Extracting the public modulus from an RSA public key?

Dr Paul Dale paul.dale at oracle.com
Wed May 6 04:48:20 UTC 2020

Might I suggest reading the documentation?

RSA_get0_n() is the function you are wanting.

Dr Paul Dale | Distinguished Architect | Cryptographic Foundations 
Phone +61 7 3031 7217
Oracle Australia

> On 6 May 2020, at 2:20 pm, Thomas Dwyer III <tomiii at tomiii.com> wrote:
> I'm porting some old legacy code from OpenSSL 1.0.2 to OpenSSL 3.0.0. A portion of this code reads X509 certificates, extracts the public key, and passes it to firmware that I cannot modify. Unfortunately, this legacy firmware API was very poorly designed such that the public key is passed in a way similar to:
>     RSA *rsa = get_pubkey_from_cert(...)
>     BIGNUM *bn = rsa->n;
>     int len = BN_num_bytes(bn);
>     unsigned char *buf = malloc(len);
>     BN_bn2bin(bn, buf);
>     pubkey_to_firmware(buf, len);
> Yuck. Ignoring the fact that this firmware appears to assume a constant exponent 'e', I cannot find a way to extract the modulus 'n' from the RSA key. I understand this is intentional. The only solution I could find is to print the key to a buffer via EVP_PKEY_print_public(), parse the result to extract the modulus into a giant hex string, and then BN_hex2bn() that back into a BIGNUM. Is there a better way?
> Thanks,
> Tom.III

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