Function hmac_sha256

Libraries/unrar/crypt5.cpp:1–79

static void hmac_sha256(const byte *Key,size_t KeyLength,const byte *Data,
                        size_t DataLength,byte *ResDigest,
                        sha256_context *ICtxOpt,bool *SetIOpt,
                        sha256_context *RCtxOpt,bool *SetROpt)
{
  const size_t Sha256BlockSize=64; // As defined in RFC 4868.

  byte KeyHash[SHA256_DIGEST_SIZE];
  if (KeyLength > Sha256BlockSize) // Convert longer keys to key hash.
  {
    sha256_context KCtx;
    sha256_init(&KCtx);
    sha256_process(&KCtx, Key, KeyLength);
    sha256_done(&KCtx, KeyHash);

    Key = KeyHash;
    KeyLength = SHA256_DIGEST_SIZE;
  }

  byte KeyBuf[Sha256BlockSize]; // Store the padded key here.
  sha256_context ICtx;

  if (ICtxOpt!=NULL && *SetIOpt)
    ICtx=*ICtxOpt; // Use already calculated first block context.
  else
  {
    // This calculation is the same for all iterations with same password.
    // So for PBKDF2 we can calculate it only for first block and then reuse
    // to improve performance. 

    for (size_t I = 0; I < KeyLength; I++) // Use 0x36 padding for inner digest.
      KeyBuf[I] = Key[I] ^ 0x36;
    for (size_t I = KeyLength; I < Sha256BlockSize; I++)
      KeyBuf[I] = 0x36;

    sha256_init(&ICtx);
    sha256_process(&ICtx, KeyBuf, Sha256BlockSize); // Hash padded key.
  }

  if (ICtxOpt!=NULL && !*SetIOpt) // Store constant context for further reuse.
  {
    *ICtxOpt=ICtx;
    *SetIOpt=true;
  }

  sha256_process(&ICtx, Data, DataLength); // Hash data.

  byte IDig[SHA256_DIGEST_SIZE]; // Internal digest for padded key and data.
  sha256_done(&ICtx, IDig);

  sha256_context RCtx;

  if (RCtxOpt!=NULL && *SetROpt)
    RCtx=*RCtxOpt; // Use already calculated first block context.
  else
  {
    // This calculation is the same for all iterations with same password.
    // So for PBKDF2 we can calculate it only for first block and then reuse
    // to improve performance.