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The Amsco Cipher is a transpostion cipher. Choose a number of columns, then write the plaintext into the columns going from left to right, alternating between writing one or two plaintext letters into each successive column. Number the columns with a random numbering scheme, then write the ciphertext by going down each numbered column in order.
Plaintext: Whoever has made a voyage up the Hudson must remember the Kaatskill mountains.
3 5 1 4 2 w ho e ve r ha s ma d ea v oy a ge u pt h eh u ds o nm u st r em e mb e rt h ek a at s ki l lm o un t ai n s
Ciphertext: EMAAE HUMBA LMNRE AUDSR RTSUN WHAVP TOEMH KITVE DGEUS TEATO SHOSO YHNME EKLAI
As with any transposition cipher, the frequency count and monographic IC will look like that of plain English, unless the plaintext is chosen carefully.
By writing the ciphertext into a column where you alternate writing one letter and writing two letters below that, you can reconstruct the enciphering columns. However, without knowing how many columns were used and what order they were put in, you don't know where to end your column and start a new one with a different alternation pattern.