text: /**********************************************************************
   * Copyright (c) 2014, 2015 Pieter Wuille                             *
   * Distributed under the MIT software license, see the accompanying   *
   * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
   **********************************************************************/
  
  #include <string.h>
  #include <secp256k1.h>
  
  #include "lax_der_privatekey_parsing.h"
  
  int ec_privkey_import_der(const secp256k1_context* ctx, unsigned char *out32, const unsigned char *privkey, size_t privkeylen) {
      const unsigned char *end = privkey + privkeylen;
      int lenb = 0;
      int len = 0;
      memset(out32, 0, 32);
      /* sequence header */
      if (end < privkey+1 || *privkey != 0x30) {
          return 0;
      }
      privkey++;
      /* sequence length constructor */
      if (end < privkey+1 || !(*privkey & 0x80)) {
          return 0;
      }
      lenb = *privkey & ~0x80; privkey++;
      if (lenb < 1 || lenb > 2) {
          return 0;
      }
      if (end < privkey+lenb) {
          return 0;
      }
      /* sequence length */
      len = privkey[lenb-1] | (lenb > 1 ? privkey[lenb-2] << 8 : 0);
      privkey += lenb;
      if (end < privkey+len) {
          return 0;
      }
      /* sequence element 0: version number (=1) */
      if (end < privkey+3 || privkey[0] != 0x02 || privkey[1] != 0x01 || privkey[2] != 0x01) {
          return 0;
      }
      privkey += 3;
      /* sequence element 1: octet string, up to 32 bytes */
      if (end < privkey+2 || privkey[0] != 0x04 || privkey[1] > 0x20 || end < privkey+2+privkey[1]) {
          return 0;
      }
      memcpy(out32 + 32 - privkey[1], privkey + 2, privkey[1]);
      if (!secp256k1_ec_seckey_verify(ctx, out32)) {
          memset(out32, 0, 32);
          return 0;
      }
      return 1;
  }
  
  int ec_privkey_export_der(const secp256k1_context *ctx, unsigned char *privkey, size_t *privkeylen, const unsigned char *key32, int compressed) {
      secp256k1_pubkey pubkey;
      size_t pubkeylen = 0;
      if (!secp256k1_ec_pubkey_create(ctx, &pubkey, key32)) {
          *privkeylen = 0;
          return 0;
      }
      if (compressed) {
          static const unsigned char begin[] = {
              0x30,0x81,0xD3,0x02,0x01,0x01,0x04,0x20
          };
          static const unsigned char middle[] = {
              0xA0,0x81,0x85,0x30,0x81,0x82,0x02,0x01,0x01,0x30,0x2C,0x06,0x07,0x2A,0x86,0x48,
              0xCE,0x3D,0x01,0x01,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F,0x30,0x06,0x04,0x01,0x00,0x04,0x01,0x07,0x04,
              0x21,0x02,0x79,0xBE,0x66,0x7E,0xF9,0xDC,0xBB,0xAC,0x55,0xA0,0x62,0x95,0xCE,0x87,
              0x0B,0x07,0x02,0x9B,0xFC,0xDB,0x2D,0xCE,0x28,0xD9,0x59,0xF2,0x81,0x5B,0x16,0xF8,
              0x17,0x98,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFF,0xFF,0xFE,0xBA,0xAE,0xDC,0xE6,0xAF,0x48,0xA0,0x3B,0xBF,0xD2,0x5E,
              0x8C,0xD0,0x36,0x41,0x41,0x02,0x01,0x01,0xA1,0x24,0x03,0x22,0x00
          };
          unsigned char *ptr = privkey;
          memcpy(ptr, begin, sizeof(begin)); ptr += sizeof(begin);
          memcpy(ptr, key32, 32); ptr += 32;
          memcpy(ptr, middle, sizeof(middle)); ptr += sizeof(middle);
          pubkeylen = 33;
          secp256k1_ec_pubkey_serialize(ctx, ptr, &pubkeylen, &pubkey, SECP256K1_EC_COMPRESSED);
          ptr += pubkeylen;
          *privkeylen = ptr - privkey;
      } else {
          static const unsigned char begin[] = {
              0x30,0x82,0x01,0x13,0x02,0x01,0x01,0x04,0x20
          };
          static const unsigned char middle[] = {
              0xA0,0x81,0xA5,0x30,0x81,0xA2,0x02,0x01,0x01,0x30,0x2C,0x06,0x07,0x2A,0x86,0x48,
              0xCE,0x3D,0x01,0x01,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFE,0xFF,0xFF,0xFC,0x2F,0x30,0x06,0x04,0x01,0x00,0x04,0x01,0x07,0x04,
              0x41,0x04,0x79,0xBE,0x66,0x7E,0xF9,0xDC,0xBB,0xAC,0x55,0xA0,0x62,0x95,0xCE,0x87,
              0x0B,0x07,0x02,0x9B,0xFC,0xDB,0x2D,0xCE,0x28,0xD9,0x59,0xF2,0x81,0x5B,0x16,0xF8,
              0x17,0x98,0x48,0x3A,0xDA,0x77,0x26,0xA3,0xC4,0x65,0x5D,0xA4,0xFB,0xFC,0x0E,0x11,
              0x08,0xA8,0xFD,0x17,0xB4,0x48,0xA6,0x85,0x54,0x19,0x9C,0x47,0xD0,0x8F,0xFB,0x10,
              0xD4,0xB8,0x02,0x21,0x00,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
              0xFF,0xFF,0xFF,0xFF,0xFE,0xBA,0xAE,0xDC,0xE6,0xAF,0x48,0xA0,0x3B,0xBF,0xD2,0x5E,
              0x8C,0xD0,0x36,0x41,0x41,0x02,0x01,0x01,0xA1,0x44,0x03,0x42,0x00
          };
          unsigned char *ptr = privkey;
          memcpy(ptr, begin, sizeof(begin)); ptr += sizeof(begin);
          memcpy(ptr, key32, 32); ptr += 32;
          memcpy(ptr, middle, sizeof(middle)); ptr += sizeof(middle);
          pubkeylen = 65;
          secp256k1_ec_pubkey_serialize(ctx, ptr, &pubkeylen, &pubkey, SECP256K1_EC_UNCOMPRESSED);
          ptr += pubkeylen;
          *privkeylen = ptr - privkey;
      }
      return 1;
  }
  