beast: daily C challenge, 25 October 2026

Your mission

Fill out with the chained blocks, each one folded into the value before it.

today's brief

Buenos Aires, September 2011. Thai Duong and Juliano Rizzo run BEAST against a live PayPal session on stage and pull the cookie out one byte at a time. TLS 1.0 chained its blocks straight across records, so the starting value for your next record was the last block of the one an eavesdropper had already watched go past. TLS 1.1 had fixed that in 2006 and almost nobody had switched. Build the chain and see it.

See it in action

chain(out, {1,1,1}, 3, 0): 1 2 3
chain(out, {200,200}, 2, 100): 44 244
chain(out, {0,0,0}, 3, 5): 5 5 5

Your checklist

blocks holds n values, each between 0 and 255, and out has room for n values.
out[0] is blocks[0] plus iv, taken modulo 256.
Every later out[i] is blocks[i] plus out[i - 1], taken modulo 256.
iv is between 0 and 255. When n is 0 there is nothing to write.

Constraints

Expected file: chain.c
Allowed functions: None. Write every helper yourself.