# Attacks on Biased OTP

I know that a perfectly random unbiased and not reused OTP cipher is perfect. An OTP under cryptanalysis has a very strong bias (up to 30 % of symbols are represented by 5 out of 255 possible symbols) but no repetition could be found. This OTP encryption is theoretically vulnerable. May I know the possible sequence of steps to exploite the vulnerability?

• Just out of interest - where did you get those numbers from? Is it hypothetical, or is there some real OTP that you're investigating? – Paul Uszak Jun 11 '17 at 21:14
• A hypothetical setup – Uraguan Jun 12 '17 at 0:09

More generally, given sufficient bias in the pad and sufficient redundancy in the plaintext, you may be able to simply guess which characters of the ciphertext are likely to be encrypted with the most common pad values, and then fill in the missing characters. For example, the ciphertext below encodes a common English pangram, encrypted with a standard mod 26 one-time pad, biased so that a bit over 50% of all the pad characters are A (effectively leaving about half of the plaintext characters unchanged).
EHUICVDPTWNFOXCUAGSOYERTHRLAZEJOG