General Remarks
Any substitution cipher, enciphered by a single alphabet composed of letters, figures or conventional signs, can be handled by the methods of case 6. For example, the messages under case 4-a and 5-a are easily solved by these methods. But note that the messages under case 4-b and 5-b cannot so be solved because several alphabets are used. We will see later that there are methods of segregating the different alphabets in some cases where several are used and then each of the alphabets is to be handled as below.
Case 6-a.
Message
QDBYP BXHYS OXPCP YSHCS EDRBS ZPTPB BSCSB PSHSZ AJHCD OSEXV HPODA PBPSZ BSVXY XSHCD
This message was received from a source which makes us sure it is in Spanish. The occurrence of B, H, P and S has tempted us to try the first two words as in case 4 and 5 but without result. We now prepare a frequency table, noting at the same time the preceding and following letter. This latter proceeding takes little longer than the preparation of an ordinary frequency table and gives most valuable information.
Frequency Table
| Prefix | Suffix | ||||
| A | 11 | 2 | ZD | JP | |
| B | 11111111 | 8 | DPRPBSPZ | YXSBSPPS | |
| C | 11111 | 5 | PHSHH | PSSDD | |
| D | 11111 | 5 | QECOC | BROA | |
| E | 11 | 2 | SS | DX | |
| F | |||||
| G | |||||
| H | 111111 | 6 | XSSJVS | YCSCPC | |
| I | |||||
| J | 1 | 1 | A | H | |
| L | |||||
| M | |||||
| N | |||||
| O | 111 | 3 | SDP | XSD | |
| P | 111111111 | 9 | YXCZTBHAB | BCYTBSOBS | |
| Q | 1 | 1 | D | ||
| R | 1 | 1 | D | B | |
| S | 111111111111 | 12 | YYCBBCPHOPBX | OHEZCBHZEZVH | |
| T | 1 | 1 | P | P | |
| U | |||||
| V | 11 | 2 | XS | HX | |
| X | 11111 | 5 | BOEVY | HPVYS | |
| Y | 1111 | 4 | BHPX | PSSX | |
| Z | 111 | 3 | SSS | PAB | |
It is clear from an examination of this table that we have to deal with a single alphabet but one in which the letters do not occur in their regular order.
We may assume that P and S are probably A and E, both on account of the frequency with which they occur and the variety of their prefixes and suffixes. If this is so, then B and H, are probably consonants and may represent R and N respectively. D and X are then vowels by the same method of analysis. Noting that HC occurs three times and taking H as N we conclude that C is probably T. Substitute these values in the last three words of the message because the letters assumed occur rather frequently there.
| PBPSZBSV | X | Y | X | SHC | D |
| I | I | I | |||
| ARAE_RE_ | _ | ENT | |||
| O | O | O |
Now Z is always prefixed by S and may be L. Taking X=I and D=O, (they are certainly vowels), V=G and Y=M, we have
| ARA EL REGIMIENTO |
Substituting these values in the rest of the message we have
| Q | DBYPBXH | YSOXPCP | YSHCSED | RBSZPTPB |
| _ | ORMARIN | ME_IATA | MENTE_O | _RELA_AR |
| BSCSB | PSHSZ | AJHCD | OSEXVHPODA | |
| RETER | AENEL | __NTO | _E_IGNA_O_ |
We may now take Q=F, O=D, E=S, R=B, T=C, A=P and J=U and the message is complete. We are assisted in our last assumption by noting that S=E and E=S, etc., and we may on that basis reconstruct the entire alphabet. The letters in parenthesis do not occur in the message but may be safely assumed to be correct.
| Ordinary | A | B | C | D | E | F | G | H | I | J | L | M | N | O | P | Q | R | S | T | U | V | X | Y | Z |
| Cipher | P | R | T | O | S | (Q) | (V) | N | (X) | (U) | (Z) | (Y) | (H) | D | A | F | B | E | C | J | G | I | M | L |
It is always well to attempt the reconstruction of the entire alphabet for use in case any more cipher messages written in it are received.——
Case 6-b.
Message
Lt. J. B. Smith, Royal Flying Corps, Calais, France.
| DACFT | RRBHA | MOOUE | AENOI | ZTIET |
| ASMOS | EOHIE | YOCKF | NOHOE | NOUTH |
| OMEAH | NILGO | OSAHU | OHOUE | APCHS |
| TLNDA | CFTEN | INTWN | BAFOH | GROHT |
| AEIOH | ABRIS | ODACF | TRREN | OSTSM |
| AYBIS | DFTEN | EFAPH | OSMNI | ZTIEA |
| HLILL | TWSOU | GDENO | UTHOM | EAHBH |
| AMOOU | EAYOE | QISUU | OLEHA | DENOE |
| NHOOQ | OBBOR | TSLHO | BAHEO | UBHOB |
| IHTSW | ENOHO | PAHIH | ITUAS | BIHTL |
Graham-White.
The address and signature indicate that this message is in English.
There are 250 letters in the cipher; the vowels AEIOU occur 109 times or 43.6%, the letters LNRST occur 62 times or 24.8%, and the letters KQVXZ occur 5 times or 2%. The proportion in the case of the vowels is somewhat too large and, in the case of the letters LRNST, it is too small. It is then questionable whether this is a transposition cipher altho, at first glance it might appear to be one.
On examination for parts of possible words we are at once struck by the occurrence at irregular intervals of recurring groups, viz:
| DACFTRR | ENO | BHAMOOUEA |
| DACFTEN | ENOUTHOMEAH | BHAMOOUEA |
| ENO | ||
| DACFTRR | DENOUTHOMEAH | IZTIE |
| FTEN | DENO | IZTIE |
| ENO |
This is a strong indication that the cipher is a substitution cipher, so, to make an examination a frequency table will be constructed.