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generalize next_iv comment

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This commit is contained in:
Thomas Waldmann 2016-08-29 19:18:46 +02:00
parent ce5c5781aa
commit ca4fc2a222
1 changed files with 3 additions and 2 deletions
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5
src/borg/crypto/low_level.pyx
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@ -476,11 +476,12 @@ cdef class _AEAD_BASE:
self.iv[i] = iv[i]
def next_iv(self):
# AES-GCM, AES-OCB, CHACHA20 ciphers all add a internal 32bit counter to the 96bit
# (12 byte) IV we provide, thus we only need to increment the IV by 1 (and we must
# not encrypt more than 2^32 cipher blocks with same IV):
assert self.blocks < 2**32
# we need 16 bytes for increment_iv:
last_iv = b'\0' * (16 - self.iv_len) + self.iv[:self.iv_len]
# gcm mode is special: it appends a internal 32bit counter to the 96bit (12 byte) we provide, thus we only
# need to increment the 96bit counter by 1 (and we must not encrypt more than 2^32 AES blocks with same IV):
next_iv = increment_iv(last_iv, 1)
return next_iv[-self.iv_len:]