improve the how-to a bit with pics
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README.md
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README.md
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# LS47 hand-computable cipher
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# LS47 hand-computable cipher
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This is a slight improvement of the ElsieFour cipher as described by Alan Kaminsky [1]. We use 7x7 characters instead of previous (barely fitting) 6x6 to be able to encrypt some structured information and add a simple key-expansion algorithm. Similar security considerations hold.
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This is a slight improvement of the ElsieFour cipher as described by Alan
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Kaminsky [1]. We use 7x7 characters instead of original (barely fitting) 6x6,
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to be able to encrypt some structured information. We also describe a simple
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key-expansion algorithm, because remembering passwords is popular. Similar
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security considerations as with ElsieFour hold.
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There's 3D-printable SCAD model of the whole thing.
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There's also a 3D-printable SCAD model of the whole thing. Yay!
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### Character board
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### Character board
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We have added some real punctuation, basic stuff for writing a bit of markdown,
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We have added some real punctuation, basic stuff for writing expressions,
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and quotes&parens for writing structured information. The letters of the board
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punctuation and quotes. The letters of the board now look like this:
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now look like this:
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```
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```
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_ a b c d e f
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_ a b c d e f
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@ -21,26 +24,29 @@ u v w x y z .
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/ : ? ! ' ( )
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/ : ? ! ' ( )
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```
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```
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Zoomed in, it's nice to have extra position information written on the tiles:
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Zoomed in, it's very practical to have extra position information written on
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the tiles:
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```
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```
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/-----\ /-----\ /-----\ /-----\ /-----\
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/-----\ /-----\ /-----\ /-----\ /-----\
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| | | | | | | | | |
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| | | | | | | | | |
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| _ 0| | a 1| | b 2| | c 3| | d 4| ...
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| _ 0| | a 1| | b 2| | c 3| | d 4| ...
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| 0 | | 0 | | 0 | | 0 | | 0 |
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| 0 | | 0 | | 0 | | 0 | | 0 |
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\-----/ \-----/ \-----/ \-----/ \-----/
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\-----/ \-----/ \-----/ \-----/ \-----/
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/-----\ /-----\
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/-----\ /-----\
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| | | |
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| g 0| | h 1| ...
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| g 0| | h 1| ...
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| 1 | | 1 |
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| 1 | | 1 |
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\-----/ \-----/
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\-----/ \-----/
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. .
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. .
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. .
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. .
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. .
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. .
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```
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```
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You also need some kind of a marker (e.g. a small shiny stone, bolt nut or similar kinds of well-shaped trash).
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To run (hand-run) the encryption/decryption, you will also need some kind of a
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marker (e.g. a small shiny stone, bolt nut or similar kind of well-shaped
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trash).
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## How-To
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## How-To
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@ -58,6 +64,58 @@ You may as well see the paper [1], there are also pictures. This is somewhat mor
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8. Update the position of the marker: `M := M + C' mod (7,7)` where `C'` are the numbers written on the ciphertext tile.
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8. Update the position of the marker: `M := M + C' mod (7,7)` where `C'` are the numbers written on the ciphertext tile.
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9. Repeat from 3 as many times as needed to encrypt the whole plaintext.
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9. Repeat from 3 as many times as needed to encrypt the whole plaintext.
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#### Encryption example with ascii images!
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```
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1,2. Symmetric key with 3,4. we want to encrypt 'y'.
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marker put on 'e' Look at the marker:
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[e]f _ a b c d /-----\
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l m g h i j k | |
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( ) / : ? ! ' | e 5|
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s t n o p q r | 0 |
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z . u v w x y \-----/
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5 6 0 1 2 3 4
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+ * 7 8 9 , -
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5. Ciphertext is 'w' 6. Rotate the plaintext 1 position
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(='y' moved by (5,0)) right, possibly carrying the marker.
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[e]f _ a b c d [e]f _ a b c d
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Output 'w'! l m g h i j k l m g h i j k
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( ) / : ? ! ' ( ) / : ? ! '
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s t n o p q r s t n o p q r
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z . u v w x y >> y z . u v w x
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5 6 0 1 2 3 4 5 6 0 1 2 3 4
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+ * 7 8 9 , - + * 7 8 9 , -
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7. Rotate the ciphertext 1 Now look at the ciphertext tile.
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position down, also carry.
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[e]f _ a b , d /-----\
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l m g h i c k | |
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( ) / : ? j ' | w 2|
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s t n o p ! r | 3 |
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y z . u v q x \-----/
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5 6 0 1 2 w 4
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+ * 7 8 9 3 -
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8. Update the marker position 9. GOTO 3.
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by ciphertext offset (2,3).
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e f _ a i c d
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l m g h ? j k
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( ) / : p ! '
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s t[n]o v q r
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y z . u 2 w x
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5 6 0 1 9 3 4
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+ * 7 8 b , -
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```
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### Decryption
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### Decryption
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Decryption procedure is basically the same, except that in step 5 you know `C`
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Decryption procedure is basically the same, except that in step 5 you know `C`
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@ -99,13 +157,13 @@ The actual expansion can be as simple as this:
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### Undistinguishable ciphertexts
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### Undistinguishable ciphertexts
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To get a different ciphertext even if the same plaintext is encrypted; prepend
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To get a different ciphertext even if the same plaintext is encrypted
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it with a nonce. A nonce is a completely random sequence of letters of a
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repeatedly; prepend it with a nonce. A nonce is a completely random sequence of
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pre-negotiated length (at least 10 tiles drawn randomly from a bag is
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letters of a pre-negotiated length (e.g. N tiles drawn randomly from a bag,
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adviseable).
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adviseable value of N is at least 10).
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You may also want to add a random number of spaces to the end of the ciphertext
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You may also want to add a random number of spaces to the end of the ciphertext
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-- it prevents the enemy from seeing the difference between ciphertexts for
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-- it prevents the enemy from seeing the difference between ciphertexts of
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'yes please' and 'no', which would otherwise encrypt to easily measurable
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'yes please' and 'no', which would otherwise encrypt to easily measurable
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gibberish like `qwc3w_cs'(` and `+v`.
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gibberish like `qwc3w_cs'(` and `+v`.
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@ -113,13 +171,15 @@ gibberish like `qwc3w_cs'(` and `+v`.
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Because ciphertext may be altered in the transfer or during the error-prone
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Because ciphertext may be altered in the transfer or during the error-prone
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human processing, it is advised to append a simple "signature" to the end of
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human processing, it is advised to append a simple "signature" to the end of
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the message; e.g. a simple string `__YourHonorableNameHere`. If the signature
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the message; which may look as simple as `__YourHonorableNameHere`. If the
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doesn't match expectations (which happens with overwhelming probability if
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signature doesn't match expectations (which happens with overwhelming
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there was any error in the process), you discard the message and ask the sender
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probability if there was any error in the process), either try again to see if
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to re-transmit.
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you didn't make a mistake, or discard the message and ask the sender to
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re-transmit.
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This works because the cipher output is message-dependent: Having a wrong bit
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This works because the cipher output is message-dependent: Having a wrong bit
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somewhere in the middle causes avalanche effect and breaks the signature.
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somewhere in the middle causes avalanche effect and erases any meaning from the
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text after several characters.
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## References
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## References
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