555 Fixed-ROM XOR Stream Engine
555 : Fixed-ROM XOR Stream Engine
- Author: Owen Deng
- Description: Ready/valid byte-stream XOR engine backed by a fixed 160-byte ROM
- GitHub repository
- Open in 3D viewer
- Open in Tiny Tapeout Explorer
- Clock: 50000000 Hz
How it works
The Fixed-ROM XOR Stream Engine stores 160 fixed bytes in combinational
standard-cell ROM. After START, a host supplies one same-position input byte
at a time. The chip XORs that byte with the addressed ROM byte and returns the
result through a ready/valid output register.
This keeps the ASIC deliberately small: it stores only the fixed ROM and one output byte at a time. Input acquisition, formatting, and display are left to external hardware. The design targets one TTIHP26b tile.
This is a one-time-pad design, not authenticated encryption. The production key must be uniformly random, exactly the ciphertext length, kept private, and never reused. A wrong key still produces output; the chip cannot detect it.
The ROM contents are public. Any application that relies on one-time-pad confidentiality must keep its external key uniformly random, private, exactly the ROM length, and never reuse it.
The physical layout also includes a passive TopMetal1 artwork macro. It is electrically disconnected and does not affect the digital interface.
How to test
- Reset the design with
rst_n=0. - Pulse
uio_in[1](START). - When
uio_out[4](KEY_READY) is high, place the next key byte onui_in[7:0]and pulseuio_in[0](KEY_VALID). - Keep
uio_in[2](OUT_READY) high and captureuo_out[7:0]wheneveruio_out[6](OUT_VALID) is high. - After the last byte transfers,
uio_out[7](DONE) stays high.
uio_in[3] (ABORT) synchronously stops a session and clears progress.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | KEY_DATA[0] | OUT_DATA[0] | KEY_VALID |
| 1 | KEY_DATA[1] | OUT_DATA[1] | START |
| 2 | KEY_DATA[2] | OUT_DATA[2] | OUT_READY |
| 3 | KEY_DATA[3] | OUT_DATA[3] | ABORT |
| 4 | KEY_DATA[4] | OUT_DATA[4] | KEY_READY |
| 5 | KEY_DATA[5] | OUT_DATA[5] | BUSY |
| 6 | KEY_DATA[6] | OUT_DATA[6] | OUT_VALID |
| 7 | KEY_DATA[7] | OUT_DATA[7] | DONE |