# MIT/GNU Scheme Blowfish Plugin Manual

Next: Introduction

# Blowfish Plugin Manual

This manual documents MIT/GNU Scheme Blowfish 1.1.

Copyright © 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022 Massachusetts Institute of Technology

> Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

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## 1 Introduction

This plugin is a dynamically loadable implementation of the Blowfish block cipher.

The Blowfish block cipher is totally obsolete and you should never use it for new data. It is provided only for compatibility with existing ciphertext.

The plugin includes a Scheme procedure to feed bytes from a binary input port to the library, and write the encrypted bytes to a binary output port.

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## 2 Procedures

Blowfish consists of a key setup phase and the actual encryption or decryption phase.

Procedure: blowfish-set-key bytes ¶

  Generate a Blowfish key from bytes, which must be 72 bytes or less in length. For text keys (strings), it was historical practice to apply `md5-string` and use the digest for bytes. This practice has always been ill-advised, instead of using a password-based key derivation function to raise the cost of brute force password search; it is mentioned only for compatibility with existing ciphertexts.

Procedure: blowfish-ecb input output key encyrpt? ¶

  Apply the Blowfish block cipher to a single 64-bit block. Input is an 8-byte byte vector. Output is an 8-byte byte vector. Key is a Blowfish key. Encrypt? specifies whether to encrypt (when `#t`) or decrypt (when `#f`).

  The misnomer ‘ECB’ stands for ‘Electronic Code Book’ for historical reasons.

The mode functions below all operate on variable length data. They all take an initialization vector which must be the same for decrypting a ciphertext as it was for encrypting the plaintext to yield that ciphertext.

Procedure: blowfish-cbc input output key init encrypt? ¶

  Apply Blowfish in Cipher Block Chaining mode. Input is a multiple of 8 bytes. Output is the same number of bytes as in Input. Key is a Blowfish key. Init is an 8 byte initialization vector; it is modified after each call to incrementally process a long message. The value from any call may be passed in to a later call to continue processing the same message. The initial value must be unpredictable in advance by an adversary. Encrypt? specifies whether to encrypt (when `#t`) or decrypt (when `#f`).

Procedure: blowfish-cfb64 input istart iend output ostart key init num encrypt? ¶

  Apply Blowfish in Cipher Feed-Back mode. Istart and Iend specify a range of bytes in Input. Ostart specifies the first byte to write in Output. Key is a Blowfish key. Init is an 8 byte initialization vector; it is modified after each call. The value from any call may be passed in to a later call to incrementally process a long message. The initial value must be unique for each message/key pair. Num is an integer from 0 to 7 inclusive, the low 3 bits of the number of bytes that have previously been processed in this stream. Encrypt? specifies whether to encrypt (when `#t`) or decrypt (when `#f`). The returned value is the new value of Num.

Procedure: blowfish-ofb64 input istart iend output ostart key init num ¶

  Apply Blowfish in Output Feed-Back mode. Istart and Iend specify a range of bytes in Input. Ostart specifies the first byte to write in Output. Key is a Blowfish key. Init is an 8 byte initialization vector; it is modified after each call to incrementally process a long message. The value from any call may be passed in to a later call to continue processing the same message. The initial value must be unique for each message/key pair. Num is an integer from 0 to 7 inclusive, the low 3 bits of the number of bytes that have previously been processed in this stream. The returned value is the new value of Num.

Two convenience procedures are also provided.

Procedure: blowfish-encrypt-port input output key init encrypt? ¶

  Reads bytes from input, which should be in blocking mode, until the end. Feeds the bytes to `blowfish-cfb64` and writes the resulting, encrypted bytes to output. Key and init are passed to `blowfish-cfb64` (which modifies the latter). Encrypt? specifies whether to encrypt (when `#t`) or decrypt (when `#f`).

Procedure: compute-blowfish-init-vector ¶

  Returns a uniform random initialization vector

The Blowfish cipher was invented and described by Counterpane (see http://www.counterpane.com/blowfish.html). Most of this manual was adapted from the OpenSSL manual pages.

Next: GNU Free Documentation License, Previous: Procedures, Up: Blowfish Plugin Manual

## 3 DES Modes

This chapter was written in large parts by Eric Young in his original documentation for SSLeay, the predecessor of OpenSSL. In turn, he attributed it to:

    AS 2805.5.2
    Australian Standard
    Electronic funds transfer - Requirements for interfaces,
    Part 5.2: Modes of operation for an n-bit block cipher algorithm
    Appendix A

### 3.1 Cipher Block Chaining Mode (CBC)

A multiple of 64 bits are enciphered at a time.

The CBC mode produces the same cipher text whenever the same plain text is encrypted using the same key and starting variable.

The chaining operation makes the cipher text blocks dependent on the current and all preceding plain text blocks and therefore blocks can not be rearranged.

The use of different starting variables prevents the same plain text enciphering to the same cipher text.

An error will affect the current and the following cipher text blocks.

### 3.2 Cipher Feedback Mode (CFB)

Any number of bits, j, up to 64, are enciphered at a time.

The CFB mode produces the same cipher text whenever the same plain text is encrypted using the same key and starting variable.

The chaining operation makes the cipher text variables dependent on the current and all preceding variables and therefore j-bit variables are chained together and can not be rearranged.

The use of different starting variables prevents the same plain text enciphering to the same cipher text.

The strength of the CFB mode depends on the size of k (maximal if j = k). In my implementation this is always the case.

Selection of a small value for j will require more cycles through the encipherment algorithm per unit of plain text and thus cause greater processing overheads.

Only multiples of j bits can be enciphered.

An error will affect the current and the following cipher text variables.

### 3.3 Output Feedback Mode (OFB)

Any number of bits, j, up to 64, are enciphered at a time.

The OFB mode produces the same cipher text whenever the same plain text enciphered using the same key and starting variable. More over, in the OFB mode the same key stream is produced when the same key and start variable are used. Consequently, for security reasons a specific start variable should be used only once for a given key.

The absence of chaining makes the OFB more vulnerable to specific attacks.

The use of different start variables values prevents the same plain text enciphering to the same cipher text, by producing different key streams.

Selection of a small value for j will require more cycles through the encipherment algorithm per unit of plain text and thus cause greater processing overheads.

Only multiples of j bits can be enciphered.

OFB mode of operation does not extend cipher text errors in the resultant plain text output. Every bit error in the cipher text causes only one bit to be in error in the deciphered plain text.

OFB mode is not self-synchronizing. If the two operations of encipherment and decipherment get out of synchronism, the system needs to be re-initialized.

Each re-initialization should use a value of the start variable different from the start variable values used before with the same key. The reason for this is that an identical bit stream would be produced each time from the same parameters. This would be susceptible to a ’known plain text’ attack.

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## Appendix A GNU Free Documentation License

Version 1.2, November 2002

    Copyright © 2000,2001,2002 Free Software Foundation, Inc.
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