On Mon, Aug 07, 2017, Daniel Andrade wrote: > Hi all, > > I'm writing functions to create a digest of a data buffer using > ECDSA+SHA256. I've been trying to work with only the high-level EVP > interface, and not use the low-level interfaces. > > My understanding is that the resulting digest is an ASN.1 structure. > I have to feed this digest to the Intel SGX runtime, which requires > a fixed-size input (they actually have two separates arrays, one for > X and one for Y, each with 32-byte length). > > Is it possible to get a fixed-sized digest using ECDSA/SHA256, I > mean does OpenSSL have functions for this? Or could someone give me > some pointers on how to proceed to convert that result to the > 64-byte fixed size? > The size will depend on the curve in use: presumably it's P-256 to produce 64 bytes. In outline you do this: 1. Decode the signature into an ECDSA_SIG structure using d2i_ECDSA_SIG. 2. Extract the two BIGNUM elements, r and s using ECDSA_SIG_get0(). 3. Generate padded encodings for r and s using BN_bn2binpad() or BN_bn2lebinpad() depending on the format required (big or little endian). Steve. -- Dr Stephen N. Henson. OpenSSL project core developer. Commercial tech support now available see: http://www.openssl.org -- openssl-users mailing list To unsubscribe: https://mta.openssl.org/mailman/listinfo/openssl-users
