We found this mysterious text in the file of one of our employees

We found this mysterious text in the file of one of our employees and wish to discover whether it is anything we should be concerned with. We know that it uses two substitution ciphers. Even-numbered letters are encoded with one substitution cipher and odd-numbered letters are with another; spaces and punctuation are not counted.

znjs, uxwd bnfvx il e toaqlcu swzjp un keaer-haet alplr'k llde. w jedo uxl
blfk ph niqwcl, nkplskwqvv ph hhre nr mv cfraorpnf trhglcu. xprhvx, onwceqoux,
psx mprwer hwvn il eanfsx, ifp w brnb env hq uxlm prl hwvnwrz un mn pvhsm hwpj
hjeuodoa hl lh. w bwnv qrwsm pjo keaer-haet, djnb kt wi fhk berp un jedo p
apru!

• The single-letter words are easy: E, P, W. Those map to either A or I in at least one of the substitutions. – Engineer Toast May 1 '15 at 20:51

I believe there's a typo: cfraorpnf should be cfraorp. But, of course, such typos happen all the time in notes like this :)

Uppercasing every second letter:

zNjS, uXwD bNfVx Il E tOaQlCu SwZjP uN kEaEr-HaEt AlPlR'k LlDe. W jEdO uXl
BlFk Ph NiQwCl, NkPlSkWqVv Ph HhRe Nr Mv CfRaOrPnF tRhGlCu. XpRhVx, OnWcEqOuX,
pSx MpRwEr HwVn Il EaNfSx, IfP w BrNb EnV hQ uXlM pRl HwVnWrZ uN mN pVhSm HwPj
HjEuOdOa Hl Lh. W bWnV qRwSm PjO kEaEr-HaEt, DjNb Kt Wi FhK bErP uN jEdO p
ApRu!


As for whether they should be concerned,

no, they're planning an innocent office prank.

How I managed to solve it: Step/hint 1:

The word with apostrophe is either weren't or <something>'s.

Step/hint 2:

I began by guessing weren't, but then the rest (especially the likely appearence of the word "the") didn't seem to work out. Assuming <something>'s, the only letter known so far gave another clue.

Step/hint 3:

The word nkplskwqvv will decrypt to .s...s..... Which word could it be?

Step/hint 4:

I performed a grep -x .s...s.... /usr/share/dict/words which gave ostensibl[ey] as the only matches without too many s's.

Step/hint 5:

From here, the rest was quite easy guesswork.

And the decrypted message (every second letter uppercased) is:

jOhN, tHiS wOuLd Be A pErFeCt NiGhT tO sArAn-WrAp PeTeR's DeSk. I hAvE tHe KeYs To OfFiCe, OsTeNsIbLy To WoRk On My CuRrEnTlY pRoJeCt. HaRoLd, ElIsAbEtH, aNd MaRiAn WiLl Be ArOuNd, BuT i KnOw AlL oF tHeM aRe WiLlInG tO gO aLoNg WiTh WhAtEvEr We Do. I wIlL bRiNg ThE sArAn-WrAp, ShOw Up If YoU wAnT tO hAvE a PaRt!

Thanks for this puzzle, Joe L! I also learned two new English words (I'm not native speaker):

ostensibly and saran-wrap

John, this would be a perfect night to saran-wrap Peter's desk. I have the keys to office, ostensibly to work on my currently [sic] project. Harold, Elizabeth and Marian will be around, but I know all of them are willing to go along with whatever we do. I will bring the saran-wrap, show up if you want to have a part!

Method:

I first went through and colour-coded all the letters, to allow me to see which letters belonged to which cipher.

I put down all four letters that appeared as a single word as the two is and two as. I also realised that was going to be d, s, or t because it followed an apostrophe. However, this didn't get me very far, so I started writing down the two- and three-letter words to try and see if I could see any patterns.

I noticed that

appeared three times and appeared twice (and some of their letters appeared in other two-letter words)

which was promising but not conclusive. Then I noticed

and ! Almost certainly was the, with as then, them or they.

Once I'd fillen in those letters, I knew that

(the two letter word that appeared three times) was t? and took a guess that it was probably to.

I also looked at . I knew that

was a or i, and was e. There are only two words of the form i?e - ice and ire - and neither seemed that likely, so I took the reasonable guess that was a. also followed (my them/they/then word) so it seemed likely that was are (it could have been ate, but less likely).

If was a, (the other red single-word letter) had to be i. That left me with

thi? near the start. Only this and thin fit that pattern, and I thought given its placement (second word of the puzzle, after a comma), this was more likely.

After that I started making some bigger jumps.

I turned

I ??o? into I know

which gave me

this wo--- -e and w-n- to to turn into this would be and want to respectively

and then

I know a-l -- the- are

into

I know all of them are

etc. etc.

Eventually giving me the answer you see at the top.