Can I generate public key from modulus & exponent?

[shacha086]

In JVM, I do RSAPublicKeySpec(BigInteger(modulus), BigInteger(publicExponent)) but refers to document, I tried CryptographyProvider.Default.get(RSA.PKCS1).keyPairGenerator().generateKey() and there is no parameter to set modulus. Am I missing sth?

[whyoleg]

Hey! Thanks for raising the issue. That's true, at this moment, there is no API directly, but it's possible to do it using cryptography-serialization-asn1-modules dependency models.

E.g.:

fun createRsaPublicKey(
    modulus: BigInt,
    publicExponent: BigInt,
    digest: CryptographyAlgorithmId<Digest> = SHA256,
): RSA.PSS.PublicKey {
    // for DER encoding, coming from `cryptography-serialization-asn1-modules`
    val rsaPublicKey = RsaPublicKey(
        modulus = modulus,
        publicExponent = publicExponent,
    )
    return CryptographyProvider.Default.get(RSA.PSS).publicKeyDecoder(digest).decodeFromByteArrayBlocking(
        format = RSA.PublicKey.Format.DER.PKCS1,
        bytes = Der.encodeToByteArray(rsaPublicKey)
    )
}

However, it could be simplified, of course, so I would try to address this in the next release.