A-Sin-To-Err

Cipher Machine Using 4 bit Nibbler CPU.

Encoding

Before encryption, plaintext needs to be converted to a stream of decimal digits. One way to do it is by using straddling checkerboard.

	0	1	2	3	4	5	6	7	8	9
	A		S	I	N		T	O	E	R
1	B	C	D	F	G	H	J	K	L	M
5	P	Q	U	V	W	X	Y	Z	.	/

Letters in the unmarked row are represented by one digit (column number), e.g., N becomes 4.

Letters in the other two rows are represented by two digits (row, then column), e.g., W becomes 54.

Numbers are enclosed in / symbols (59), then each digit is repeated twice, e.g., 42 becomes 59442259.

Example encoding of text as stream of digits using straddling checkerboard:

Plaintext	H	E	L	L	O	W	O	R	L	D
Encoded	15	8	18	18	7	54	7	9	18	12

From the command line: echo "Hello world" | ./straddling_checkerboard 1581 8187 5479 1812

Encryption

Message begins with 8 digit nonce followed by stream of encrypted digits. Nonce is not secret, but every value should only be used once. It is similar to page identifier of one time pad.

To encrypt, type nonce followed by encoded stream and record encrypted message shown on the LED display.

Nonce	5551	8424
Encoded			1581	8187	5479	1812
Message	5551	8424	3279	9875	7770	9178

From the command line:

echo "5551 8424 1581 8187 5479 1812" | ./a_sin_to_err
5551 8424 3279 9875  7770 9178

Decryption

To decrypt, type message and record decrypted stream shown on the LED display.

Message	5551	8424	3279	9875	7770	9178
Decrypted			1581	8187	5479	1812

From the command line:

echo "5551 8424 3279 9875  7770 9178" | ./a_sin_to_err
5551 8424 1581 8187  5479 1812

Decoding

Ignore first 8 digits (nonce) and decode the rest using straddling checkerboard. Example:

Decrypted	15	8	18	18	7	54	7	9	18	12
Decoded	H	E	L	L	O	W	O	R	L	D

From the command line:

echo "1581 8187  5479 1812" | ./straddling_checkerboard
HELLOWORLD

Building

Start by changing the key in a_sin_to_err.cpp. The key has to be random, e.g., 128 coin tosses. Then build code generator:

make a_sin_to_err
cp asintoer.asm asintoer2.asm
./a_sin_to_err codegen >>asintoer2.asm 

Compile asintoer2.asm on Windows to create ROM for Nibbler:

assembler -o asintoer2.bin asintoer2.asm

Compile optional encoder/decoder:

make straddling_checkerboard

Implementation Details

Encryption is RC5 in CTR mode. For ease of implementation only one nibble from each block is used, and nibbles that do not correspond to a BCD are simply discarded.

Encryption function applied to each digit is (20 - plaintext - key) % 10. It is involutory, meaning that f(f(x)) = x, so decryption function is the same as encryption: (20 - ciphertext - key).

Known Limitations

Expanded key is stored in ROM because Nibbler's lack of indirect addressing and limited ROM size makes it difficult to port rc5_setup. Besides, entering 39 digit key every time is impractical.

Hardware Changes

The only hardware changes compared to the original Nibbler are input and output:

• Four seven-segment LED digits instead of LCD. HEF4543B decoders are connected to OUT0, /LE lines to OUT1. Rightmost digit is bit 0 of OUT1.
• 12 key telephone-style keypad instead of four pushbuttons. Key switches are arranged in a matrix. Columns are connected to IN0. Rightmost column is bit 0. Rows are connected to OUT0. Top row is bit 0.

Clock

When no keys are pressed after device is turned on or reset, it works as a clock. Time is displayed in 24 hour (military) standard, hours and minutes only. Time can be set using the keypad. Clock runs only when no or 4 digits have been entered so that it does not interfere with encryption.

Other keys

"*" (star) key resets the device. "#" (hash) key shows debugging information (currently four least significant digits of CTR, hex values above 9 decoded as blanks).