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The decryption code is PKCS#1 v1.5 non-compliant - no padding length check #164

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tomato42 opened this issue Oct 26, 2020 · 1 comment
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@tomato42
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@tomato42 tomato42 commented Oct 26, 2020

the current code:

python-rsa/rsa/pkcs1.py

Lines 246 to 267 in 6f59ff0

blocksize = common.byte_size(priv_key.n)
encrypted = transform.bytes2int(crypto)
decrypted = priv_key.blinded_decrypt(encrypted)
cleartext = transform.int2bytes(decrypted, blocksize)
# Detect leading zeroes in the crypto. These are not reflected in the
# encrypted value (as leading zeroes do not influence the value of an
# integer). This fixes CVE-2020-13757.
if len(crypto) > blocksize:
raise DecryptionError('Decryption failed')
# If we can't find the cleartext marker, decryption failed.
if cleartext[0:2] != b'\x00\x02':
raise DecryptionError('Decryption failed')
# Find the 00 separator between the padding and the message
try:
sep_idx = cleartext.index(b'\x00', 2)
except ValueError:
raise DecryptionError('Decryption failed')
return cleartext[sep_idx + 1:]

doesn't check that the length of the padding is at least 8 bytes.

See https://tools.ietf.org/html/rfc8017#section-7.2.2 step 3

@sybrenstuvel
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@sybrenstuvel sybrenstuvel commented Nov 11, 2020

Thanks for the report. This is fortunately easy enough to fix after #165 (comment) is implemented. Changing the line

sep_idx_bad = sep_idx < 0

to

sep_idx_bad = sep_idx < 10

should do the trick.

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