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The second encrypted message from the Zodiac Killer is one of the world’s most famous unsolved cryptograms. According to computational linguist Tom Juzek, it will never be solved because no solution exists.

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Murder victim Ricky McCormick left behind two encrypted notes. Both the crime and the cryptograms are unsolved to date.

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A recent TV documentary reported on a spy radio transmitter that was placed near an East German army facility. The transmission schedule of this device was computed with a method that is not publicly known. Can a reader reverse-engineer this algorithm?

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Today I am going to introduce two encrypted diaries. Only one of these is solved. Can a reader decipher the other one?

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Only three of the many encrypted postcards I have covered on this blog so far have remained unsolved. The one I am going to introduce today might be number four.

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US artist Andy Bauch hid messages in his Lego artworks. Blog reader Christian Baumann has now solved two of the codes.

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In 1934, a US magazine published an encrypted message a reader had found in an old document. The solution is not known to me.

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Two postcards from the early 20th century are written in Morse code or something similar. One of these cryptograms is still unsolved.

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A Facebook user has posted a number of scans of a 19th century pocket lexicon with about 80 pages of code in it. Can a reader solve it?

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The Museum für Kommunikation in Frankfurt owns a cipher tool from the 19th century. Not much is known about it. Can a reader find out more?