DCPcrypt v2: Users Guide - Ciphers

DCPcrypt Cryptographic Component Library v2
Copyright © 1999-2002 David Barton
http://www.cityinthesky.co.uk/
crypto@cityinthesky.co.uk

Ciphers - TDCP_cipher

All ciphers are inherited from the TDCP_cipher component either directly for stream ciphers (such as RC4) or via the TDCP_blockcipher component.

The TDCP_cipher component implements key initialisation features and the basic encryption/decryption interface. Functions available are:

          property Initialized: boolean;
          property Id: integer;
          property Algorithm: string;
          property MaxKeySize: integer;

          class function SelfTest: boolean;

          procedure Init(const Key; Size: longword; InitVector: pointer);
          procedure InitStr(const Key: string; HashType: TDCP_hashclass);
          procedure Burn;
          procedure Reset;
          procedure Encrypt(const Indata; var Outdata; Size: longword);
          procedure Decrypt(const Indata; var Outdata; Size: longword);
          function EncryptStream(InStream, OutStream: TStream; Size: longword): longword;
          function DecryptStream(InStream, OutStream: TStream; Size: longword): longword;
          function EncryptString(const Str: string): string;
          function DecryptString(const Str: string): string;

Example usage:

- Example 1 - String encryption.
- Example 2 - File encryption.
- Example 3 - General encryption.

Function descriptions

property Initialized: boolean;

Once key initialization has been performed this property is set to true, otherwise it is set to false. Calling Burn will immediately set this to false.

property Id: integer;

Every algorithm I implement gets given a unique ID number so that if I use several different algorithms within a program I can determine which one was used. This is a purely arbitrary numbering system.

property Algorithm: string;

This contains the name of the algorithm implemented within the component.

property MaxKeySize: integer;

This is the maximum size of key you can pass to the cipher (in bits!).

class function SelfTest: boolean;

In order to test whether the implementations have all been compiled correctly you can call the SelfTest function. This compares the results of several encryption/decryption operations with known results for the algorithms (so called test vectors). If all the tests are passed then true is returned. If ANY of the tests are failed then false is returned. You may want to run this function for all the components when you first install the DCPcrypt package and again if you modify any of the source files, you don't need to run this everytime your program is run. Note: this only performs a selection of tests, it is not exhaustive.

procedure Init(const Key; Size: longword; InitVector: pointer);

This procedure initializes the cipher with the keying material supplied in Key. The Size of the keying material is specified in BITS. The InitVector is a pointer to chaining information (only used for block ciphers). The variable that this points to should be equal to the block size of the algorithm. If nil is specified then (if necessary) an initialization vector is automatically generated from the key. Note: the method for generating automatic IVs is different from DCPcrypt v1.31, if this is a problem uncomment the DCPcrypt v1.31 compatibility mode line in DCPcrypt2.pas.

Init example: use the hash of a string to initialize the cipher

      procedure TForm1.Button1Click(Sender: TObject);
      var
        Cipher: TDCP_rc4;
        Hash: TDCP_sha1;
        Digest: array[0..19] of byte;  // SHA-1 produces a 160bit (20byte) output
      begin
        Hash:= TDCP_sha1.Create(Self);
        Hash.Init;                     // initialize the hash
        Hash.UpdateStr(Edit1.Text);    // generate a hash of Edit1.Text
        Hash.Final(Digest);            // save the hash in Digest
        Hash.Free;
        Cipher:= TDCP_rc4.Create(Self);
        Cipher.Init(Digest,Sizeof(Digest)*8,nil);  // remember size is in BITS (hence sizeof*8)
        ...

procedure InitStr(const Key: string; HashType: TDCP_hashclass);

This procedure initializes the cipher with a hash of the key string using the specified hash type (in a way similar to the example above). To replicate the behaviour from DCPcrypt v2 Beta 1 use Cipher.InitStr(KeyStr,TDCP_sha1).

InitStr example: prompt the user for a passphrase to initialize the cipher

      procedure TForm1.Button1Click(Sender: TObject);
      var
        Cipher: TDCP_rc4;
      begin
        Cipher:= TDCP_rc4.Create(Self);
        Cipher.InitStr(InputBox('Passphrase','Enter a passphrase',''),TDCP_sha1); // prompt for a passphrase
        ...

procedure Burn;

Once you have finished encrypting/decrypting all your data call Burn to erase all keying information. This is automatically called once the cipher is freed, however it is a good habit to call this procedure explicitly.

procedure Reset;

Stream ciphers (and block ciphers in chaining modes) generally store chaining information that is dependant on the information already encrypted. Consequently decrypting a block of information immediately after encrypting it won't result in the original information because when you called the decrypt procedure the chaining information was different from when you called the encrypt procedure. Hence use Reset to restore the chaining information to it's original state.

Remember that calling EncryptString, DecryptString, EncryptStream and DecryptStream will also affect the chaining information.

Reset example: encrypting and decrypting

      function TestCipher: boolean;
      const
        InData: array[0..9] of byte= ($01,$23,$45,$56,$67,$78,$89,$10,$AB,$FF);
      var
        Cipher: TDCP_rc4;
        Data: array[0..9] of byte;
      begin
        Cipher:= TDCP_rc4.Create(nil);
        Cipher.InitStr('Hello World',TDCP_sha1);   // initialize the cipher
        Cipher.Encrypt(InData,Data,Sizeof(Data));  // encrypt some known data
        Cipher.Decrypt(Data,Data,Sizeof(Data));    // now decrypt it
        Cipher.Burn;                               // clear keying information
        Cipher.Free;
        Result:= CompareMem(@InData,@Data,Sizeof(Data));  // compare input and output
      end;

The above will ALWAYS result in false due to the chaining information.

      function TestCipher: boolean;
      const
        InData: array[0..9] of byte= ($01,$23,$45,$56,$67,$78,$89,$10,$AB,$FF);
      var
        Cipher: TDCP_rc4;
        Data: array[0..9] of byte;
      begin
        Cipher:= TDCP_rc4.Create(nil);
        Cipher.InitStr('Hello World',TDCP_sha1);   // initialize the cipher
        Cipher.Encrypt(InData,Data,Sizeof(Data));  // encrypt some known data
        Cipher.Reset;                              // reset chaining information
        Cipher.Decrypt(Data,Data,Sizeof(Data));    // now decrypt it
        Cipher.Burn;                               // clear keying information
        Cipher.Free;
        Result:= CompareMem(@InData,@Data,Sizeof(Data));  // compare input and output
      end;

The above should always return true.

procedure Encrypt(const Indata; var Outdata; Size: longword);

Encrypt Size bytes from Indata and place it in Outdata. Block ciphers encrypt the data using the method specified by the CipherMode property. Also see the notes on Reset.

procedure Decrypt(const Indata; var Outdata; Size: longword);

Decrypt Size bytes from Indata and place it in Outdata. Block ciphers decrypt the data using the method specified by the CipherMode property. Also see the notes on Reset.

function EncryptStream(InStream, OutStream: TStream; Size: longword): longword;

Encrypt Size bytes from the InStream and place it in the OutStream, returns the number of bytes read from the InStream. Encryption is done by calling the Encrypt procedure. Also see the notes on Reset.

function DecryptStream(InStream, OutStream: TStream; Size: longword): longword;

Decrypt Size bytes from the InStream and place it in the OutStream, returns the number of bytes read from the InStream. Decryption is done by calling the Decrypt procedure. Also see the notes on Reset.

function EncryptString(const Str: string): string;

Encrypt the string Str then Base64 encode it and return the result. For stream ciphers the Encrypt procedure is called to do the encryption, for block ciphers the CFB8bit method is always used. Base64 encoding is used to ensure that the output string doesn't contain non-printing characters.

function DecryptString(const Str: string): string;

Base64 decode the string then decrypt it and return the result. For stream ciphers the Decrypt procedure is called to do the decryption, for block ciphers the CFB8bit method is always used.

Example 1: String encryption

This example shows how you can encrypt the contents of a TMemo and leave the contents printable.

      procedure TForm1.btnEncryptClick(Sender: TObject);
      var
        i: integer;
        Cipher: TDCP_rc4;
        KeyStr: string;
      begin
        KeyStr:= '';
        if InputQuery('Passphrase','Enter passphrase',KeyStr) then  // get the passphrase
        begin
          Cipher:= TDCP_rc4.Create(Self);
          Cipher.InitStr(KeyStr,TDCP_sha1);         // initialize the cipher with a hash of the passphrase
          for i:= 0 to Memo1.Lines.Count-1 do       // encrypt the contents of the memo
            Memo1.Lines[i]:= Cipher.EncryptString(Memo1.Lines[i]);
          Cipher.Burn;
          Cipher.Free;
        end;
      end;

      procedure TForm1.btnDecryptClick(Sender: TObject);
      var
        i: integer;
        Cipher: TDCP_rc4;
        KeyStr: string;
      begin
        KeyStr:= '';
        if InputQuery('Passphrase','Enter passphrase',KeyStr) then  // get the passphrase
        begin
          Cipher:= TDCP_rc4.Create(Self);
          Cipher.InitStr(KeyStr,TDCP_sha1);         // initialize the cipher with a hash of the passphrase
          for i:= 0 to Memo1.Lines.Count-1 do       // decrypt the contents of the memo
            Memo1.Lines[i]:= Cipher.DecryptString(Memo1.Lines[i]);
          Cipher.Burn;
          Cipher.Free;
        end;
      end;

Example 2: File encryption

This example shows how you can encrypt the contents of a file, takes the input and output file names from two edit boxes: boxInputFile and boxOutputFile.

      procedure TForm1.btnEncryptClick(Sender: TObject);
      var
        Cipher: TDCP_rc4;
        KeyStr: string;
        Source, Dest: TFileStream;
      begin
        KeyStr:= '';
        if InputQuery('Passphrase','Enter passphrase',KeyStr) then  // get the passphrase
        begin
          try
            Source:= TFileStream.Create(boxInputFile.Text,fmOpenRead);
            Dest:= TFileStream.Create(boxOutputFile.Text,fmCreate);
            Cipher:= TDCP_rc4.Create(Self);
            Cipher.InitStr(KeyStr,TDCP_sha1);              // initialize the cipher with a hash of the passphrase
            Cipher.EncryptStream(Source,Dest,Source.Size); // encrypt the contents of the file
            Cipher.Burn;
            Cipher.Free;
            Dest.Free;
            Source.Free;
            MessageDlg('File encrypted',mtInformation,[mbOK],0);
          except
            MessageDlg('File IO error',mtError,[mbOK],0);
          end;
        end;
      end;

      procedure TForm1.btnDecryptClick(Sender: TObject);
      var
        Cipher: TDCP_rc4;
        KeyStr: string;
        Source, Dest: TFileStream;
      begin
        KeyStr:= '';
        if InputQuery('Passphrase','Enter passphrase',KeyStr) then  // get the passphrase
        begin
          try
            Source:= TFileStream.Create(boxInputFile.Text,fmOpenRead);
            Dest:= TFileStream.Create(boxOutputFile.Text,fmCreate);
            Cipher:= TDCP_rc4.Create(Self);
            Cipher.InitStr(KeyStr,TDCP_sha1);              // initialize the cipher with a hash of the passphrase
            Cipher.DecryptStream(Source,Dest,Source.Size); // decrypt the contents of the file
            Cipher.Burn;
            Cipher.Free;
            Dest.Free;
            Source.Free;
            MessageDlg('File decrypted',mtInformation,[mbOK],0);
          except
            MessageDlg('File IO error',mtError,[mbOK],0);
          end;
        end;
      end;

Example 3: General encryption

This hypothetical example shows how you might encrypt a packet of information before transmission across a network.

      type
        TSomePacket= record
          Date: double;
          ToUserID: integer;
          FromUserID: integer;
          MsgLen: integer;
          Msg: string;
        end;

      procedure EncryptPacket(Cipher: TDCP_cipher; var Packet: TSomePacket);
      // encrypt the information packet with the cipher
      // if the cipher isn't initialized then prompt for passphrase
      begin
        if Cipher= nil then
          raise Exception.Create('Cipher hasn''t been created!')
        else
        begin
          if not Cipher.Initialized then        // check the cipher has been initialized
            Cipher.InitStr(InputBox('Passphrase','Enter passphrase',''),TDCP_sha1);
          if Cipher is TDCP_blockcipher then    // if a block cipher use CFB 8bit as encrypting small packets
            TDCP_blockcipher(Cipher).CipherMode:= cmCFB8bit;
          // encrypt the record part by part, could do this in one go if it was a packed record
          Cipher.Encrypt(Packet.Date,Packet.Date,Sizeof(Packet.Date));
          Cipher.Encrypt(Packet.ToUserID,Packet.ToUserID,Sizeof(Packet.ToUserID));
          Cipher.Encrypt(Packet.FromUserID,Packet.FromUserID,Sizeof(Packet.FromUserID));
          Cipher.Encrypt(Packet.MsgLen,Packet.MsgLen,Sizeof(Packet.MsgLen));
          Cipher.Encrypt(Packet.Msg[1],Packet.Msg[1],Length(Packet.Msg));  // slightly different for strings
          // don't bother resetting the cipher, instead keep the chaining information
        end;
      end;

Index, Block Ciphers, Hashes

DCPcrypt is copyrighted © 1999-2002 David Barton.
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