/*************************************************
* X9.42 PRF Source File                          *
* (C) 1999-2004 The Botan Project                *
*************************************************/

#include <botan/kdf.h>
#include <botan/der_enc.h>
#include <botan/oids.h>
#include <botan/lookup.h>
#include <memory>

namespace Botan {

namespace {

/*************************************************
* Encode an integer as an OCTET STRING           *
*************************************************/
MemoryVector<byte> encode_x942_int(u32bit n)
   {
   byte n_buf[4];
   for(u32bit j = 0; j != 4; j++)
      n_buf[j] = get_byte(j, n);

   DER_Encoder encoder;
   DER::encode(encoder, n_buf, 4, OCTET_STRING);
   return encoder.get_contents();
   }

}

/*************************************************
* X9.42 PRF                                      *
*************************************************/
OctetString X942_PRF::derive(u32bit key_len,
                            const byte secret[], u32bit secret_len,
                            const byte salt[], u32bit salt_len) const
   {
   std::auto_ptr<HashFunction> hash(get_hash("SHA-1"));
   const OID kek_algo(key_wrap_oid);

   SecureVector<byte> key;
   u32bit counter = 1;

   while(key.size() != key_len)
      {
      DER_Encoder encoder;
      encoder.start_sequence();
        encoder.start_sequence();
          DER::encode(encoder, kek_algo);
          encoder.add_raw_octets(encode_x942_int(counter));
        encoder.end_sequence();

        if(salt_len)
           {
           encoder.start_explicit(ASN1_Tag(0));
           DER::encode(encoder, salt, salt_len, OCTET_STRING);
           encoder.end_explicit(ASN1_Tag(0));
           }

        encoder.start_explicit(ASN1_Tag(2));
          encoder.add_raw_octets(encode_x942_int(8 * key_len));
        encoder.end_explicit(ASN1_Tag(2));
      encoder.end_sequence();

      hash->update(secret, secret_len);
      hash->update(encoder.get_contents());
      SecureVector<byte> digest = hash->final();
      key.append(digest, std::min(digest.size(), key_len - key.size()));

      counter++;
      }

   return key;
   }

/*************************************************
* X9.42 Constructor                              *
*************************************************/
X942_PRF::X942_PRF(const std::string& oid)
   {
   if(OIDS::have_oid(oid))
      key_wrap_oid = OIDS::lookup(oid).as_string();
   else
      key_wrap_oid = oid;
   }

}
