bleichenbacher: daily C challenge, 22 October 2026

Your mission

Return the message length inside a padded block, or reject the block when the padding is not right.

today's brief

Crypto '98, Santa Barbara. Daniel Bleichenbacher shows that a server which answers bad padding differently from bad data will decrypt anything you like, one question at a time. About a million questions, hence the name. Nineteen years later ROBOT found the same hole still open in load balancers that were shipping at the time. Read the block before you trust what is inside it.

See it in action

message_length({2,9,9,9,9,9,9,9,9,0,65,66,67}, 13): 3
message_length({2,9,9,9,9,9,9,9,0,65}, 10): -1
message_length({2,9,9,9,9,9,9,9,9,0}, 10): 0

Your checklist

block holds len bytes, each between 0 and 255.
A well formed block starts with the byte 2, then eight or more non-zero bytes, then a single zero byte. Everything after that zero byte is the message.
Return how many bytes the message holds.
Return -1 when the block is not well formed.

Constraints

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