Transcript case study in embedde..

PORTING A NETWORK CRYPTOGRAPHIC
SERVICE TO THE RMC2000 : A CASE
STUDY IN EMBEDDED SOFTWARE
DEVELOPMENT
Porting a Network Cryptographic Service to RCM2000
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About RCM2000
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Porting a Network Cryptographic Service to RCM2000
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About Dynamic C
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Porting a Network Cryptographic Service to RCM2000
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Reference site
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http://www.rabbitsemiconductor.com/products/rcm2000/docs.shtml
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Porting a Network Cryptographic Service to RCM2000
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Introduction
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Experience porting a transport-layer cryptography service to an embedded
microcontroller
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Some key development issues and techniques involved in porting networked
software to a connected , limited resource device such as the Rabbit RCM2000
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The effectiveness of a few proposed porting strategies by examining important
program and run-time characteristics
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NETWORK CRYPTOGRAPHIC SERVICES
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SSL (Secure Sockets Layer)
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a protocol that layers on top of TCP/IP to provide secure communications
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e.g. , to encrypt web pages with sensitive information
not cheap
negotiating an SSL session can degrade server performance
iSSL
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a cryptographic library that layers on top of the Unix sockets layer to provide
secure point-to-point communications
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NETWORK CRYPTOGRAPHIC SERVICES
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Ported iSSL service to the RCM2000
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SSL forms a layer above TCP
→ easily moved from the server to other hardware
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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
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THE RCM2000 ENVIRONMENT
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The RCM2000 TCP/IP Development Kit
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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)
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THE RCM2000 ENVIRONMENT
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Dynamic C
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developed along with the Rabbit microcontrollers
→ support the Rabbit 2000 in embedded system applications
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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
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PORTING AND DEVELOPMENT ISSUES
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Three broad classes of porting problems that demanded code rewrites
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the absence of certain libraries and operating system facilities
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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
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PORTING AND DEVELOPMENT ISSUES
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differing APIs with similar functionality
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the protocol for accessing the RCM2000's TCP/IP stack differs quite a bit from the
BSD sockets used within iSSL
sloppy memory management
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memory leaks
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remove logging altogether
to make logging write to a circular buffer rather than a file
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PORTING AND DEVELOPMENT ISSUES
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Interrupts
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used the serial port on the RCM2000 board for debugging
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configured the serial interface to interrupt the processor when a character
arrived
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In response , the system either replied with a status messages or reset the
application , possibly maintaining program state
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to set up the interrupt from the serial port
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enable interrupts from the serial port
register the interrupt routine
enable the interrupt receiver
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PORTING AND DEVELOPMENT ISSUES
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Memory
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not support the standard library functions malloc and free
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provides the xalloc function that allocates extended memory
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remove all references to malloc and statically allocate all variables
→ drop support of multiple key and block sizes in the iSSL library
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PORTING AND DEVELOPMENT ISSUES
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Program structure
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use of high-level operating system functions such as fork that were not
provided by the RCM2000 environment
→ restructure the program
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provide neither the standard Unix fork nor an equivalent of accept
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the socket bound to the port handles the request
→ connection is required to have a corresponding call to tcp
listen
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easily increase the number of processes ( and hence simultaneous connections )
by adding more costatements
→ the program would have to be re-compiled
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EXPERIMENTAL RESULTS
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Compared the C implementation of the AES algorithm ( Rijndael )
included with the iSSL library with a hand-coded assembly version
supplied by Rabbit Semiconductor
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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%
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EXPERIMENTAL RESULTS
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Problems
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lack of experience with Dynamic C
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lack of experience with the RCM2000 platform
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CONCLUSIONS
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Problems
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support for some hardware idiosyncrasies
API for TCP/IP both differed substantially from the Unix-like behavior the
service originally used
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the substantial difference between BSD-like sockets and the provided TCP/IP
implementation
the simple absence of a file-system
Solutions
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writing substantial amounts of additional code to implement the missing
library functions
reworking the original code to use or simply avoid the API
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