Last updated: 2020-06-08

Checks: 7 0

Knit directory: bioinformatics_tips/

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File Version Author Date Message
Rmd 9019e42 davetang 2020-06-08 Shared-key cryptosystem
html f851e18 davetang 2020-06-07 Build site.
Rmd a36c1e8 davetang 2020-06-07 Security basics

The Internet has become an integral part of our lives. When exchanging data over the internet, the data passes through various networks and devices. There are four problems that can occur when data is transferred from one party to another:

  1. Interception
  2. Spoofing
  3. Falsification
  4. Repudication

These four problems are countered by:

  1. Encryption
  2. Message authenitcation codes (MACs) or digital signatures
  3. MACs or digital signatures
  4. Digital signatures and certificates

Encryption

Encryption means performing an operation on data such that a computer cannot decipher into something meaningful, i.e. turn data into ciphertext. A key is typically used to perform the encryption’s numeric calculation and the same key is used to decrypt the encrypted data. One way of achieving this is by using a XOR cipher; XOR (exclusive or) is an operation that works like OR but returns zero when both conditions are true.

0 XOR 0 = 0
1 XOR 1 = 0
1 XOR 0 = 1
0 XOR 1 = 1

If our data (in binary) is 00110011 and our key is 11110000 then:

data       = 00110011
key        = 11110000
ciphertext = 11000011

If we use the same key on the ciphertext, we obtain the original data:

ciphertext = 11000011
key        = 11110000
data       = 00110011

Hash functions

A hash function converts data into a random string of fixed length. The MD5 message-digest algorithm is a widely used (but outdated) hash function that produces a 128-bit hash value.

echo hello world | md5
6f5902ac237024bdd0c176cb93063dc4

The output is in hexadecimal (0-9 then A-F), which requires 4 bits to represent because F in hexadecimal is 1111 in binary. Therefore the 32 long hexadecimal number is 32*4 bits. Any data used as input into the MD5 hash function will return a 128-bit hash value or a length 32 hexadecimal number.

echo abc | md5
0bee89b07a248e27c83fc3d5951213c1

When given the same input, a hash function will invariably produce the same output.

echo hello world | md5
6f5902ac237024bdd0c176cb93063dc4

However, if the input data only differs by a single bit, the output is very different.

echo hell world | md5
a3723e12600ef5c0456c201f5e8c7a37

Sometimes, completely different data can produce identical hash values but this has a very low probability and is known as a hash collision. Finally, it is impossible to convert hash values back into their original data.

Shared-key cryptosystem

Shared-key or symmetric-key cryptosystems use the same key for encryption and decryption. The Advanced Encryption Standard is the first (and only) publicly accessible cipher approved by the National Security Agency (NSA).

The problem with shared-key systems is that in order for the receiving party to decrypt the encrypted file, the key needs to be transferred as well. A secure method is necessary for transmitting keys, i.e. performing a key-exchange. There are two types of methods:

  1. Methods using key-exchange protocols
  2. Methods using the public-key cryptosystem

To be continued…


sessionInfo()
R version 4.0.0 (2020-04-24)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Catalina 10.15.5

Matrix products: default
BLAS:   /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRblas.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] workflowr_1.6.2

loaded via a namespace (and not attached):
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 [5] R6_2.4.1        backports_1.1.7 git2r_0.27.1    magrittr_1.5   
 [9] evaluate_0.14   stringi_1.4.6   rlang_0.4.6     fs_1.4.1       
[13] promises_1.1.0  whisker_0.4     rmarkdown_2.1   tools_4.0.0    
[17] stringr_1.4.0   glue_1.4.1      httpuv_1.5.3.1  xfun_0.14      
[21] yaml_2.2.1      compiler_4.0.0  htmltools_0.4.0 knitr_1.28