1 Topic by ENOTTY (edited by ENOTTY 2021-07-20 11:14:54)

  • Registered: 2019-09-16
  • Posts: 7

Topic: Implementing rs256 in an SGX enclave

Hi all,

I'm trying to implement a rs256 signature (for JWTs) using WolfSSL 4.8.0 compiled for an SGX enclave environment. Broadly, the code I implemented will read an array containing a PEM-encoded RSA-4096 private key and convert it to a DER (using `wc_KeyPemToDer`), then decode that into an `RsaKey` object (using `wc_RsaPrivateKeyDecode`), initializes the RNG and a signature buffer, and finally calls `wc_SignatureGenerate`.

The call to `wc_SignatureGenerate` is failing with an error code of -112, which is `MP_EXPTMOD_E`. Recompiling WolfSSL with debug and -O0 and -g, I believe the error originates from `fp_exptmod` where it is checking for "modulus of zero and prevent overflows":

if (fp_iszero(P) || (P->used > (FP_SIZE/2))) {

I believe it is the second test that is failing `(P->used > (FP_SIZE/2))`. In my case, `P->used` is currently set to 64 and `FP_SIZE/2` seems to be equal to 36 (if I am reading the disassembly correctly).

I generated the key I'm testing with using: `openssl req -new -newkey rsa:4096 -nodes -keyout xxx.key -out xxx.csr`, converted to a C include header with xxd. Also happy to post my minimized test code.

Any advice as to debugging this would be greatly appreciated!

Thanks for reading!

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  • Registered: 2019-09-16
  • Posts: 7

Re: Implementing rs256 in an SGX enclave

Doing some further searching on the forums, am I running into the issue where the FP_MAX_BITS is not set high enough? By default WolfSSL only supports up to 2048-bit RSA keys (even on the SGX build), right? So to support 4096-bit, I would need to edit FP_MAX_BITS in tfm.h.

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  • Registered: 2015-10-07
  • Posts: 430

Re: Implementing rs256 in an SGX enclave

Hi ENOTTY,

The FP_MAX_BITS can be overridden as a build-time pre-processor macro and should be set to double the max RSA/DH key size. So for 4096-bit it should be `-DFP_MAX_BITS=8192` or `./configure CFLAGS="-DFP_MAX_BITS=8192` or `#define FP_MAX_BITS 8192`.

Thanks,
David Garske, wolfSSL

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  • Registered: 2019-09-16
  • Posts: 7

Re: Implementing rs256 in an SGX enclave

Thanks!

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