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ECPrivateKey does not zero-pad private key, violates rfc5915

See original GitHub issue

Hi,

https://tools.ietf.org/html/rfc5915:

privateKey is the private key. It is an octet string of length ceiling (log2(n)/8) (where n is the order of the curve) obtained from the unsigned integer via the Integer-to-Octet-String- Primitive (I2OSP) defined in [RFC3447].

I.e. the encoded number must be padded with zeros. Best rationale I can think of is that an encrypted ec key would leak information (key size) otherwise.

asn1crypto uses IntegerOctetString which cannot be told the encoded width of the number. I.e. the number is not padded. IntegerOctetString is only used in ECPrivateKey.

Code that wants to generate a correct ECPrivateKey structure would need to pass the intended length of the key. I don’t see any way around this; asn1crypto cannot determine the width by any other means.

I’m not sure how the API should look like. (31337, 32)? Or {'value': 31337, 'width': 32}? Or add a new class method which takes value + width?

A breaking change (i.e. do not accept a plain int) might be acceptable; existing code is incorrect anyway.

.native should still return an int. Actual encoding (and its length) can be read with .contents.

Thanks!

Incorrect key (generated by asn1crypto):

Correct key (generated by openssl):

Code to generate above key:

from asn1crypto.keys import ECPointBitString, ECPrivateKey, NamedCurve, PrivateKeyInfo
from asn1crypto.pem import armor

key = PrivateKeyInfo.wrap(
    ECPrivateKey({
        'version': 1,
        'private_key': 31337,
        'parameters': NamedCurve('secp256r1'),
        'public_key': ECPointBitString.from_coords(
            0x2faab997dc7495a56a141fc767498ea0b063ae4a91bc4055453fed67e79f055f,
            0x39a20a2c5ab56087e243fea98e6afeb2855ea0922685eb6966f135ec480c5843,
        ),
    }),
    'ec'
)
print(armor('PRIVATE KEY', key.dump()).decode(), end="")