GNU/Linux and MacOS X, with a Step-by-Step Introduction
to Signing and Encrypting E-mails and Files with OpenPGP
Peter Jockisch,
Freiburg i. Br.
peterjockisch.de
English: PDF DIN A4 • A4 two-column • US Letter • US Letter two-column
German: PDF DIN A4 • A4 two-column • US Letter • US Letter two-column
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Computer files can be manipulated in many ways unnoticed. Cryptographic checksums, hash values, serve to protect your data: By forming an electronic fingerprint of a file, an always constant numerical value is created. If this value deviates at a later point in time, there is damage or manipulation. With a single mouse click, the integrity of a file can be checked at any time.
Cryptographic checksums form the basis for cryptographic signing and encryption, for website- and e-mail certificates, for the qualified electronic signature, and for the technical understanding of revision-proof e-mail archiving, to which all merchants are legally obliged.
This introduction presents two free graphical programs for checksum generation, CyoHash and Jacksum, for file manager operation.
Console-based programs are also described, they are available across operating system platforms and are pre-installed on MS-Windows 10 as well as most Unix/BSD and GNU/Linux systems (see 2.3 for instructions). This means that no programs need to be installed at all, the existing operating system resources are sufficient to calculate checksums.
Humans are complex creatures. In order to identify them quickly and easily, fingerprints are often created. Computer files can be identified according to the same principle: by generating an “electronic fingerprint”, the so-called cryptographic checksum, an always constant number.
By means of standardized procedures, a fast integrity and authenticity check of files of any kind can be carried out.
Human fingerprints are created with stamp pads, electronic fingerprints with a checksum program.
Fig.1: Proof of authenticity for human and computer files
We consider cryptographic checksums. They are based on hash functions, which provide hash values as a result for any file. This value is also called hash code.
A file, as well as identical copies of it, always has the same hash checksum. However, if only a single bit or character changes due to damage or manipulation, a completely different hash code should be created.
A hash-function-checksum-procedure should therefore always return different values for different computer files. Depending on the method used, the calculated checksum always has the same length. Therefore, of course, only a limited number of numbers can be depicted: There are practically an infinite number of computer files, so that it is impossible to assign a different value to each of these files with a number of fixed length.
From a security point of view, there are various attack scenarios, including forgery of documents. An attacker
would like to create a fake version of a given original file, for example a business order, with a manipulated, increased order quantity that has the same hash value checksum. After making the changes to the document, he then tries to obtain a file version with a cryptographic checksum identical to that of the original file by trial and error, perhaps by inserting invisible control characters. Such an attack, of course, uses supporting computer programs.
If an attacker actually succeeds in creating a second file (at a reasonable cost in terms of time) containing the desired manipulations and which has the same cryptographic checksum of the original file, the hash function procedure in question is “broken”. Once such a weakness becomes known, it should no longer be used. Due to continuous research work, weaknesses are detected a long time in advance.
Fig.2: Checksum collision
If there were a computer with infinite computational power, theoretically any method could be broken by simply trying out all the possibilities (brute force attack). In practice, such an approach is not considered practicable in the majority of cases, since the necessary calculations are almost never feasible in a reasonable time.
Most hash functions have had a limited lifetime and have been replaced by successor functions for security
reasons. Computer generations with higher computing power contribute to shortening the service life. In addition to computational force-based attacks, however, there are also attacks with a different orientation, and it can never be ruled out that mathematical creativity can be used to launch practicable attacks today.
In the background a huge army of mathematicians works and researches, especially for intelligence services. Not all scientific findings are published.
Until 2016, the Western IT infrastructure was predominantly based on the SHA-1 algorithm (Secure Hash Algorithm 1). Since 2017, this algorithm has been regarded as finally broken, and the computing time required to corrupt it has fallen drastically. Experts now recommend the SHA-2 variants SHA256, SHA384, or SHA512.
The recommended successor algorithm to SHA-2, SHA-3 [1], has been officially established since 2012.
In Russia and many other CIS states, GOST R 34.11-94 resp. GOST 34.311-95 were the previous hash standards in authorities and various economic sectors. [2] As with SHA-1, structural weaknesses were also found in this standard.
All computational power-related statements in this introduction refer to publicly available computer systems and research work released to the general public. The use of the latest, most advanced computer technology is probably currently still reserved for intelligence services in order to guarantee them a computing power advantage for an effective leveraging of established encryption technology.
The widely approved encryption procedures may not be readily breakable for lower levels of government. However, at the top of the hierarchy, at the intelligence level, there should be unrestricted access to the latest computer technology. In addition, all data transferred via the Internet will probably be archived for automatic evaluation. Under this aspect, the resilience of files sent over the Internet that have been encrypted using publicly standardized technology is put into perspective.
For a long time there have been considerations that certain cryptographic algorithms raised to official standards
might have inherent mathematical weaknesses which are only known to the experts of the intelligence services. A possibly existing influence of the secret services on the design of security products (software and possibly hardware backdoor problems, open questions about standards, etc.) is the subject of numerous articles on computer security, for example in “Did NSA Put a Secret Backdoor in New Encryption Standard?”. Several renowned companies have already directly or indirectly confirmed that they cooperate with intelligence services in their product development. One of the official reasons for this was the intention to optimize the technical safety of company products. It remains to be seen how much pressure was exerted on “cooperation”.
Corrupted electronics, known or unknown “advanced” hardware architectures with factory-built “remote maintenance” functions, possibly even with a wireless system built into the processor, represent the other side of the problem.
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