[openssl-users] Padding for RSA signatures

[Gelareh Taban]

Hi all,

Any help would be *much* appreciated. I am playing around with RSA
signatures with different padding options and I have some questions.

I am trying to define different padding options and so am defining and
using a EVP_PKEY_CTX . However I am not sure if this padding is getting
used in the signature since  my Verify outputs OK regardless of which
option my Sign uses. Which leads to:

1 - Do I need to use a EVP_PKEY_CTX with the same options when doing
verify? If so, I assume I can't reuse the same PKey_Ctx and I have to
define another one. Right now even when I don't use any EVP_PKEY_CTX in
Verify, I still verify OK, which makes me question if the padding option
has been set.

2 - Is there a way to figure out what padding/hashing/etc option was used
for the Sign/verify operation? This way I can be sure what algorithm or
standard is being used.

3 - Do I need to set the hash function I am using in both EVP_PKEY_CTX  as
well as EVP_MD_CTX ? Or the latter is what defines this for the signing
option?

4 - In general, is there a way of making the Signature/Encryptions in
OpenSSL be deterministic for debugging/testing purposes?

5 - I noticed that there are two ways of determining the signature size:
(a) by calling EVP_PKEY_size(rsaKeypair) as I am doing below, as well as
(b) calling EVP_DigestSignFinal(md_ctx, nil, &sig_len) . Is one better than
the other?

My sample code is below for reference. It's in Swift (but it should still
be close enough to C to be readable). Also in Swift, some of the complex
macros in OpenSSL have to be broken down to be compilable hence my usage of
EVP_DigestUpdate instead of EVP_DigestVerifyUpdate .

Thanks in advance for any insight in the above.

cheers!
Gelareh


        let md_ctx = EVP_MD_CTX_create()

        let md_ctx_verify = EVP_MD_CTX_create()



        // To define padding option used in signature

        let pkey_ctx = EVP_PKEY_CTX_new(rsaKeypair, nil)



        // EVP_PKEY_CTX_set_rsa_padding(pkey_ctx, RSA_PKCS1_PADDING) -
complex macro needs to be replaced

        EVP_PKEY_CTX_ctrl(pkey_ctx, EVP_PKEY_RSA, -1,
EVP_PKEY_CTRL_RSA_PADDING, RSA_X931_PADDING, nil)


        // EVP_PKEY_CTX_set_signature_md() When should this be set?



        //  SIGN

        var rc = EVP_DigestSignInit(md_ctx, &pkey_ctx, EVP_sha256(), nil,
myRSA.rsaKeypair)

        // EVP_DigestSignUpdate(md_ctx, message, message.count)

        // Complex macro needs to be replaced

        rc = EVP_DigestUpdate(md_ctx, message, message.count)



        // allocate memory for signature

        var sig_len: Int = Int(EVP_PKEY_size(rsaKeypair))

        let sig = UnsafeMutablePointer<UInt8>.allocate(capacity: sig_len)


        rc = EVP_DigestSignFinal(md_ctx, sig, &sig_len)





        // VERIFY

        rc = EVP_DigestVerifyInit(md_ctx_verify, nil, EVP_sha256(), nil,
rsaKeypair)


        //        rc = EVP_DigestVerifyUpdate(md_ctx_verify, message,
message.count)

        rc = EVP_DigestUpdate(md_ctx_verify, message, message.count)



        rc = EVP_DigestVerifyFinal(md_ctx_verify, sig, sig_len)

        print("signature verified = \(rc == 1 ? "OK" : "FAIL")")
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