Method and arrangement for the transmission of enciphered texts.

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SIEMENS AKTIENGESELLSCHAFT Our sign

Berlin and Munich VPA op ρ ι ι g 3 nc

Method and arrangement for transmitting encrypted texts

The invention relates to a method for transmitting encrypted texts according to the preamble of claim 1. Furthermore, the invention relates to an arrangement for carrying out the method. 5

From DE-OS 29 26 013 a method for the transmission of encrypted texts from a transmitting station to a receiving station is known, in which the texts are stored in a memory in the receiving station and are issued only to reception-authorized persons. The transmitted texts are provided with a message header containing the name of the person authorized to receive and an identifier which signals the text as confidential. If the identifier is detected in the receiving station, the texts are stored in a receive memory and a message is issued, indicating that the recipient is a confidential text. After entering a password by the authorized person, the texts are decrypted in the receiving station and output to an output unit. By this known method, a relatively high access security is achieved. However, there is a danger that an unauthorized person may come into possession of the password and issue texts that are addressed to another person.

Ret 1 Obh / 15.3.1982

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The invention is therefore based on the object ,, a method and an arrangement for the encrypted transfer of texts specify, in their use, the access security is further improved. According to the invention the object is achieved in the method of the type mentioned by the features stated in the characterizing part of patent claim 1.

The invention is based on the idea of ​​uniquely identifying the recipient by a codeword. This code word can be contained in a subscriber directory and be publicly available, for example. The codeword is used to calculate the current key for the encrypted transmission of the texts. With the help of this current key, test marks are generated from the code word and transmitted in the message header to the receiving station. There they are stored together with the encrypted texts in the receive memory. At the receiving station, it is possible to check at any time whether the correct key has been decrypted for decrypting the stored texts and whether the codeword transmitted in the message header has been manipulated. After checking whether the codeword is assigned to the corresponding password, the stored texts are output.

To generate the check marks, it may be favorable if the current key is encrypted with the random-sequence codeword prior to encryption.

As linking unit, a well-known DES encryption block is suitably used. This can operate according to a mapping mode or a feedback mode. The assignment mode advantageously results in the generation of the current key, the check result and the generation of the check vector

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used. The use in the feedback mode proves to be useful when the pseudo-random sequence for the encryption of the plaintext is generated. In this case, an input vector for the encryption unit is expediently generated from the test key.

From the test vector, the check marks are generated in a simple manner by linking the low-order bits with the high-order bits in accordance with a modulo-2 addition. The modulo-2 addition also proves useful in the encryption of the random sequence and the codeword of the generation of the current key and before the generation of the test vector.

The basic key can be formed as a permanent basic key, which is permanently assigned to a particular subscriber group, which is connected to an in-house transmission network, for example Identification card is arranged to also at a receiving station that does not contain the basic key, to be able to output the encrypted texts. This is important, for example, if the person entitled to receive is on a business trip and the receiving station is connected to a network other than the in-house network and thus has another basic key.

An implementation of the method and an arrangement according to the invention are explained in more detail below with reference to drawings. Show it

1 is a schematic representation of an arrangement for the encrypted transmission of texts,

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2 is a schematic representation of the message format in the transmission of the encrypted texts,

3 is a flow chart for the encryption of the texts and the access protection,

Fig. 4 shows an arrangement for carrying out the method.

FIG. 1 shows a data source D1 at the transmitting end, a first key device SG1 and a data transmitter SE and at the receiving end a data receiver EM₃ a second key device SG2 and a data sink D2. The key device SG1 contains a key processor S1, which encrypts the plain text KT delivered by the data source D1 and outputs encrypted texts to the data transmitter SE. The key device SG1 also transmits to the data transmitter SE a codeword associated with the person authorized to receive the encrypted text. Furthermore, the key device SG1 generates a random sequence and check characters which are formed from the code word and the random sequence using a current key. The data transmitter SE generates a message header and an end identifier. These form the message together with the codeword, the random sequence, the checkmark and the encrypted text

The data receiver EM removes the message header NK and the end identifier from the message N and supplies the remaining characters to the key device SG2. If it is stated in the message header that the message N contains an encrypted text, this message is stored in a receive memory provided in the key device SG2. In addition, the test symbols are generated from the received codeword and the random sequence in the receiving station and compared with the received test symbols, in order to be able to detect a manipulation of the codeword.

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After entering a password into the key device SG2 and a corresponding check, the encrypted text is decrypted using the key processor S2 and output as clear text KT at the data sink D2. 5

The data source D1, the data transmitter SE, the data receiver EM and the data sink D2 are preferably each components of a telex typewriter, in addition to. the key device S61 or SG2 is provided. The telex typewriter can be designed so that it is intended for normal office use, and the texts can be transmitted unencrypted in a known manner, bypassing the key devices SG1 and SG2.

The key devices SG1 or SG2 of a particular subscriber group, for example, an in-house transmission network are preferably provided with the same basic key. This key may be changed by security officers from time to time. It is also possible to assign a temporary basic key to individual authorized persons, with the help of which texts can be transmitted even if the teleprinters are connected to a network other than the internal network. This is the case, for example, when the recipient is on a business trip to a third-party company and wants to exchange encrypted texts with his own company. The temporary basic key is arranged, for example, on an identity card and in this case can then be dispensed with the input of a password, of course, since the authorized person has already identified by the temporary basic key. In any case, however, the codeword is transmitted, encrypted with the random sequence and checked in the receiving station.

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^VPA82 P ί 1 9 3DE In the illustration in Fig. 2, the structure of the message N is shown schematically. The message N begins with the message header NK, which indicates, for example, that in the message N encrypted texts VT are included, which are to be stored in the receiving station in the receive memory. The message header NK is followed by the code word CW, which contains, for example, 40 or 45 bits and is assigned to the receiving subscriber. The code word CW can for example be taken from a generally accessible list of participants. This is followed by a random sequence ZF, which is generated in the transmitting station. From the code word CW and the random sequence ZF a current key for the encryption of the plain text KT is generated using the basic key. In addition, a test vector is generated from the code word CW and the random sequence ZF, possibly using the current key, which in turn is encrypted and results in the test character PZ. Subsequent to the check mark PZ, the encrypted text VT follows and the message N is terminated by an end identifier EK.

The illustration in FIG. 3 schematically shows the processes which are carried out in the key device SG1 during the generation of the check characters PZ and in the generation of the encrypted text VT from a plaintext KT. Corresponding processes also take place in the key device SG2, wherein the plaintext KT is again generated from the encrypted text VT, and additionally wherein the determined check characters are compared with the transmitted check characters PZ in order to prevent output of the transmitted text at the receiving station.

After the input of the codeword CW at the data source D1, the key computer S1 generates a random sequence ZF and combines this according to a modulo-2 addition M1 with the codeword CW. The result is stored in an encrypted

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encrypting unit according to the well-known DES algorithm using a basic key GS encrypted. The encryption result forms the current key AS for the intended transmission of texts. The random sequence ZF is encrypted for the generation of the check mark PZ using the current key AS also in accordance with the DES algorithm in an encryption unit. The result is a test key PS. This is encrypted with the codeword CW using a modulo 2 addition M2. The result is then encrypted again according to the DES algorithm to generate a test vector PV. The test vector PV is divided into two halves, which are linked together in accordance with a modulo-2 addition M3 and form the test marks PZ transmitted within the message N from the transmitting station to the receiving station. In the receiving station's key processor S2, these check characters PZ are compared with the received check characters PZ in order to determine whether the access authorization is available or whether the code word CW has been manipulated.

Using the current key AS, a pseudorandom sequence PZF is generated by means of the encryption unit in accordance with the DES algorithm, the test key PS forming an input vector EV. By means of this pseudo-random sequence PZF, the plaintext KT is encrypted in accordance with a modulo-2 addition M4 in order to generate the encrypted text VT.

In the generation of the current key AS, the Prüfschlüssls PS and the test vector PV, the encryption unit operates as a block coder without feedback (code book). On the other hand, in the generation of the pseudo-sequence PZF, the encryption unit operates in a cipher-feedback mode as a pseudo-random generator.

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These modes of operation are known in the art and are feasible, for example, by an integrated circuit commercially available under the designation WD2001 from Western Digital. 5

The code word CW contains, for example, eight or nine alphanumeric characters, which are represented by 40 or 45 bits, for example, according to the international telegraph alphabet number 2. The random sequence ZF has, for example, 64 bits, while the check characters PZ have, for example, 32 bits. The basic key 6S₅ For example, the current key AS and the check key PS each have 56 bits.

In the modulo-2 addition M2, the random sequence ZF can also be used instead of the check key PS. Furthermore, it is also conceivable to generate the checkmark PZ by encrypting the codeword CW with the part of the random sequence ZF using the current key AS, and encrypting the results in turn according to a modulo-2 addition.

The arrangement shown in Fig. 4 for carrying out the method can be used both as a transmitting and receiving station. In this case, an input-output unit EA assumes the function of the data source D1 or of the data sink D2 and a transmission unit UE corresponds to the data transmitter SE or the data receiver EM. The key device SG with its key computer S corresponds to the key devices SG1 and SG2 and the key computers S1 and S2. The input-output unit EA and the transmission unit UE can be components of a known telex typewriter, which is additionally provided with the reasoning device SG.

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The key device SG contains three switches A, B and C, which are brought by a control unit ST in various switching Stel settings. If the switches A, B and C respectively have the switching position 1, the key device SG is turned off and the telex machine operates in a known manner by the input by means of the input-output unit EA texts are switched to the transmission unit UE and passed from there to the trunk FL become. In a corresponding manner, received texts are switched through by the transmission unit UE to the input-output unit EA and output there.

The entered texts can be stored in the transmission memory SS and be delivered from this in line operation to the trunk FL, when the switch A takes the switch position 2. In local mode, the content of the transmit memory SS can also be output. For this purpose, the switch B assumes the switching position 4. With the help of the send memory SS it is possible to edit created messages.

In the encryption mode, the content of the transmit memory SS is supplied to the key processor S, which encrypts the plaintext and provides the encrypted text VT with the codeword CW, the random sequence ZF and the checkmark PZ and additionally generates the message header NK and the end identifier EK. When the switch A assumes the switching setting 3, the message N generated in this way is delivered via the transmission unit UE to the trunk FL. When the control unit ST recognizes that an encrypted text VT is to be received, the switches B and C assume the switch position 2, and a part of the message N, for example, the message header NK and the codeword CW are output at the input-output unit EA the rest of the Natiricht, in particular the encrypted text VT in the receive memory ES a

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is stored. An output of the text is only possible if a password PW or the temporary key TS is entered by an authorized person. In this case, the text stored in the receiving memory ES is decrypted in the key computer S and output at the input-output unit EA when the switch B assumes the switching position 3. The password PW is checked in the control unit ST for its correctness or for the correct assignment to the received codeword CW. In the event of a mismatch, the output of the text is prevented. This is done even if the check characters determined in the key computer S do not match the received check marks PZ.

When using the temporary key TS, the password PW is not checked and the encrypted text VT is decrypted using the temporary key TS in the key processor S and output to the input-output unit EA. With the aid of the key device SG it is also possible to decrypt texts in local mode. For this purpose, the switches B and C assume the switching position 3. It is also possible, in principle, to store all received messages in the receive memory ES when the switch C assumes the switch position 4.

10 Claims 4 figures

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