Convert raw format ecdsa sha384 csr signature into ASN1 format

Question

I have a 96 bytes long ecdsa signature created with sha384 algorithm by a smart card in raw format.It is composed of two 48 bytes long integers r and s. The ecdsa signature is in a buffer pointed to by sign_ptr. I'm converting the raw format signature into buf_out in ASN1 format with this function (in C):

int convert_ecdsa_sha384_sign(char **buf_out, char *sign_ptr)
{
   buf_out[0]=0x30;                            /* Type = Sequence of */
   buf_out[2]=0x02;                            /* Type = Integer */
   /* Verify if negative bit is set */
   if (!(sign_ptr[0] & 0x80))
   {
       buf_out[3]=0x30;                        /* Length */
       memcpy(&(buf_out[4]), sign_ptr, 48);    /* Copy first integer */
   }
   else
   {
       /* Negative bit is set. Add one padding byte */
       buf_out[3]=0x31;                        /* Length */
       buf_out[4]=0x00;                        /* Padding */
       memcpy(&(buf_out[5]), sign_ptr, 48);    /* Copy first integer */
       sign_offset += 1;
   }

   buf_out[52+sign_offset]=0x02;                                      /* Type = Integer */
   /* Verify if negative bit is set */
   if (!(sign_ptr[48] & 0x80))
   {
       buf_out[53+sign_offset]=0x30;                                  /* Length */
       memcpy(((&(buf_out[54]))+ sign_offset), sign_ptr + 48, 48);    /* Copy second integer */
   }
   else
   {
       /* Negative bit is set. Add one padding byte */
       buf_out[53+sign_offset]=0x31;                                 /* Length */
       buf_out[54+sign_offset]=0x00;                                 /* Padding */
       memcpy(((&(buf_out[55]))+ sign_offset), sign_ptr + 48, 48);   /* Copy second integer */
       sign_offset += 1;
   }
   buf_out[1]= 100 + sign_offset;                                    /* Total signature length */
   return 1;

}

I am wondering if there is an equivalent openssl function that can help me to do this in a more elegant way? I did look at many d2i functions (d2i_ASN1_xxxx, ASN1_item_d2i, ASN_d2i_func, etc.) but it is not clear which one suits.

Answer 1

Here is the answer I got from Dr Stephen N. Henson. OpenSSL project core developer in openssl-users forum:

"The structure ECDSA_SIG is the one you need.

In outline: allocate the structure using ECDSA_SIG_new, set the r and s values using BN_bin2bn, encode the result with i2d_ECDSA_SIG and finally free with ECDSA_SIG_free".