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64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
### Tiny AES in C
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This is a small and portable implementation of the AES ECB and CBC encryption algorithms written in C.
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You can override the default block-size of 128 bit with 192 or 256 bit by defining the symbols AES192 or AES256 in `aes.h`.
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The API is very simple and looks like this (I am using C99 `<stdint.h>`-style annotated types):
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```C
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void AES_ECB_encrypt(uint8_t* input, const uint8_t* key, uint8_t* output);
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void AES_ECB_decrypt(uint8_t* input, const uint8_t* key, uint8_t* output);
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void AES_CBC_encrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);
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void AES_CBC_decrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);
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```
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You can choose to use one or both of the modes-of-operation, by defining the symbols CBC and ECB. See the header file for clarification.
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There is no built-in error checking or protection from out-of-bounds memory access errors as a result of malicious input. The two functions AES_ECB_xxcrypt() do most of the work, and they expect inputs of 128 bit length.
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The module uses less than 200 bytes of RAM and 2.3K ROM when compiled for ARM (<2K for Thumb but YMMV).
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It is one of the smallest implementation in C I've seen yet, but do contact me if you know of something smaller (or have improvements to the code here).
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I've successfully used the code on 64bit x86, 32bit ARM and 8 bit AVR platforms.
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GCC size output when only ECB mode is compiled for ARM (using 128 bit block size):
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$ arm-none-eabi-gcc -Os -c aes.c -DCBC=0
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$ size aes.o
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text data bss dec hex filename
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2071 0 184 2255 8cf aes.o
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.. and when compiling for the THUMB instruction set, we end up just above 1.7K in code size.
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$ arm-none-eabi-gcc -mthumb -Os -c aes.c -DCBC=0
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$ size aes.o
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text data bss dec hex filename
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1499 0 184 1683 693 aes.o
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I am using the Free Software Foundation, ARM GCC compiler:
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$ arm-none-eabi-gcc --version
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arm-none-eabi-gcc (4.8.4-1+11-1) 4.8.4 20141219 (release)
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Copyright (C) 2013 Free Software Foundation, Inc.
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This is free software; see the source for copying conditions. There is NO
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warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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This implementation is verified against the data in:
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[National Institute of Standards and Technology Special Publication 800-38A 2001 ED](http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf) Appendix F: Example Vectors for Modes of Operation of the AES.
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A heartfelt thank-you to all the nice people out there who have contributed to this project.
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All material in this repository is in the public domain.
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