Abstract
This chapter provides an overview of modern symmetric encryption and asymmetric encryption.
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Notes
- 1.
Note: This acronym will mean different things in different chapters, so pay close attention to context.
- 2.
Actually, Rijndael is a family of algorithms and had a range of configurations. Only a subset are certified as AES.
- 3.
Note, however, it does not work in the reverse direction. A change made to the last block only changes the last block.
- 4.
Although, in practice, block ciphers must do the same thing. As you saw in the previous section, if each block is encrypted exactly the same, patterns emerge. Modes of operation like Cipher Block Chaining mode carry over the output from one block’s encryption into the input of the next. In short, how the cipher encrypts the block is changed or influenced by the previous block’s encryption. For this reason, CBC mode may be considered a form of stream cipher even though it is not usually referred to as such.
- 5.
Some machines cannot generate that much random data quickly.
- 6.
Even though this is true from a purely theoretical perspective, an attacker may have some contextual knowledge that would permit them to recognize the “correct” plaintext. In practice, however, for a message of even a relatively short length, the odds of getting a zero key (key of all zeros) are very unlikely. For the short 14-character message I used as an example, the odds of getting the zero key are \(2^{14*8} = 2^{112} = 5{,}192{,}296{,}858{,}534{,}827{,}628{,}530{,}496{,}329{,}220{,}096\).
- 7.
In order to make the image displayable, the header portion of the BMP file is not modified. But all of the data that comprises the visible image is XORed.
- 8.
There are other modes of operation that produce a stream cipher, but counter mode is the most common and probably the easiest to understand.
- 9.
Although AES-CTR mode and other modes like it do have limits on how large the key stream can safely get, these details are outside the scope of the book.
- 10.
Sometimes, the term key exchange is used as a synonym for key agreement. However, I am going to follow the Handbook of Applied Cryptography’s nomenclature. Key establishment is getting the parties to share a key. Key transport and key agreement are two forms of key establishment. The term key exchange can refer to various algorithms including the Diffie-Hellman algorithm discussed in this section, but it is not a synonym.
- 11.
Diffie-Hellman Ephemeral is abbreviated DHE. However, it is such a common mode for DH that even when it is not expressly stated, or even when the DH abbreviation is used, it may very well be DHE. You may need to ask for clarification if it is not clear from context. This is also true for ECDHE.
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Nielson, S.J. (2023). Core Cryptography Technology. In: Discovering Cybersecurity. Apress, Berkeley, CA. https://doi.org/10.1007/978-1-4842-9560-1_5
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