RSA - relation between message size and key size

[Karl Magdsick]

There are a variety of attacks against RSA when used in this manner.  You
really should use OAEP (
https://en.m.wikipedia.org/wiki/Optimal_asymmetric_encryption_padding ) and
you almost certainly should use RSA to exchange keys for a symmetric
authenticated encryption algorithm (such as ChaCha20-Poly1305 or AES-GCM).

It goes without saying that playing around with encryption is fun, but for
anything serious, use a high-level well-reviewed library implementing
well-studied protocols.  libgnutls, libgpgme, and libsodium are good
choices, depending on your use case.  libgcrypt is a low-level library
meant as a building block for high-level end-user libraries.


Cheers,
Karl

On Tue, Jun 26, 2018, 23:33 Steffen Bingel, pi4 <sbi at pi4.de> wrote:

> Hi,
>
> at first, this is the first time for me using a mailing list and I
> apologize in advance for any violation of rules I may not know yet.
>
> I'm playing around with the private/public key functions of libgcrypt
> and ran into an behavior I couldn't find an explanation for. If my
> message that I try to encrypt is larger than the key I use for
> encryption the pk_encrypt seems to generate random data without throwing
> an error. The following code is a condensed copy from
> https://github.com/vedantk/gcrypt-example/blob/master/main.cc. If my
> message contains 32 characters (256 bit) this works fine but if I pass
> 33 or more characters the decrypted messages makes no sense at all. I
> was also playing around with bigger keys where I could observe the same
> behavior (msg bigger than key not working).
>
> So if the function is not intended to take data larger than the key, why
> is it not returning an error?
>
> What is the correct way to encrypt large, at least larger than the key,
> binary data I have in memory?
>
> Thanks a lot
>
>      gcry_error_t err;
>
>      #define _assert(cmd) {\
>          err = cmd;\
>          if (err != GPG_ERR_NO_ERROR) {\
>              L("ERR: command returned: %s",gcry_strerror(err));\
>          }}
>
>      /* generate key pair */
>      gcry_sexp_t rsa_keypair;
>      gcry_sexp_t parms;
>      _assert(gcry_sexp_build( &parms, NULL, "(genkey(rsa(nbits
> %d)))",256));
>
>      _assert(gcry_pk_genkey( &rsa_keypair,parms ));
>
>      gcry_sexp_t pubk = gcry_sexp_find_token(rsa_keypair, "public-key", 0);
>      gcry_sexp_t privk = gcry_sexp_find_token(rsa_keypair,
> "private-key", 0);
>
>      /* Create a message. */
>      gcry_mpi_t msg;
>      gcry_sexp_t data;
>      const unsigned char* s = (const unsigned char*)
>          "uweoirdnd1iejfkslrm2kdleirjfm3xss";
>      _assert(gcry_mpi_scan(&msg, GCRYMPI_FMT_USG, s, strlen((const
> char*) s), NULL));
>
>      gcry_mpi_dump(msg);
>
>      _assert(gcry_sexp_build(&data, NULL,"(data (flags raw) (value
> %m))", msg));
>
>      gcry_sexp_dump(data);
>
>      /* Encrypt the message. */
>      gcry_sexp_t ciph;
>      _assert(gcry_pk_encrypt(&ciph, data, pubk));
>
>      gcry_sexp_dump(ciph);
>
>      /* Decrypt the message. */
>      gcry_sexp_t plain;
>      _assert(gcry_pk_decrypt(&plain, ciph, privk));
>
>      /* Pretty-print the results. */
>      gcry_mpi_t out_msg = gcry_sexp_nth_mpi(plain, 0, GCRYMPI_FMT_USG);
>      L("Original:");
>      gcry_mpi_dump(msg);
>      L("\n" "Decrypted:");
>      gcry_mpi_dump(out_msg);
>
>      if (gcry_mpi_cmp(msg, out_msg)) {
>          L("data corruption!");
>      } else {
>          L("Messages match.\n");
>      }
>
>
>
>
>
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