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Revert to digest sized salts in rsassa-pss signing #436

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Oct 14, 2022
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Revert to digest sized salts in rsassa-pss signing #436

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dd42718
Revert to digest sized salts in rsassa-pss signing
lukpueh Oct 13, 2022
de3d1e5
Remove unused import
lukpueh Oct 13, 2022
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5 changes: 2 additions & 3 deletions securesystemslib/rsa_keys.py
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Original file line number Diff line number Diff line change
Expand Up @@ -326,11 +326,10 @@ def create_rsa_signature(private_key, data, scheme='rsassa-pss-sha256'):
if scheme.startswith('rsassa-pss'):
# Generate an RSSA-PSS signature. Raise
# 'securesystemslib.exceptions.CryptoError' for any of the expected
# exceptions raised by pyca/cryptography. 'salt_length' is set to
# the maximum length available.
# exceptions raised by pyca/cryptography.
signature = private_key_object.sign(
data, padding.PSS(mgf=padding.MGF1(digest_obj.algorithm),
salt_length=padding.PSS.MAX_LENGTH), digest_obj.algorithm)
salt_length=padding.PSS.DIGEST_LENGTH), digest_obj.algorithm)
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The original code had digest_obj.algorithm.digest_size instead, is there a difference here?

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See PR description / commit mesesage:

Note that we now use the padding.PSS.DIGEST_LENGTH constant instead of passing the actual digest length, as we did before. Using the constant has the same result, but is recommended by the library documentation.

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I locally created two signature one with the constant and one with the digest length and can confirm that they do the same thing.

I also checked the code.

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https://github.com/pyca/cryptography/blob/25d794e079b44e6fac23791ca51da3a3fb9cb526/src/cryptography/hazmat/backends/openssl/rsa.py#L53-L54

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@trishankatdatadog trishankatdatadog Oct 13, 2022 •
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Right: I think padding.PSS.DIGEST_LENGTH is simply an internal cryptography constant that does look like it's used to end up using digest_obj.algorithm.digest_size in the end. This flag is also much easier to understand.

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Thanks! Oof my bad I didn't read the description first 🤦


elif scheme.startswith('rsa-pkcs1v15'):
# Generate an RSA-PKCS1v15 signature. Raise
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26 changes: 15 additions & 11 deletions tests/test_rsa_keys.py
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Expand Up @@ -157,26 +157,30 @@ def test_verify_rsa_signature(self):
scheme, public_rsa, data))


def test_verify_rsa_pss_sha256(self):
def test_verify_rsa_pss_different_salt_lengths(self):
rsa_scheme = 'rsassa-pss-sha256'
data = 'The ancients say the longer the salt, the more provable the security'.encode('utf-8')
data = 'The ancients say, salt length does not matter that much'.encode('utf-8')
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Mathematically speaking, apparently this is not strictly true, although practically it might be the case 😉

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Yeah, I would also not trust every word the ancients say. ;)


private_key = load_pem_private_key(private_rsa.encode('utf-8'),
password=None, backend=default_backend())
digest = securesystemslib.hash.digest_from_rsa_scheme(rsa_scheme, 'pyca_crypto')

# Old-style signature: use the hash length as the salt length.
old_signature = private_key.sign(data,
padding.PSS(mgf=padding.MGF1(digest.algorithm), salt_length=padding.PSS.DIGEST_LENGTH),
digest.algorithm)
# Make sure digest size and max salt length are not accidentally the same
self.assertNotEqual(digest.algorithm.digest_size,
padding.calculate_max_pss_salt_length(private_key, digest.algorithm))

# New-style signature: use the automatic salt length.
new_signature, _ = securesystemslib.rsa_keys.create_rsa_signature(private_rsa, data)
# Sign with max salt length (briefly available in sslib v0.24.0):
max_salt_sig = private_key.sign(data,
padding.PSS(mgf=padding.MGF1(digest.algorithm), salt_length=padding.PSS.MAX_LENGTH),
digest.algorithm)

# Verify both old-style and new-style signatures.
for signature in (old_signature, new_signature):
# Sign with salt length == digest length
fix_salt_sig, _ = securesystemslib.rsa_keys.create_rsa_signature(private_rsa, data)

# Verification infers salt length automatically and so works for both
for signature in (max_salt_sig, fix_salt_sig):
verified = securesystemslib.rsa_keys.verify_rsa_signature(signature, rsa_scheme,
public_rsa, data)
public_rsa, data)
self.assertTrue(verified)


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