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Variable-length key cryptosystem

  • US 7,224,795 B2
  • Filed: 03/26/2002
  • Issued: 05/29/2007
  • Est. Priority Date: 10/20/1999
  • Status: Expired due to Fees
First Claim
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1. A variable-length key cryptosystem performing stream cryptographic processing in which, in an encryption of plain text, a plain text and a cryptographic key are added to each other by considering respective characters as numerical values, a string obtained by the addition is used as an encrypted text, and in a decryption, values of the cryptographic key obtained by considering respective characters as numerical values are subtracted from values of the encrypted text by considering respective characters thereof as numerical values, the cryptosystem comprising:

  • a computer including a variable-length key generation processing part generating a cryptographic key having a first length; and

    a data string length detection part detecting a length of a character string, wherein the variable-length key generation processing part further comprises;

    a storage storing an initial character string and a set of conversion rules of converting a first character string to a second character string that has a length exceeding a length of the first character string, anda generation processor repeating a character conversion by using the set of conversion rules from the initial character string, and generating a new character string,wherein,in the encryption of the plain text;

    the data string length detection part detects a length of the plain text, andthe variable-length key generation processing part generates a character string having the same length as the detected length of the plain text by using the initial character string and the conversion rule, and generates a cryptographic key based on the generated character string, the generated cryptographic key being stored in the computer and used to encrypt the plain text in stream encryption processing, andin the decryption of the encrypted text;

    the data string length detection part detects a length of the encrypted text, andthe variable-length key generation processing part generates a cryptographic key that is the same as the cryptographic key used in the encryption of the plain text by using the same initial character string and the set of conversion rules as those used in the encryption of the plain text, and based on the detected length of the encrypted text, the generated cryptographic key being used to decrypt the encrypted text in stream decryption processing, andupon a length of a generated converted character string reaching a length of a plain text that is a length that has already been used in generating a cryptographic key used in encryption, the generation processor;

    continues character conversion to generate a converted character string with a length that has not been used in generating a cryptographic key by;

    adopting a portion of the converted character string that has a length equal to that of a plain text, anddiscarding a remaining portion thereof, the decrypted text being stored on the computer.

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