MD5 and HMAC-MD5 algorithm implemented in C language.

make

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

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

• https://www.rfc-editor.org/rfc/rfc1321.html