add NULL parameter to AlgorithmIdentifier of encoded keys

[clenimar]

as per the RFC 8017 [1], the NULL parameter MUST be present if the OID in
AlgorithmIdentifier is rsaEncryption. this commit adds an explicit
NULL to the AlgorithmIdentifier so that RSA keys encoded by this crate
are compliant to the RFC:

The object identifier rsaEncryption identifies RSA public and private
keys as defined in Appendices A.1.1 and A.1.2. The parameters field
has associated with this OID in a value of type AlgorithmIdentifier
SHALL have a value of type NULL.

[1] https://tools.ietf.org/html/rfc8017#appendix-A

fixes #91

[randombit]

The public key should also have an explicit NULL included in the algorithm identifier (line ~335 of encode.rs)

Can be confirmed by openssl genrsa | openssl rsa -pubout | openssl asn1parse

[tarcieri]

Yes, it should. Note that this is correctly handled (or at east, easily handled correctly) by the pkcs8 crate.

I've also partially written (but have not yet published) a pkcs1 crate as well. Once I finish that up and release it I can look at migrating the rsa crate over to using it.

[clenimar]

Thanks, @randombit, I completely missed it. Now fixed.

@tarcieri: I didn't integrate to pkcs8 right away because it's currently not a dependency of this crate. If you feel strongly against using the simple fix proposed here, I could look into it. Thanks, great work!

[tarcieri]

The simple fix is fine and I approved this PR. @dignifiedquire generally maintains this crate though, so I was waiting for his sign-off.

[dignifiedquire]

can you rebase on master to fix ci please?

[dignifiedquire]

@clenimar did you verify these with openssl?

[tarcieri]

@dignifiedquire I haven't verified the encoded bytes, but this looks structurally the same as a PKCS#8-encoded RSA key generated by OpenSSL

https://github.com/RustCrypto/utils/blob/master/pkcs8/tests/private_key.rs#L81-L91

[est31]

@dignifiedquire I tried out this PR locally with the example from #95 and it works. So with this PR, ring can successfully import the key.

[clenimar]

@clenimar did you verify these with openssl?

@dignifiedquire yes, I did. Also been using this PR with ring, as @est31 mentioned. Rebased now.

Thanks!