One-time pad encryption of ASCII characters I can't understand a question I came across in a course.


  You observe three ciphertexts all resulting from encryption of ASCII
  plaintexts containing English letters and spaces only, using the
  one-time pad and the same key. The 10th byte of the first ciphertext
  is 0xA8, the 10th byte of the second ciphertext is 0xED, and the 10th
  byte of the third ciphertext is 0xBD. What is the 10th ASCII character
  of the third plaintext?


I XORed 0xA8 and 0xBD and got 00010101
I XORed 0xED and OxBD and got 01010000
so clearly 0xED is a space and 0xBD is a character.

What I don't understand is why it is the letter 'p'.

Thank you in advance.
So you were really close. You are right that the 3rd ciphertext's 10th byte(0xED) in the 3rd message is a space. So we know 0x20 in hex is a space in ASCII. So now to find the 10th byte of the key you need to xor 0xED and 0x20 to get 0xCD(which is the 10th byte of the key). Well now that you know the key, you can xor the 10th byte of the 3rd ciphertext 0xBD with 0xCD. So you get 0x70 in hex and 'p' in ASCII.