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Blinding uses slow algorithm #162

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tomato42 opened this issue Oct 18, 2020 · 0 comments
Closed

Blinding uses slow algorithm #162

tomato42 opened this issue Oct 18, 2020 · 0 comments

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@tomato42
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@tomato42 tomato42 commented Oct 18, 2020

The private key operations generate a new blinding factor for every new signature:

python-rsa/rsa/key.py

Lines 440 to 453 in 4beb68d

def blinded_encrypt(self, message: int) -> int:
"""Encrypts the message using blinding to prevent side-channel attacks.
:param message: the message to encrypt
:type message: int
:returns: the encrypted message
:rtype: int
"""
blind_r = self._get_blinding_factor()
blinded = self.blind(message, blind_r) # blind before encrypting
encrypted = rsa.core.encrypt_int(blinded, self.d, self.n)
return self.unblind(encrypted, blind_r)

Given that blinding requires calculating inverse modulo of of the blinding factor, it is a slow operation (rule of thumb is that inverse is as costly as calculating 100 modulo multiplications).

The existing literature[1] (section 9) suggests generating a blinding factor, its inverse, and then squaring both of them for next operation.

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