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README.md

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MD5 & HMAC-MD5

MD5 and HMAC-MD5 algorithm implemented in C language.

Usage

make
./bin/md5 <INPUT_STRING>
./bin/hmacmd5 <INPUT_STRING> -key <KEY>

How It Works

  1. Padding Message

    • First fill with a 1, and then fill with multiple 0s, until the information length is equal to 448 modulo 512
    • Pad data length of 64 bits. Note that the data length represents the number of bits, not bytes

  2. Initialize MD Buffer

    • Initialize 4 32-bit buffers A, B, C, D

  3. Divide Block

    • Divide the padded information into blocks of 512 bits

  4. Loop Compression

    • 4 cycles, 16 iterations per cycle, 64 iterations in total
    • Each operation modifies the data in the buffer, and the data in the buffer is used as the input for the next iteration
    • 4 cycles using 4 (F, G, H, I) functions
    • In each of the 16 iterations, we take a different part of the block (g table), add a different fixed value (K table), and loop left by a different number of bits (s table).

  5. Output

    • Concatenate 4 32-bit buffers to get 128-bit result

Reference