Question

[Update] I am offering a bonus for this. Frankly, I don’t care which encryption method is used. Preferably something simple like XTEA, RC4, BlowFish … but you chose.

I want minimum effort on my part, preferably just drop the files into my projects and build.

Idealy you should already have used the code to en/de-crypt a file in Delphi and C (I want to trade files between an Atmel UC3 micro-processor (coding in C) and a Windows PC (coding in Delphi) en-and-de-crypt in both directions).

I have a strong preference for a single .PAS unit and a single .C/.H file. I do not want to use a DLL or a library supporting dozens of encryption algorithms, just one (and I certainly don’t want anything with an install program).

I hope that I don’t sound too picky here, but I have been googling & trying code for over a week and still can’t find two implementations which match. I suspect that only someone who has already done this can help me …

Thanks in advance.

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As a follow up to my previous post, I am still looking for some very simple code with why I can – with minimal effort – en-de crypt a file and exchange it between Delphi on a PC and C on an Atmel UC3 u-processor.

It sounds simple in theory, but in practice it’s a nightmare. There are many possible candidates and I have spend days googling and trying them out – to no avail.

Some are humonous libraries, supporting many encryption algorithms, and I want something lightweight (especially on the C / u-processor end).

Some look good, but one set of source offers only block manipulation, the other strings (I would prefer whole file en/de-crypt).

Most seem to be very poorly documented, with meaningless parameter names and no example code to call the functions.

Over the past weekend (plus a few more days), I have burned my way through a slew of XTEA, XXTEA and BlowFish implementations, but while I can encrypt, I can’t reverse the process.

Now I am looking at AES-256. Dos anyone know of an implementation in C which is a single AES.C file? (plus AES.H, of course)

Frankly, I will take anything that will do whole file en/de-crypt between Delphi and C, but unless anyone has actually done this themselves, I expect to hear only “any implementation that meets the standard should do” – which is a nice theoory but just not working out for me 🙁

Any simple AES-256 in C out there? I have some reasonable looking Delphi code, but won’t be sure until I try them together.

Thanks in advance …

Answer 1

Here is RC4 code. It is very lightweight.

The C has been used in a production system for five years.

I have added lightly tested Delphi code. The Pascal is a line-by-line port with with unsigned char going to Byte. I have only run the Pascal in Free Pascal with Delphi option turned on, not Delphi itself. Both C and Pascal have simple file processors.

Scrambling the ciphertext gives the original cleartext back.

No bugs reported to date. Hope this solves your problem.

rc4.h

#ifndef RC4_H
#define RC4_H

/*
 * rc4.h -- Declarations for a simple rc4 encryption/decryption implementation.
 * The code was inspired by libtomcrypt.  See www.libtomcrypt.org.
 */
typedef struct TRC4State_s {
  int x, y;
  unsigned char buf[256];
} TRC4State;

/* rc4.c */
void init_rc4(TRC4State *state);
void setup_rc4(TRC4State *state, char *key, int keylen);
unsigned endecrypt_rc4(unsigned char *buf, unsigned len, TRC4State *state);

#endif

rc4.c

void init_rc4(TRC4State *state)
{
  int x;
  state->x = state->y = 0;
  for (x = 0; x < 256; x++)
    state->buf[x] = x;
}

void setup_rc4(TRC4State *state, char *key, int keylen)
{
  unsigned tmp;
  int x, y;

  // use only first 256 characters of key 
  if (keylen > 256) 
    keylen = 256;

  for (x = y = 0; x < 256; x++) {
    y = (y + state->buf[x] + key[x % keylen]) & 255;
    tmp = state->buf[x]; 
    state->buf[x] = state->buf[y]; 
    state->buf[y] = tmp;
  }
  state->x = 255;
  state->y = y;
}

unsigned endecrypt_rc4(unsigned char *buf, unsigned len, TRC4State *state)
{
  int x, y; 
  unsigned char *s, tmp;
  unsigned n;

  x = state->x;
  y = state->y;
  s = state->buf;
  n = len;
  while (n--) {
    x = (x + 1) & 255;
    y = (y + s[x]) & 255;
    tmp = s[x]; s[x] = s[y]; s[y] = tmp;
    tmp = (s[x] + s[y]) & 255;
    *buf++ ^= s[tmp];
  }
  state->x = x;
  state->y = y;
  return len;
}

int endecrypt_file(FILE *f_in, FILE *f_out, char *key)
{
  TRC4State state[1];
  unsigned char buf[4096];
  size_t n_read, n_written;

  init_rc4(state);
  setup_rc4(state, key, strlen(key));
  do {
    n_read = fread(buf, 1, sizeof buf, f_in);
    endecrypt_rc4(buf, n_read, state);
    n_written = fwrite(buf, 1, n_read, f_out);
  } while (n_read == sizeof buf && n_written == n_read);
  return (n_written == n_read) ? 0 : 1;
}

int endecrypt_file_at(char *f_in_name, char *f_out_name, char *key)
{
  int rtn;

  FILE *f_in = fopen(f_in_name, "rb");
  if (!f_in) {
    return 1;
  }
  FILE *f_out = fopen(f_out_name, "wb");
  if (!f_out) {
    close(f_in);
    return 2;
  }
  rtn = endecrypt_file(f_in, f_out, key);
  fclose(f_in);
  fclose(f_out);
  return rtn;
}

#ifdef TEST

// Simple test.
int main(void)
{
  char *key = "This is the key!";

  endecrypt_file_at("rc4.pas", "rc4-scrambled.c", key);
  endecrypt_file_at("rc4-scrambled.c", "rc4-unscrambled.c", key);
  return 0;
}
#endif

Here is lightly tested Pascal. I can scramble the source code in C and descramble it with the Pascal implementation just fine.

type
  RC4State = record
    x, y : Integer;
    buf : array[0..255] of Byte;
  end;

  KeyString = String[255];

procedure initRC4(var state : RC4State);
var
  x : Integer;
begin
  state.x := 0;
  state.y := 0;
  for x := 0 to 255 do
    state.buf[x] := Byte(x);
end;

procedure setupRC4(var state : RC4State; var key : KeyString);
var
  tmp : Byte;
  x, y : Integer;
begin
  y := 0;
  for x := 0 to 255 do begin
    y := (y + state.buf[x] + Integer(key[1 + x mod Length(key)])) and 255;
    tmp := state.buf[x];
    state.buf[x] := state.buf[y];
    state.buf[y] := tmp;
  end;
  state.x := 255;
  state.y := y;
end;

procedure endecryptRC4(var buf : array of Byte; len : Integer; var state : RC4State);
var
  x, y, i : Integer;
  tmp : Byte;
begin
  x := state.x;
  y := state.y;
  for i := 0 to len - 1 do begin
    x := (x + 1) and 255;
    y := (y + state.buf[x]) and 255;
    tmp := state.buf[x];
    state.buf[x] := state.buf[y];
    state.buf[y] := tmp;
    tmp := (state.buf[x] + state.buf[y]) and 255;
    buf[i] := buf[i] xor state.buf[tmp]
  end;
  state.x := x;
  state.y := y;
end;

procedure endecryptFile(var fIn, fOut : File; key : KeyString);
var
  nRead, nWritten : Longword;
  buf : array[0..4095] of Byte;
  state : RC4State;
begin
  initRC4(state);
  setupRC4(state, key);
  repeat
    BlockRead(fIN, buf, sizeof(buf), nRead);
    endecryptRC4(buf, nRead, state);
    BlockWrite(fOut, buf, nRead, nWritten);
  until (nRead <> sizeof(buf)) or (nRead <> nWritten);
end;

procedure endecryptFileAt(fInName, fOutName, key : String);
var
  fIn, fOut : File;
begin
  Assign(fIn, fInName);
  Assign(fOut, fOutName);
  Reset(fIn, 1);
  Rewrite(fOut, 1);
  endecryptFile(fIn, fOut, key);
  Close(fIn);
  Close(fOut);
end;

{$IFDEF TEST}
// Very small test.
const
  key = 'This is the key!';
begin
  endecryptFileAt('rc4.pas', 'rc4-scrambled.pas', key);
  endecryptFileAt('rc4-scrambled.pas', 'rc4-unscrambled.pas', key);
end.
{$ENDIF}