Variable security level encryption

• US 5,594,797 A
• Filed: 02/22/1995
• Issued: 01/14/1997
• Est. Priority Date: 02/22/1995
• Status: Expired due to Term

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Mobile Phones Incorporated (Nokia Corporation)
• Inventors: Alan ar a, Seppo, Berson, Thomas
• Primary Examiner(s): Cain, David C.
• Application Number:
  US08/394,537
• Time in Patent Office: 692 Days
• Field of Search: 380/28, 380/29, 380/37, 380/42, 380/43, 380/49
• US Class Current: 380/28
• CPC Class Codes: H04L 2209/046 of operations, operands or results of the operations
• CPC Class Codes: H04L 2209/34 Encoding or coding, e.g. Huffman coding or error correction
• CPC Class Codes: H04L 2209/805 Lightweight hardware, e.g. radio-frequency identification [RFID] or sensor
• CPC Class Codes: H04L 9/0618 Block ciphers, i.e. encrypting groups of characters of a plain text message using fixed encryption transformation

Abstract

A method and apparatus for digital cellular telephone encryption includes a switch when a choice between compatibility security and a higher security is required. Plaintext is converted into ciphertext using a three-stage transformation process. In a first stage, the plaintext is transformed using a secret key to control encrypting by an invertible transformation process. In a second stage, the output of the first stage is transformed by an involutory transformation, which is unkeyed when the compatibility switch is in a compatibility setting and keyed when the compatibility switch is in a secure setting. In a third stage, the output of the second stage is transformed by the inverse of the transformation performed in the first stage. The compatibility switch is used to toggle between compatibility with a less secure encryption standard and a key-based secure encryption.

Claims

1. A method of securely encrypting a plaintext data block into a ciphertext data block, the method comprising the steps of:

converting the plaintext data block into a first intermediate data block according to a first encrypting process which uses a first key signal as its key and the plaintext data block as its data input;

determining a state of a compatibility switch, said state being one of a compatible state or a secure state;

converting said first intermediate data block into a second intermediate data block according to a second encrypting process which uses said first intermediate data block as its data input and uses a second key signal as its key when said state is said secure state and a compatibility value as its key when said state is said compatible state, wherein said second encrypting process is an involutory transformation of its data input; and

converting said second intermediate data block into the ciphertext data block according to an encrypting process which is an inverse transformation of said first encrypting process using said first key signal as its key and said second intermediate data block as its data input.

2. The method of claim 1, wherein said compatibility value is one.

3. The method of claim 1, wherein the plaintext data block is a message data block and the ciphertext data block is an encrypted message data block.

4. The method of claim 1, wherein the plaintext data block is plaintext only relative to said ciphertext block, said plaintext data having been previously encrypted for additional security.

5. A cryptographic system for transforming a message data block into an encrypted message data block, comprising:

first transformation means for transforming the message data block into a first intermediate data block, said first transformation means configured to transform a data input thereto through a first transformation according to a key input at a key input thereto;

a compatibility switch which indicates one state of a compatible state or a secure state;

second transformation means, coupled to receive said first intermediate data block from said first transformation means, for transforming said first intermediate data block into a second intermediate data block, said second transformation means configured to transform a data input thereto through a second transformation according to a key input thereto, said second transformation is an involutory transformation of its said data input;

third transformation means, coupled to receive said second intermediate data block from said second transformation means, for transforming said second intermediate data block into the encrypted message data block, said third transformation means configured to transform a data input thereto through an inverse of said first transformation according to a key input at a key input thereto;

first key application means, coupled to said key inputs of said first and third transformation means, for applying a first key to said key inputs of said first and third transformation means; and

second key application means, coupled to said key input of said second transformation means and said compatibility switch, for applying a second key to said key input of said second transformation means when said compatibility switch indicates a secure state and applying a compatibility value to said key input of said second transformation means when said compatibility switch indicates a compatible state.

6. The cryptographic system of claim 5, wherein said compatibility value is one.

7. The cryptographic system of claim 5, wherein the plaintext data block is a message data block and the ciphertext data block is an encrypted message data block.

8. The cryptographic system of claim 5, wherein the plaintext data block is plaintext only relative to said ciphertext block, said plaintext data having been previously encrypted for additional security.

9. A method of securely encrypting a plaintext data block into a ciphertext data block, the method comprising the steps of:

converting the plaintext data block into a first intermediate data block according to a first encrypting process which uses a first key signal as its key and the plaintext data block as its data input;

converting said first intermediate data block into a second intermediate data block according to a second encrypting process which uses said first intermediate data block as its data input and uses a second key signal as its key, wherein said second encrypting process is an involutory transformation of its data input; and

converting said second intermediate data block into the ciphertext data block according to an encrypting process which is an inverse transformation of said first encrypting process using said first key signal as its key and said second intermediate data block as its data input.