GB2501315A

GB2501315A - Generation of ciphertext using cipher block chaining (CBC) with padding

Info

Publication number: GB2501315A
Application number: GB1206995.1A
Authority: GB
Inventors: David Sallis
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-04-20
Filing date: 2012-04-20
Publication date: 2013-10-23
Also published as: GB201206995D0

Abstract

A method of generating a ciphertext sequence from a first key and plaintext comprises generating a first initialisation vector (IV1) and a padding sequence, combining the plaintext and the padding sequence to generate a first intermediate sequence, and generating ciphertext by encrypting the first intermediate sequence using the key and the initialisation vector in a standard cipher-block chaining (CBC) process having a block length of M bytes. The ciphertext is decrypted by generating a second initialisation vector (IV2), decrypting the ciphertext using the first key and the second initialisation vector, and then removing the first N bytes. The padding sequence and initialisation vectors may be randomly generated. The padding sequence may have a length of M bytes and may be added to the beginning of the plaintext. The decryption does not require any knowledge of IV1 (IV2 being unrelated to IV1), or the padding.

Description

Methods for Generatin2 and Decrvptin2 Ciphertext

Field of the Invention

The present invention relates to an enhancement of the Cipher Block Chaining (CBC) cryptography method patented by IBM ("Message verification and transmission error detection by block chaining", US Patent 4074066, 1976), and subsequent enhancements to the CBC method.

BackQround to the Invention As is well known to thosc skilled in the art, the CBC algorithm rcquircs, in addition to a key (Key) for encryption and decryption of a message, an Initialisation Vector (IV).

It is also well known that for a constant Key, known to both the sender and receiver of the message, the IV must be different for each message if the security of the cipher is to be reasonably safe from attempts to decrypt the encrypted message (Ciphertext) by a third party (Attacker). This necessitates the generation and sending of a new IV along with each Ciphertext, and if the Ciphertext is to be stored, necessitates storing the IV in addition to the Ciphertext. These necessities are an inconvenience in computer systems that transmit and/or store messages encrypted using the CBC algorithm and its variants.

Summary of the Invention

Tn a first aspect, the present invention provides a method of generating a ciphertext sequence from a first key and a plaintcxt sequence, the method comprising: generating a first initialisation vector and a padding sequence; combining the plaintext sequence and padding sequence to generate a first intermediate sequence; generating the ciphertext sequence by encrypting the first intermediate sequence using a cipher-block chaining process having a cipher block length of M bytes, the first key and the first initialisation vector.

In a preferred embodiment, the present invention provides a method of encrypting a message using a CBC method, transmitting or otherwise sharing the Ciphertext, and decrypting the Ciphertext, all without any know'edge by the decrypting party of the IV, nor any requirement of the encrypting party to retain the IV used for the encryption of the message. This is all without any prejudice to the security of the method with respect to decryption by an Attacker.

In further preferred embodimcnts, the method results in a Ciphertext that is in general different for each instance of encryption of the same message with the same Key, save for accidental eases that will in practice be extremely rare provided a reasonably long Cipher Block size is employed, for example less than one chance in 300,000,000,000,000,000,000,000,000,000,000,000,000 of two successive Ciphertexts being the same for a Cipher Block size of 16 bytes. This aspect is of considerable utility, for example where a commonly occurring message such a person's name appears encrypted in multiple records within a database, in which case the method prevents the association of one record with another by the observation of a common Ciphertext. Note that this aspect is possessed by the standard CBC method provided a different IV is used for each encryption using the same key, but in practice this requirement for a different IV is often not properly observed because of the inconvenience of generation, transmission and/or storage of the different IV for each case.

In a second aspect, the present invention provides a method of generating a plaintext sequence by decrypting a ciphertext sequence using a first key, the method comprising: generating a second initialisation vector; generating a second intermediate sequence by decrypting the ciphertext sequence using the a cipher-block chaining process, the first key and the second initialisation vector; and generating the plaintext sequence by removing the first N bytes from the second intermediate sequence.

Brief Description of the Drawings

The present invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows the standard CBC encryption method known from the prior art; and Figure 2 shows the standard CBC decryption method known from the prior art.

Detailed Description of Embodiments of the Invention Encryption A Key, known both to thc scndcr and the receiver but to 110 other party, and a mcssage (Plaintcxt) known initially only to thc scndcr, arc givcn.

To encrypt the Plaintext the sender first generates an IV at random (IVI) usillg one of the secure random string generation algorithms that will be known to those skilled in the art.

A second independent random string (the Padding) equal in length to that of the Cipher Block is similarly generated by the sender.

The sender creates a Padded Message by concatenating the Padding and the Plaintext.

A Ciphertext is created by enclypting the Padded Message using the standard CBC method with lvi and the Key.

The sender transmits or otherwise shares the Ciphertext with the receiver. The sender does not retain any knowledge of lvi or the Padding, nor does the sender transmit or othcrwisc share with the recciver the IV1 or the Padding.

Decryption To decrypt the Ciphcrtcxt the rcceivcr first generates an IV at random (1V2) using one of the secure random string generation algorithms that will be known to those skilled in the art. 1V2 is necessarily unrelated to Wi since Wi is unknown to the receiver.

Using the Key, known to the receiver, and 1V2, the receiver uses the standard CBC method to decrypt the Ciphertext, resulting in a string (Padded Message 2). The receiver removes a certain number (N) bytes from the beginning of Padded Message 2, resulting in the original Plaintext.

The number N will in general depend on the Cipher Block size employed and the particular implementation of the CBC method and the underlying cipher employed.

The number N is easy to determine by examination of an example Plaintext and Padded Message 2 resulting from the particular combination of Block size and underlying cipher employed. For example, in the present embodiment with a Block size of 16 bytes and the commonly used AES256 cipher, then N is equal to 24.

Claims

Claims 1. A method of generating a ciphertext sequence from a first key and a plaintext sequence, the method comprising: generating a first initialisation vector and a padding sequence; combining the plaintext sequence and padding sequence to generate a first intermediate sequence; generating the ciphertext sequence by encrypting the first intermediate sequence using a cipher-block chaining process having a cipher block length of M bytes, the first key and the first initialisation vector.
2. A method of generating a ciphertext sequence according to claim 1, wherein the length of the padding sequence is M bytes.
3. A method according to claims 1 or 2, wherein the first initialisation vector and the padding sequence are generated at random.
4. A method according to any of claims 1 to 3, wherein the cipher-block chaining process is a standard cipher-block chaining process.
5. A method according to any preceding claim, wherein the padding sequence is combined with the plaintext sequence by adding the padding sequence to the beginning of the plaintext sequence.
6. A method of generating a plaintcxt sequence by decrypting a ciphcrtext sequence using a first key, the method comprising: generating a second initialisation vector; generating a second intermediate sequence by decrypting the ciphertcxt sequence using the a cipher-block chaining process, the first key and the second initialisation vector; and generating the plaintext sequence by removing the first N bytes from the second intermediate sequence.
7. A method according to claim 6, whcrcin thc ciphertext sequence is generated according to the method of any of claims Ito 5.
8. A computer implemented method according to any of claims I to 7.
9. A computer program or a suite of computer programs configured to carry out the method of any of claims I to 7.
10. A computcr-rcadablc medium having the computer program or suite of computer programs according to claim 9 stored thereon.
11. A computing dcvicc configurcd to carry out thc steps of any of claims ito 7.