Slidefair Tutorial

Introduction

The Slidefair cipher is a bit different than the ciphers attacked in the past. It is a periodic substitution ciphers similar to Vigenère or Beaufort. However, the major difference between this cipher and those mentioned above is that encipherment is by digraphs (two letter combinations) as opposed to single letters. The Slidefair system can be used with any type of periodic system. In the Cryptogram, examples usually use the Vigenère system unless otherwise stated. I’m going to assume you are familiar with enciphering/deciphering with Slidefair. So if you haven’t already done so, break out your Phoebee disk (Or equivalent), and either ‘ELCY’ or “The ACA and You’ and get familiar with the system.

The example we are going to study is ND03 E-06:

CU OB OR QS JE HF AK GV II YQ WT TQ YV PE TC CE PZ BS OV ZG MF AR
YY NE GQ QM YV XN JM CE KF ZS LG AT EP BQ BH BE EJ QR CZ QB SB DX
OC IN FH XN JM CE KF ZS LG AT EP BQ XH FB KW VL ST FC RU ZV FG AU
WO BR HV UM AQ ZC LR RS TC ZM JZ

Period Determination

As with any periodic cipher, the first order of business is to determine the period. I normally use the IC method with Slidefair. However, the Kasiski method also works well with Slidefair. What we need to find are repeating digraphs in the cipher text and see whether any common period can be found by the usual analysis method. The major difference between this analysis and the normal Kasiski method is that we count positions by digraphs and not single letters. This is necessary since the cipher operates on letter pairs (i.e., digraphs), not on single letters.

A frequency analysis finds the following repeated digraphs (And frequencies):

CE(3), TC(2), KF(2), JM(2), XN(2), EP(2), BQ(2), ZS(2), AT(2), YV(2)

A Kasiski analysis of these repeats provides:

Digraph (Pos#1) (Pos#2) Difference Factors Possible Periods
YV 13 37 24 2, 2, 2, 3 4, 6, 8
TC 15 75 60 2, 2, 3, 5 4, 5, 6, 10
CE 16 30 14 2, 7 7
 
XN 28 48 20 2, 2, 5 4, 5
JM 29 49 20 2, 2, 5 4, 5
CE 30 50 20 2, 2, 5 4, 5
KF 31 51 20 2, 2, 5 4, 5
ZS 32 52 20 2, 2, 5 4, 5
 
AT 34 54 20 2, 2, 5 4, 5
FP 35 55 20 2, 2, 5 4, 5
BQ 36 56 20 2, 2, 5 4, 5

Note: The repeated sequence ‘XN JM CE KF ZS’ starts at both positions 28 and 48. The repeated sequence ‘AT FP BQ’ starts at both positions 34 and 54.

This analysis, and the repeats, strongly point toward a period of either 4 or 5. A comparison of the ACA standard (15-18 periods deep) with the total number of digraphs (77) provides an estimated period for the cipher between 77/18 = 4.28 and 77/15 = 5.13. This points strongly toward a period of 5. Therefore, 5 will be our initial guess at the period.

Keyword Determination

Before beginning the work necessary to find the key word, arrange the cipher into groups of period length. This will greatly assist the determination of which digraph belongs to which keyword letter. This provides:

01 02 03 04 05   01 02 03 04 05   01 02 03 04 05

CU OB OR QS JE   HF AK GV II YQ   WT TQ YV PE TC
CE PZ BS OV ZG   MF AR YY NE GQ   QM YV XN JM CE
KF ZS LG AT EP   BQ BH BE EJ QR   CZ QB SB DX OC
IN FH XN JM CE   KF ZS LG AT EP   BQ XH FB KW VL
ST FC RU ZV FG   AU WO BR HV UM   AQ ZC LR RS TC
ZM JZ

So, how do we recover the keyword? The salient point here is the manner in which the digraph pairs are enciphered. If you examine the method used in Slidefair you will notice the following important facts:

  1. Both letters of a digraph are encoded with the slide position (i.e., The same letter as a key).
  2. If a Vigenère based Slidefair system is used, the first letter of a plain text digraph is enciphered via Vigenère as the second letter of the cipher text digraph. The second letter of a plain text digraph is enciphered via Variant Beaufort as the first letter of the cipher text digraph.
  3. If a Variant based Slidefair system is used, the first letter of a plain text digraph is enciphered via Variant as the second letter of the cipher text digraph. The second letter of a plain text digraph is enciphered via Vigenère as the first letter of the cipher text digraph.
  4. If a Beaufort based Slidefair system is used, the first letter of a plain text digraph is enciphered via Beaufort as the second letter of the cipher text digraph. The second letter of a plain text digraph is also enciphered via Beaufort as the first letter of the cipher text digraph.

In our case, we will assume this is a Vigenère based Slidefair since that appears to be the ACA standard. We can now take the digraphs from each group and do an independent frequency analysis of the first and second letters. If we assume the first letters are Variant enciphered and the second letters are Vigenère enciphered, we should find they both correspond to the same key letter. Here, we will use the shortcut method (Based on the SENORITA letters) described by ROT13 in the ND2002 issue of the Cryptogram to identify the key (N.B., Other methods can be used as well).

For example, for 01 group letters we get:

Pos System Letters (Frequency) Possible Keys (etaonirsh)
1 Variant 3C 2ABK 1HIMQSWZ M (SHOCBWFGV) (5 Hits)
2 Vigenère 4F 3Q 2MTUZ 1ENZ M (QFMAZUDET) (7 Hits)

We can assume the first letter of the key is ‘M’ and do a partial decipherment. This provides:

01 02 03 04 05   01 02 03 04 05   01 02 03 04 05
 M  *  *  *  *    M  *  *  *  *    M  *  *  *  *

CU OB OR QS JE   HF AK GV II YQ   WT TQ YV PE TC
io ** ** ** **   tt ** ** ** **   hi ** ** ** **

CE PZ BS OV ZG   MF AR YY NE GQ   QM YV XN JM CE
so ** ** ** **   ty ** ** ** **   ac ** ** ** **

KF ZS LG AT EP   BQ BH BE EJ QR   CZ QB SB DX OC
tw ** ** ** **   en ** ** ** **   no ** ** ** **

IN FH XN JM CE   KF ZS LG AT EP   BQ XH FB KW VL
bu ** ** ** **   tw ** ** ** **   en ** ** ** **

ST FC RU ZV FG   AU WO BR HV UM   AQ ZC LR RS TC
he ** ** ** **   im ** ** ** **   em ** ** ** **

ZM JZ
al **

While the first digraph is a bit unusual (Unless the plain text starts with the word “I”), most of the recovered plain text looks good (i.e., No low frequency digraphs like jy or zq). Lets move on to recover the second letter of the key word. For group 02 letter we get:

Pos System Letters (Frequency) Possible Keys (etaonirsh)
1 Variant 3Z 2AF 1BJOPQTWXY O (QFMAZUDET) (4 Hits)
2 Vigenère 3H 2BCSZ 1KQRV O (SHOCBWFGV) (5 Hits)

 

Once again, we substitute and get:

01 02 03 04 05   01 02 03 04 05   01 02 03 04 05
 M  O  *  *  *    M  O  *  *  *    M  O  *  *  *

CU OB OR QS JE   HF AK GV II YQ   WT TQ YV PE TC
io nc ** ** **   tt wo ** ** **   hi ch ** ** **

CE PZ BS OV ZG   MF AR YY NE GQ   QM YV XN JM CE
so ld ** ** **   ty do ** ** **   ac hm ** ** **

KF ZS LG AT EP   BQ BH BE EJ QR   CZ QB SB DX OC
tw en ** ** **   en tp ** ** **   no ne ** ** **

IN FH XN JM CE   KF ZS LG AT EP   BQ XH FB KW VL
bu tt ** ** **   tw en ** ** **   en tl ** ** **

ST FC RU ZV FG   AU WO BR HV UM   AQ ZC LR RS TC
he ot ** ** **   im ak ** ** **   em on ** ** **

ZM JZ
al lx

It is pretty clear now we are on the right track. Whole words (e.g., Sold, two) have appeared. Plenty of partial words are available to help recover the remainder of the key word (e.g., The partial ‘onc’ in group one, the partial ‘hich’ in group three, and the partial ‘twen’ in groups 7 and 11).

You should be able to finish this decipherment without further help.