Simple MD5 implementation in VHDL

MD5 hash function is implemented as a finite state machine.

Entity description

• clk, reset - clock and reset.
• data_in - 32-bit input. Used to load message length and 32-bit chunks of message itself.
• data_out - 32-bit output. Used to output 32-bit chunks of calculated hash.
• done - 1-bit output. Set high when hash is done calculating and can be shifted out.
• start - 1-bit input. Depending on current state of the machine, it can either trigger loading of input message (states load_length and load_data) or it can trigger the machine to output calculated hash.

Functional description

Initially, machine resides in idle state. The only way to exit this state is to set input start to logical 1. Doing so will trigger state change to load_length. In this state, machine expects to receive length of the message to process, in bits.

One clock cycle later we're in load_data state. Actual loading of input data will start in the next cycle. From this point on, 32-bit chunks of input message are latched on each rising edge, until they reach or exceed message length specified previously.

MD5 specification requires input message to be padded in a specific way:

• append one '1' bit
• add zeroes to fill the buffer
• set last 64 bits to message_length (in bits) All those steps are executed in state pad.

Following that, endianness of input message needs to be swapped (state rotate).

Now computation of the hash can finally begin. There are 4 main stages: 1, 2, 3 and 4. Each of those is further divided into F and B. F is computed first, because it's required to compute B. State stageN_F (N is the stage number) only computes F, whereas stageN_B computes B and performs required shifts (A to D and so on). Machine jumps back and forth between stageN_F and stageN_B, each time incrementing loop_counter. Once 16 iterations are done, machine goes to the next stage.

Exiting stage4_B, machine enters stage5, where constants a0, b0, c0, d0 are added to A, B, C, D respectively. Following that, in stage6 endianness of ABCD is swapped. This concludes computation stage. Machine enters state finished, sets output done high, and awaits for the hash to be read.

Setting start high again triggers machine to enter store_data and output computed hash. User can expect first chunk one rising edge later. Once four 32-bit chunks are shifted out, machine enters idle state.

State graph