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I would like to learn more about RE.

I wrote a simple program on a STM32F107 which does nothing else than encrypting and decrypting a text once using AES128-ECB.

Here is the C code (I intentionally left out the key so far):

    struct AES_ctx TestAes;
uint8_t key[16] =
        { MY_KEY_IS_HERE };
uint8_t InputText[16] =
        { 1, 2, 3, 4, 5, 6, 7, 8, 9, 0xa, 0xb, 0xc, 0xd, 0xe, 0xf, 0 };
AES_init_ctx(&TestAes, key);
AES_ECB_encrypt(&TestAes, InputText);
AES_ECB_decrypt(&TestAes, InputText);

Now I want to find the 16 byte private key in my binary.

When I open the binary in a hex editor and search for my key I find all 16 bytes in a row.

I loaded the binary in Ghidra, installed FindCrypt before and now run the analysis.

FindCrypt now finds AES_Decrytion_SBox_Inverse and AES_Ecryption_SBox.

But both are not my AES key but the SBox. How does it go on after that? In all tutorials I find it looks quite simple, because the Functions Finder finds the AES functions - but since the project is Bare Metal this will probably not work.

I thought FindCrypt looks for some kind of hex pattern which could result in a key...

I have attached the binary. endian is little, architecture is ARM Cortex (I think?!)

Ghidra FindCrypt Analysis

7

FindCrypt is not supposed to find your key, it's supposed to find fixed, known constants associated with well-known cryptographic algorithms. You can see the byte patterns it's looking for in its database.json. It has done exactly what it was supposed to do. Your job as a reverse engineer is to look at the cross-references to the tables that it found and see if you can determine the key being passed as argument to the encryption or decryption routines.

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