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Abstract

An apparatus and method for decrypting an encrypted MS Office file using a key other than a password used for encryption, based on a time-memory trade-off (TMTO) technique. The apparatus for decrypting an encrypted file includes a table generation unit for generating a table corresponding to an encryption algorithm used in an encrypted file. A data extraction unit extracts an encryption header from the encrypted file, and extracts encrypted fixed plaintext of a block corresponding to the extracted encryption header. A data search unit generates a key chain based on the encrypted fixed plaintext, generates final key candidates corresponding to the generated key chain, and searches for a start key using the final key candidates and the table. A key verification unit verifies validity of an encryption key using the start key. A reencryption unit reencrypts the encrypted file using the encryption key.

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First Claim