Re: 3-DES size

[Dr. Stephen Henson]

On Sun, Sep 28, 2003, Pierre De Boeck wrote:

 I would like to know the overhead in size when encrypting
 a message of n bytes with 3DES.
 
 A simple rule of thumb says that it will be n bytes plus
 the padding needed for n to be a multiple of 64 bits (8 bytes).
 
 Is it correct? and how the different modes (CBC,..)+ initialization
 vector influences that rule?

Block ciphers encrypt data in multiples of the block size so if the input data
isn't a multiple of the block size it has to be padded.

Standard block padding (aka PKCS#5 padding) adds k bytes each of value k where

k = block_size - input_size%block_size

note this means that if the input is already a multiple of the block size then
block_size bytes are added. This is so the decrypter can unambiguously
determine the size of the input data.

That assumes standard block padding. You can disable standard block padding
in the EVP calls but then you have to provide your own scheme to ensure the
input is a multiple of the block size.

Steve.
--
Dr Stephen N. Henson.
Core developer of the   OpenSSL project: http://www.openssl.org/
Freelance consultant see: http://www.drh-consultancy.demon.co.uk/
Email: [EMAIL PROTECTED], PGP key: via homepage.
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RE: 3-DES size

[Pierre De Boeck]

Ok, thanks.

 -Original Message-
 From: [EMAIL PROTECTED]
 [mailto:[EMAIL PROTECTED] Behalf Of Dr. Stephen Henson
 Sent: Sunday, September 28, 2003 8:37 PM
 To: [EMAIL PROTECTED]
 Subject: Re: 3-DES size
 
 
 On Sun, Sep 28, 2003, Pierre De Boeck wrote:
 
  I would like to know the overhead in size when encrypting
  a message of n bytes with 3DES.
  
  A simple rule of thumb says that it will be n bytes plus
  the padding needed for n to be a multiple of 64 bits (8 bytes).
  
  Is it correct? and how the different modes (CBC,..)+ initialization
  vector influences that rule?
 
 Block ciphers encrypt data in multiples of the block size so if 
 the input data
 isn't a multiple of the block size it has to be padded.
 
 Standard block padding (aka PKCS#5 padding) adds k bytes each of 
 value k where
 
 k = block_size - input_size%block_size
 
 note this means that if the input is already a multiple of the 
 block size then
 block_size bytes are added. This is so the decrypter can unambiguously
 determine the size of the input data.
 
 That assumes standard block padding. You can disable standard 
 block padding
 in the EVP calls but then you have to provide your own scheme to 
 ensure the
 input is a multiple of the block size.
 
 Steve.
 --
 Dr Stephen N. Henson.
 Core developer of the   OpenSSL project: http://www.openssl.org/
 Freelance consultant see: http://www.drh-consultancy.demon.co.uk/
 Email: [EMAIL PROTECTED], PGP key: via homepage.
 __
 OpenSSL Project http://www.openssl.org
 User Support Mailing List[EMAIL PROTECTED]
 Automated List Manager   [EMAIL PROTECTED]
__
OpenSSL Project http://www.openssl.org
User Support Mailing List[EMAIL PROTECTED]
Automated List Manager   [EMAIL PROTECTED]