The invention represents a method for creating a ciphertext block from a plaintext block consisting of more than one consecutive plaintext character strings (M1, M2, . . . Mn), which are encrypted with an encryption block operating on counter mode. When encrypting a plaintext character string (M3, for example) a hash is formed from the preceding plaintext character string (M2). Preferably the hash is message authentication code MAC or CMAC, the generation algorithm of which uses as a key (Key2) the hash value formed from the plaintext character string (M1) preceding string M2. The hash formed from the plaintext character string (M2) is Counter input to encryption block (Ek) that outputs a key stream (Keystream 3). It is combined in XOR operation with the plaintext character string (M3) wherein the result is a cipher text character string (C3). The invention makes it possible to truncate a file size without losing information stored in the rest of the file.