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RSA PKCS#1 v1.5 signature scheme verification incompatibility issue #10

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RSA PKCS#1 v1.5 signature scheme verification incompatibility issue #10

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15 changes: 13 additions & 2 deletions pkcs1/emsa_pkcs1_v15.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,11 +10,22 @@
hashlib.sha512: b'\x30\x51\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x03\x05\x00\x04\x40',
}

def encode(message, em_len, ps=None, hash_class=hashlib.sha1):
DIGEST_INFO_PREFIXES_IMP = {
hashlib.md5: b'\x30\x1e\x30\x0a\x06\x08\x2a\x86\x48\x86\xf7\x0d\x02\x05\x04\x10',
hashlib.sha1: b'\x30\x1f\x30\x07\x06\x05\x2b\x0e\x03\x02\x1a\x04\x14',
hashlib.sha256: b'\x30\x2f\x30\x0b\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x04\x20',
hashlib.sha384: b'\x30\x3f\x30\x0b\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x02\x04\x30',
hashlib.sha512: b'\x30\x4f\x30\x0b\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x03\x04\x40',
}

def encode(message, em_len, ps=None, hash_class=hashlib.sha1, explicit_null_param=True):
halgo = hash_class(message)
h = halgo.digest()
try:
t = DIGEST_INFO_PREFIXES[hash_class] + h
if explicit_null_param:
t = DIGEST_INFO_PREFIXES[hash_class] + h
else:
t = DIGEST_INFO_PREFIXES_IMP[hash_class] + h
except KeyError:
raise NotImplementedError('hash algorithm is unsupported', hash_class)
if em_len < len(t) + 11:
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4 changes: 3 additions & 1 deletion pkcs1/rsassa_pkcs1_v15.py
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,8 @@ def verify(public_key, message, signature, hash_class=hashlib.sha1):
try:
em_prime = emsa_pkcs1_v15.encode(message, public_key.byte_size,
hash_class=hash_class)
em_prime_imp = emsa_pkcs1_v15.encode(message, public_key.byte_size,
hash_class=hash_class, explicit_null_param=False)
except ValueError:
raise exceptions.RSAModulusTooShort
return primitives.constant_time_cmp(em, em_prime)
return primitives.constant_time_cmp(em, em_prime) or primitives.constant_time_cmp(em, em_prime_imp)
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@bmurray7 bmurray7 Jan 22, 2022

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Is this still constant time?

In an or statement, Python only evaluates the second condition if the first condition evaluates to True. See the Python specification for Boolean operations..

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@bdauvergne bdauvergne Jan 22, 2022

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You are right. But don't expect this lib to give you real constant time even with this change. It's a toy implementation, if you need a really hardened implementation of RSA, there are a lot around written in C or ASM.