Alice generates a million cryptograms, each of a different type. She sends them to Bob with instructions.
Bob picks one and solves it, i.e. finds the key. He uses the key to encrypt the message "secret key test" and sends it to Alice.
Alice checks her million keys and finds the one which Bob used. Now Bob and Alice have agreed on a secret key which they can use to send secret messages.
If Eve is listening to the conversation, she has to solve all the million puzzles in order to figure out which key Bob used.
Cryptogram I: every letter has been shifted forward by k places, where k is between 1 and 3.
VJKU KU ETARVQITCO PWODGT QPG.
Cryptogram II: every letter has been shifted forward by k places, where k is between 4 and 6.
XLMW MW XLI WIGSRH TYDDPI.
Cryptogram III: every letter has been shifted forward by k places, where k is between 7 and 9.
CQRB RB CQN CQRAM VNBBJPN.
Cryptogram IV: every other letter, starting with the second, has been shifted forward by k places, where k is between 1 and 3.
IT TIIT TIE GOVRUH NETSBGF?
Cryptogram V: every other letter, starting with the second, has been shifted forward by k places, where k is between 4 and 6.
FNVJ IX TME SURBJR TF YHNS REXSFGJ.
Cryptogram VI: every other letter, starting with the second, has been shifted forward by k places,where k is between 7 and 9.
TPE UEASIGM YWU IRM RMALIVG QS VUUBMR AIF.
Cryptogram VII: every other letter, starting with the first, has been shifted forward by k places, where k is between 1 and 3
WHHRH IV A FRBPWOJRDM QUPBHRHD VEYEQ HHRH.
Cryptogram VIII: every other letter, starting with the first, has been shifted forward by k places, where k is between 4 and 6.
OF EOA CGN XEGD ZHOS, OT OS IREPZOMRGM KIMHZ.
Cryptogram IX: every other letter, starting with the first, has been shifted forward by k places,where k is between 7 and 9.
AHL LHSA CYYWTVGYAT IZ NBMIEYEK NPNL.
I: UGETGV MGA VGUV
II: WIGVIX OIC XIWX
III: BNLANC TNH CNBC
IV: SFCSEU KFY UETT
V: SJCWEY KJY YEXT
VI: SMCZEB KMY BEAT
VII: VEFRHT NEB THSW
VIII: YEIRKT QEE TKSZ
IX: ZEJRLT REF TLSA