[openssl-users] Padding for RSA signatures

[Gelareh Taban]

Hi Matt,

Thanks so much for your replies. Got my signature verification to fail
which makes me happy :-)

The only remaining question is:

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

Any ideas or thoughts?

cheers,
Gelareh


On Fri, Dec 29, 2017 at 10:07 AM, Matt Caswell <matt at openssl.org> wrote:

> Some comments inserted below.
>
> Matt
>
> On 29/12/17 15:20, Gelareh Taban wrote:
> > 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.
>
> It hasn't. The call to EVP_DigestSignInit() expects an EVP_PKEY_CTX **.
> This is because that function creates its *own* EVP_PKEY_CTX * and
> stores it in the location you provide (if you give one). In your code
> the EVP_PKEY_CTX you are creating is being overwritten by the one
> created by EVP_DigestSignInit(). From the documentation:
>
>   EVP_DigestSignInit() sets up signing context ctx to use digest type
>   from ENGINE impl and private key pkey. ctx must be created with
>   EVP_MD_CTX_new() before calling this function. If pctx is not NULL the
>   EVP_PKEY_CTX of the signing operation will be written to *pctx: this
>   can be used to set alternative signing options.
>
> Try removing the creation of your own EVP_PKEY_CTX *, and moving the
> EVP_PKEY_CTX_set_rsa_padding() call to after EVP_DigestSignInit().
>
> >
> > 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?
>
> You only need to specify the hash function in
> EVP_DigestSignInit()/EVP_DigestVerifyInit().
>
> To answer your question in the code, there is no need to call
> EVP_PKEY_CTX_set_signature_md() directly in this scenario. It is called
> for you by EVP_DigestSignInit()/EVP_DigestVerifyInit().
>
> >
> > 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?
>
> The former gives you a maximum bound on the size of the signature before
> the signature is created. The latter gives you the *actual* size of the
> signature that is generated (which could be smaller).
>
>
> >
> > 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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