Cryptography

From the beginnings of encryption to the present day — The history of cryptography and a look into the future.

Today, encryption is mainly thought of as an IT term, because data, e-mails, computers etc. are encrypted. But that was not always so. Encryption actually has its origins back in the year 480, but of course at that time, there was no IT that needed to be encrypted. And until a few years ago, encryption was primarily used in espionage or in top-secret government communications.

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The year is 480, and Roman generals vie for rule of the Roman kingdom. Intrigue, murders and other fraudulent activities need to be planned and carried out. But how can an attack plan be sent undetected to the contractor?

Have you ever heard of lemon juice on parchment? It is an example of a classic secret communication method. The text is written on the parchment using lemon juice. After the lemon juice has dried, the parchment gives the impression of a blank page. But the recipient of the message can still decrypt the message very easily by holding a candle behind the parchment to make the lemon juice visible and read the message.

Another ancient method was to shave the hair from a slave’s head, tattoo the message on the back of the head and wait until the hair grew back in order to send the message to the rightful recipient. Without question, this was a radical means of communication and obviously was not suitable for urgent messages.

Such methods as these belong to steganography, which must be clearly differentiated from cryptography. Steganography is based on the fact that an outsider does not even notice that two communication partners are communicating with each other.

The Caesar cipher is a simple symmetrical encryption method based on a substitution. This means that every letter used in the message is replaced by a new letter. The replacement letter results from a letter offset within the alphabet that is determined in advance, e.g. a shift of three places. “Thank you” then becomes “Gdqnh.” A cipher disk was often used for decryption in order not to have to constantly go through the alphabet. With this type of encryption, the recipient only had to be informed of the offset as a secret key in advance.

 An unauthorized person could not decode the message without the key. But if he has dealt with the cipher before, it is easy to decrypt any message within 25 attempts, since you need to check the alphabet at most once to discover the correct letter offset. Today’s computers would take less than a second to do this.

 The Caesar cipher is therefore no longer considered secure and has been replaced by newer methods.

All aboard, we are off to France in the 16th century!

Our next stop is Germany in the 1930s. As already described with the Caesar cipher, encryption methods were primarily used in a military context. It is therefore hardly surprising that Germany also relied on encrypted communication during the Second World War. The special thing about this method was that it was encrypted and decrypted using a machine. The encryption key was changed every day so it would no longer be valid after 24 hours. The machine we’re talking about is Enigma.

 The Enigma machine for routine encryption and decryption was invented by Arthur Scherbius in 1918. The basic working principle goes back to the years of the First World War. World War I is considered to be the first war in which cryptography was used systematically. During the war and the years that followed, the first machines were developed that offered significantly higher security than the manual methods. Enigma was offered for sale after its manufacture but met with very little interest from both business and government agencies. It was not until 1933 that the Enigma became part of the standard equipment of the National Socialists under Hitler. But how exactly does this wonderful machine work?

 At first glance, it is reminiscent of a classic typewriter. But a rather complicated system is hidden inside. To put it simply, the operating principle is based on simple electrical circuits, each of which connects a letter key on the keypad with an electric lamp that lights up a letter on the display. However, the “A” is not linked to the “A” of the display, as all the reels are interlaced according to a certain system. The message can only be decrypted if the recipient knows all the settings of the sending Enigma.

 Sounds like insurmountable encryption, doesn’t it? But it too was cracked by a British computer scientist in 1941. Alan Turing declared war on Enigma and actually won it with the “Turing machine” he developed. Historians claim that this ended World War II prematurely and saved millions of lives.

DES was developed in the 1970s after the NSA issued a call for tender to develop a uniform standard for the inter-agency encryption of confidential data.

An IBM development team led by Walter Tuchman, Don Coppersmith and Alan Konheim then submitted a promising proposal for a corresponding encryption procedure and was promptly commissioned. On March 17, 1975, the algorithm designed by IBM was published in the Federal Register and approved as an encryption standard just one year later.

DES uses a 56-bit key and a combination of diffusion and confusion elements. The information to be encrypted is divided into many blocks of equal size. Each block is individually encrypted using a round key and “scrambled” in 16 rounds, also called iterations. In order to decrypt the message again, the blocks must be put back into the correct order.

Questions about the role of the NSA in this project arose many times. The NSA is said to have deliberately built in a back door so that they can read the encrypted information. The main reason for these suspicions were the discussions about the key length: IBM preferred a key length of 64 bits while the NSA considered a key length of 48 bits to be sufficient. The agreement was then reached on 56 bits.

As DES had not been sufficiently protected against brute force attacks with its 56-bit key since the 1990s, the American Department of Commerce issued an invitation to tender for a successor algorithm on January 2, 1997. To ensure a certain level of security of the Advanced Encryption Standard, the algorithm had to meet certain criteria.

Selection criteria AES:

– Symmetric algorithm, block cipher

– Use of 128 bit long blocks

– 128, 192 and 256 bit long keys can be inserted

– Above-average performance in hardware and software

– Resistance to cryptanalysis

By the deadline of 15 June 1998, fifteen proposals had been submitted. Among the five best candidates were the algorithms MARS, RC6, Rijndael, Serpent and Twofish. Since all candidates met the required criteria, additional requirements were set up to select a winner. Since Rijndael convinced mainly by its simple software implementation, security and speed, the Belgian algorithm was announced as winner on October 02, 2000.

But the choice of the winner was not without controversy. Critics also saw weaknesses in the advantages of the structure and efficiency of the Rijndael algorithm. They argued that the simpler the structure of an algorithm is, the easier it is for hackers to understand and hack it. Practically relevant attacks, however, do not exist until today, which makes AES very secure. The use of AES encryption is widespread, for example in wireless LAN, VPNs, VoIP telephony or the encryption of files.

If a message is to be sent encrypted to a recipient, the recipient’s public key is required first. The public key can be characterized as a kind of “one-way key”, because it can only encrypt but not decrypt. A very common example to illustrate this is the mailbox in traditional letter dispatch: The sender knows the address of the recipient and can drop the letter in his mailbox. However, the recipient cannot get the letter out this way. Therefore, an additional key is needed to open the mailbox and then get the letter. Similarly, the recipient can only decrypt the message with his private key.

In 1974 Ralph Merkle took the first step towards the development of the asymmetric cryptosystem with the Merkles Puzzle, named after him. The first asymmetric encryption method was developed by cryptographers at MIT in 1977: the RSA method. However, this method is very susceptible to attacks, since it is deterministic, i.e. pre-determined, and thus easy to crack.

In addition to encryption, the public key can also be used for signature generation, which can be used to confirm the authenticity of the message. Thus, the authenticity, integrity and confidentiality of the message can be guaranteed. To achieve this, the public key must be authenticated with the private keys of the other communication partners. This procedure is also called Web of Trust (WoT). This type of trust model is advantageous if you want to remain anonymous. Prominent users of PGP encryption are therefore whistleblowers like Edward Snowden. The security of PGP is only guaranteed as long as the users keep their private keys secret.

We take a leap from PGP, developed in 1991, to 1999, the birth of another asymmetric encryption standard.

The encryption method, S/MIME, which was published in 1999, is also based on the main applications signing and encryption and is therefore similar to the way PGP works. However, if you want to encrypt and sign your e-mails with S/MIME, you have to register with an appropriate certificate authority and apply for a certificate. The authentication of the public key by the other communication partners, as is the case with PGP, is thus replaced here by a formal certificate. The communication partner can then see in the header information of the message from which certification authority the sender has received his certificate and, if necessary, have his identity confirmed via this authority. Companies in particular use S/MIME for their e-mail encryption.

Basically, both encryption methods are only secure as long as the private key is kept secret. In 2018, however, a research team from the University of Applied Sciences Münster, the Ruhr University Bochum and the University of Leuven published a document questioning the security of the encryption standards PGP and S/MIME and thus attracted attention. However, the research results did not focus on the protocols themselves, but rather on a weakness in mail clients such as Thunderbird, Apple Mail, etc. The topic was taken up in media reports worldwide under “Efail”. Also Hornetsecurity dedicated an own blog post to Efail.

Slowly but surely we want to leave the terrain of the different encryption methods and take a look at the next stations to see which attacks both encryption methods and the data traffic itself are exposed to.

In a man-in-the-middle attack, also known as Janus attack (Roman mythology), a third party switches unnoticed between two communication partners. In doing so, he pretends to be the other’s actual counterpart. The aim of this attack is to view or even manipulate the data traffic at will. Depending on the area of application, the attack scenarios are manifold. A popular attack vector is, for example, an open WLAN network set up by the attacker, to which the victim connects. This allows the cybercriminal to read any data traffic while the victim is surfing the Internet recklessly. Encrypted e-mail communication can also be attacked using the same principle:

The attacker transmits his public key to the sender, but pretends that it is the public key of the legitimate recipient. The sender now encrypts the message with the public key, which is also available to the attacker, who decrypts it with his private key and can read and manipulate the message. To ensure that the communication partners are not aware of this, the message is encrypted by the attacker with the public key of the legitimate recipient and forwarded to him so that he can receive and decrypt the now manipulated message.

In order to avoid this kind of attack, the public keys should be checked for authenticity, for example by using appropriate certificates, as is the case with S/MIME.