shortest code to encrypt a string

BrainFuck: 21

>,[+<[->+>+<<]>.>+>,]

Assumes EOF=zero

echo -n 'My secret password is 123456!' | beef encrypt.bf
==> N{#wjiym}*{mv36HJLNPR>

C — 67 56 49 characters

During compilation it might warn that q is missing a type...

q;main(p){for(;p=~getchar();)putchar(q++%255-p);}

Thanks to Josh for the significantly shorter rewrites!

$ echo 'My secret password is 123456!' | ./_encrypt
N{#wjiym}*{m����v3}�6HJLNPR>(

Confirm it handles the rollover from 255 to 1 correctly by encrypting 343 xs and checking for no xs in the output:

$ echo xxxxxxx | sed -e 's/x/xxxxxxx/g' -e 's/x/xxxxxxx/g' | ./_encrypt | grep -o x | wc -l
0

Befunge 98 - 15 bytes

:~+'U3*%1+,1+#@

Keeps a counter (starting at 0) and adds that to the ~ input. Then, computes 255 (as 85 * 3 in the form of 'U3*), then mods (%). Then, it adds 1 and prints it, then adds one to the counter, then uses # to skip over the @. At the end of the input, the IP goes the other way and hits the @, ending the program.

Alternatively, if your interpreter supports unicode, this 14 byte (13 char) solution works:

:~+'ÿ%1+,1+#@

Sample run (in command prompt, so some of the unicode characters are actually some ascii value):

This string will be encrypted using the predetermined method!2.7182818284590452353602874713526624

Output:

Ujlw%y{zrxr,äw{|1tx4zäzèÆèÅüü>öôèÉèDÖÄîHÖ£ÉÉÆóöó₧¢íÖÖVñ¥¡ó¬á^pmwrzu|v~yÇ}⌂ä|üâüâåàëäçÄÄîÉïÄæÅöòÆò