Question regarding `X509_PUBKEY` and ASN.1 SubjectPublicKeyInfo.

[Matt Caswell]

On 07/02/2024 10:43, Thomas Bailleux wrote:
> Hello OpenSSL,
> 
> I'm working on a program that generates pairs of keys and then writes 
> the**ASN.1 encoded SubjectPublicKeyInfo (RFC 5280[1]) to a file.
> 
> I turned to API `X509_PUBKEY`, and especially `X509_PUBKEY_set`[2], 
> which seems to do the trick by taking an `EVP_PKEY` object that contains 
> the public key:
> 
>     The X509_PUBKEY structure represents the ASN.1 SubjectPublicKeyInfo
>     structure defined in RFC5280 and used in certificates and
>     certificate requests.
> 
> 
> However, I'm kind of lost when it comes to encoding it to an ASN.1 document.
> The documentation states the following:
> 
>     i2d_PUBKEY() encodes an *EVP_PKEY* structure using
>     *SubjectPublicKeyInfo* format. 
> 
> 
> So it seems that I can use `i2d_PUBKEY` with an `EVP_PKEY` directly 
> without having to deal with an intermediate `X509_PUBKEY` object. 
> However, `i2d_X509_PUBKEY` also exists but it doesn't have a dedicated 
> manpage (only the generic manpage[3] about encoding/decoding from/to ASN.1).
> 
> My question is: which one should I use?

Either should be fine - but i2d_PUBKEY() is usually more convenient 
because it does not require you to go through the intermediate 
X509_PUBKEY structure.

Matt