

2. Porting a Network Cryptographic Service to RCM2000 • About RCM2000

3. Porting a Network Cryptographic Service to RCM2000 • About Dynamic C

4. Porting a Network Cryptographic Service to RCM2000 • Reference site • http://www.rabbitsemiconductor.com/products/rcm2000/docs.shtml

5. Porting a Network Cryptographic Service to RCM2000 • Introduction • Experience porting a transport-layer cryptography service to an embedded microcontroller • Some key development issues and techniques involved in porting networked software to a connected , limited resource device such as the Rabbit RCM2000 • The effectiveness of a few proposed porting strategies by examining important program and run-time characteristics

6. NETWORK CRYPTOGRAPHIC SERVICES • SSL (Secure Sockets Layer) • a protocol that layers on top of TCP/IP to provide secure communications • e.g. , to encrypt web pages with sensitive information • not cheap • negotiating an SSL session can degrade server performance • iSSL • a cryptographic library that layers on top of the Unix sockets layer to provide secure point-to-point communications

7. NETWORK CRYPTOGRAPHIC SERVICES • Ported iSSL service to the RCM2000 • SSL forms a layer above TCP → easily moved from the server to other hardware • use coprocessor cards for performance • uses the RSA and AES cipher algorithms • the RSA algorithm uses a difficult-to-port bignum package → we only ported the AES cipher (uses the Rijndael algorithm ) • only implemented 128-bit keys and blocks • referred to the AESCrypt implementation developed by Eric Green and Randy Kaelber

8. THE RCM2000 ENVIRONMENT • The RCM2000 TCP/IP Development Kit • 512k of flash RAM • 128k SRAM • runs a 30 MHz • 8-bit Z80-based microcontroller • access up to 1MB through bank switching • 10 Base-T network interface • software implementing (TCP/IP, UDP and ICMP)

9. THE RCM2000 ENVIRONMENT • Dynamic C • developed along with the Rabbit microcontrollers → support the Rabbit 2000 in embedded system applications • ANSI C variant • support cooperative and preemptive multitasking • support battery-backed variables • support atomicity guarantees for shared multi-byte variables • local variables → static • default storage class for variables → static • not support the #include → using instead #use • Bit fields and enumerated types are not supported

10. PORTING AND DEVELOPMENT ISSUES • Three broad classes of porting problems that demanded code rewrites • the absence of certain libraries and operating system facilities • Dynamic C does not provide the standard random function • the protocols include timeouts , but Dynamic C does not have a timer • the iSSL library makes some use of a file-system , something not provided by the RCM2000 environment • writing a random function • changing the program logic so it no longer read a hash value from a file • final port did not implement the RSA cipher because it relied on a fairly complex bignum library that we considered too complicated to rework

11. PORTING AND DEVELOPMENT ISSUES • differing APIs with similar functionality • the protocol for accessing the RCM2000's TCP/IP stack differs quite a bit from the BSD sockets used within iSSL • sloppy memory management • memory leaks • remove logging altogether • to make logging write to a circular buffer rather than a file

12. PORTING AND DEVELOPMENT ISSUES • Interrupts • used the serial port on the RCM2000 board for debugging • configured the serial interface to interrupt the processor when a character arrived • In response , the system either replied with a status messages or reset the application , possibly maintaining program state • to set up the interrupt from the serial port • enable interrupts from the serial port • register the interrupt routine • enable the interrupt receiver

13. PORTING AND DEVELOPMENT ISSUES • Memory • not support the standard library functions malloc and free • provides the xalloc function that allocates extended memory • remove all references to malloc and statically allocate all variables →  drop support of multiple key and block sizes in the iSSL library

14. PORTING AND DEVELOPMENT ISSUES • Program structure •  use of high-level operating system functions such as fork that were not provided by the RCM2000 environment → restructure the program • provide neither the standard Unix fork nor an equivalent of accept • the socket bound to the port handles the request → connection is required to have a corresponding call to tcp listen • easily increase the number of processes ( and hence simultaneous connections ) by adding more costatements → the program would have to be re-compiled

15. EXPERIMENTAL RESULTS • Compared the C implementation of the AES algorithm ( Rijndael ) included with the iSSL library with a hand-coded assembly version supplied by Rabbit Semiconductor • pumped keys through the two implementations of the AES cipher → faster than the C port by a factor of 15-20 • A variety of optimizations on the C code • including moving data to root memory • unrolling loops • disabling debugging • enabling compiler optimization → only improved run time by perhaps 20%

16. EXPERIMENTAL RESULTS • Problems •  lack of experience with Dynamic C • lack of experience with the RCM2000 platform

17. CONCLUSIONS • Problems • support for some hardware idiosyncrasies • API for TCP/IP both differed substantially from the Unix-like behavior the service originally used • the substantial difference between BSD-like sockets and the provided TCP/IP implementation • the simple absence of a file-system • Solutions • writing substantial amounts of additional code to implement the missing library functions • reworking the original code to use or simply avoid the API