Extract an Public RSA KEY from the Private one ?

[emoiroux]

Could you show me how to extract a public RSA key from a private key? I looked but couldn’t find the steps.
Thanks !

[whyoleg]

Unfortunately, at this point, there is no API for that. But, for RSA, you can do it using cryptography-serialization-asn1-modules models. It could look like this:

private fun getPrivateKey(
    privateKey: RSA.PSS.PrivateKey,
    digest: CryptographyAlgorithmId<Digest>,
): RSA.PSS.PublicKey {
    val decoder = CryptographyProvider.Default.get(RSA.PSS).publicKeyDecoder(digest)
    val rsaPrivateKey = Der.decodeFromByteArray<RsaPrivateKey>(
        privateKey.encodeToByteArrayBlocking(RSA.PrivateKey.Format.DER.PKCS1)
    )
    val rsaPublicKey = RsaPublicKey(
        modulus = rsaPrivateKey.modulus,
        publicExponent = rsaPrivateKey.publicExponent,
    )
    return decoder.decodeFromByteArrayBlocking(
        format = RSA.PublicKey.Format.DER.PKCS1,
        bytes = Der.encodeToByteArray(rsaPublicKey)
    )
}

I believe it should be possible to support this in the library itself. I've added it to my shortlist.

[emoiroux]

Great work just fine thanks !!!