Cryptography

• Steganography
• Caesar Encryption
• Vigenere Cipher
• Enigma Code & Turing-Machine
• The Principles of Modern Cryptography
• Symetric Encryption
• Data Encryption Standard (DES)

• Advanced Encryption Standard (AES)
• Asymmetrical Encryption
• PGP
• S/MIME
• Brute-Force-Attacks
• Man-in-the-Middle-Attacks
• Views of the Future

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.

In contrast to steganography, cryptographic communication between two or more communication partners can be visible and still remains protected. Only the message is unreadable by outsiders because of the encryption.

Let’s stay in Rome and dive into the world of the Gauls and Romans.

A popular encryption technique was developed by a historically well-known personality: Gaius Julius Caesar. The famouse Roman emperor communicated with his military leaders in encrypted messages,  known today as the Caesar cipher. Neither unauthorized persons nor the enemy, in this case the Gauls, knew how to decipher the news. But over time, this encryption method could be cracked with fairly simple means.

One of the methods that replaced the Caesar cipher as a secure alternative was that of the French diplomat and cryptographer Blaise de Vigenère in the 16th century, also known as the Vigenère cipher. It is comparable to the Caesar cipher and is also based on the substitution of letters, but several ciphertext alphabets are used.

 How many alphabets are used is determined by a key. Instead of a number, a keyword is chosen here, which is written under the message. The keyword determines the letter offset for each letter. The first letter of the key word determines the alphabet for the first letter of the plain text, the second letter of the key word determines the alphabet for the second plain text letter.

The “G” provides a letter offset by seven letters, since the “G” is in the seventh position in the alphabet. The “I” is offset by five letters and so on. This is how the “WE” becomes “CM”.

 The security of this encryption method is strongly related to the key length and whether the key is used repeatedly. The keyword from our example is therefore not especially secure.

 But after a few years, this encryption method also proved to be decipherable. Next, we’ll look at another encryption method that was considered unbreakable for a long time.

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.

Many questions were raised about the role of NSAs in this project. The NSA is said to have deliberately installed a back door so that they can read the encrypted information. The main reason for these suspicions was the discussion 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.

DES was widely used in ATMs and was therefore considered a very secure encryption system. In 1998, however, “Deep Crack” succeeded in cracking the 56-bit key for the first time. The device was able to decrypt the DES algorithm within a few days using a brute force method. Today, this would be possible in a very short time. DES was therefore attested to be highly susceptible to brute force attacks.

In 2001 the Advanced Encryption Standard replaced DES as its successor. This brings us to our next station.

In the 1990s, the US Senate decided that a back door should be built into any encryption software or hardware. Pretty Good Privacy, PGP for short, was then specified by Philip Zimmermann in 1991. With the development he pursued the goal that all US citizens and citizens’ movements could exchange encrypted messages and avoid access by secret services.

Since the US export law prevented a license-free export of the PGP source code, it was typed as a book by more than 60 volunteers. At that time, the book was not subject to export restrictions and could therefore be legally exported from the USA to make the code known worldwide.

In 1997 PGP was bought by McAfee and integrated into their product portfolio. In 2002, the company abandoned the brand. The reason for this is said to have been the strong criticism of the non-disclosed source code of PGP. Until 2010, the newly founded PGP Corporation held all rights to PGP until it was sold to Symantec in 2010.

PGP is based on an encryption concept with two main functions: Encryption and signing of a message. As explained above, encryption uses a key pair. However, PGP does not encrypt the entire message asymmetrically, but only the session key used. The actual message is encrypted symmetrically. The reason for this hybrid encryption is the too high computational effort for a purely asymmetric encryption.

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.

Brute force attacks are used to obtain information by means of the trial and error method. With the help of appropriate software, various combinations of characters can be tested within a short time in order to gain illegal access to the desired information. This method is often used for passwords, but also for deciphering ciphertext. In this complete key search, all possible key combinations are tested “exhaustively”.

As already explained in the last article, the DES encryption algorithm was cracked by means of a brute force attack, since with a key length of 56 bits “only” about 72 quadrillion combinations are possible. Thus, a brute force attack can be counteracted with modern encryption algorithms by using a sufficiently long key. It would be hopeless to start a brute force attack here, since the necessary computational effort would be too high. There are, however, mathematical attacks that considerably reduce the complexity of the key, such as the square root attack. In addition, there is another variant to secure these attacks, which you probably already know: after X failed login attempts on your smartphone, it is locked for a certain time.

Our next station is dedicated to an attack technique that can tap any data traffic and is therefore particularly dangerous.

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.