|WILPOHNFBR BPMQRJKGTC QDVASSZGVF|
|HNLOOQBSLM QUOBPFVHMF DUULLTWMAY VCLBXFUZXR|
|REPPMTOSKF RXALDFDSVS EFYLYYLAHB QXPQRTNHDL|
This type of cipher was considered very secure at one time (), but is not really difficult. Suppose we guess that the first letter is T. This implies the eleventh letter is Y, the 21st letter is N, etc. Now look at the two-letter combinations that occur from different possiblilities for the second letter:
|TI YP ND EN NU AU SC OE OX BF NX OX TP (no shift of 2nd letter)|
|TJ YQ NE EO NV AV SD OF OY BG NY OY TQ|
|TK YR NF EP NW AW SE OG OZ BH NZ OZ TR|
|TL YS NG EQ NX AX SF OH OA BI NA OA TS|
|(skipping over some in the middle)|
|TF YM NA EK NR AR SZ OB OU BE NU OU TM|
|TG YN NB EL NS AS SA OC OV BD NV OV TN|
|TH YO NC EM NT AT SB OD OW BE NW OW TO|
It is necessary to know the first letter and the length of the key-sequence. If we assume the length is not too large, a program can just try all possibilities, eventually choosing the plaintext which looks best.1
A long key-sequence makes this approach more difficult, since we have fewer rows. The extreme case is that in which the key-sequence is as long as the plaintext itself. This leads to a theoretically unbreakable cipher. For any possible plaintext, there is a key for which the given ciphertext comes from that plaintext.
This type of cipher has reportedly been used by spies, who were furnished with notebooks containing page after page of randomly generated key-sequence. Notice that it is essential that each key-sequence be used only once (hence the name of the system). Otherwise the approach for Vigenère systems described above could be tried, since we would have at least two rows to work with.
One-time pads seem practical in situations where one agent is communicating with a central command. They become less attractive if several agents may need to communicate with each other. The one-time feature is lost if X and Y inadvertently use the same page to talk as W and Z are using. Also capture of X's equipment makes it possible to overhear a conversation between Y and Z.