The document explains public key cryptography, specifically focusing on RSA's use of asymmetric encryption with public and private keys, emphasizing security, key generation, and certification by trusted parties. It also discusses the Advanced Encryption Standard (AES), a symmetric key cryptography developed to secure U.S. government data, detailing its algorithm, key sizes, and applications in secure messaging and IoT devices. AES replaced DES due to its larger key sizes and stronger encryption, processing data in blocks while employing various transformation steps to encrypt messages.

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Public Key Cryptography is a form of asymmetric encryption
that makes use of two separate sets of keys- a public key and a
private key (Cryptography.io, n.d.). Public key cryptographic
algorithms with RSA will be discussed considering they use
encryption and decryption to function. RSA has three major
parts key generation, encryption, and decryption. In this
algorithm each receiver has their own unique decryption key.
This receiver will then publish their public key. A public key is
utilized to avoid spoofing. With public key with RSA a trusted
third party is used that certifies the public key only belongs to a
specific person. It’s not possible to figure out the private key
from the public key. RSA itself is used to secure sensitive data
when sent over an insecure network. The signatures use a
specific hash function and encryption power is tied to key size
(Cryptography.io, n.d.).
Cryptography.io. (n.d.) Asymmetric Algorithms. Retrieved
from https://cryptography.io/en/latest/hazmat/primitives/asymm
etric/
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The Advanced Encryption Standard (AES) – also known as
the Rijndael (pronounced “rain-doll”) cipher – is a form of
secret key cryptography that was developed as a collaborative
project between the National Institute of Standards and
Technology (NIST) and a team of public and private sector
cryptography experts. The goal of the project was to develop
encryption powerful enough to protect sensitive U.S.
government data and communications for the foreseeable future
(NIST, 2019). “The Rijndael algorithm…is a symmetric block
cipher that can process data blocks of 128 bits, using cipher
keys with lengths of 128, 192, and 256 bits” (NIST, 2001, p5).
Both the input and output of AES consists of 128-bit sequences
(“blocks”). As mentioned above there are three different

2. lengths of cipher key that can be applied to each block (128,
192, or 256 bits) so the encryption levels are described as AES-
128, AES-192, or AES-256. Obviously, the longer the cipher
key is the heavier the blocks are encrypted and the more
difficult it is to crack the encryption.
Federal Information Processing Standards (FIPS) publications
are the standard by which the U.S. government directs its
various agencies to manage their information systems and data.
Since the publication of FIPS 197 on November 26, 2001, AES
became the approved standard for cryptographic protection of
sensitive but unclassified government information, and its
adoption was also encouraged in non-government organizations
as well (NIST, 2001). AES is particularly useful in secure
messaging and encrypting large databases, due to its
mathematical stability and relatively low processing and power
costs. These advantages are becoming even more useful with
the rise of the Internet of Things (IoT). IoT devices often
require a delicate balance of processing speed and low power
consumption, which coupled with the need to collect and
securely transmit user data makes variations of AES ideal for
IoT purposes (Hung & Hsu, 2018).
Reference
Hung, C. & Hse, W. (2018). Power consumption and calculation
requirement analysis of AES for WSN IoT. Sensors, 18(6),
1675. Retrieved
from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6022145/#
sec5-sensors-18-01675
National Institute of Standards and Technology (NIST). (2001).
Federal Information Processing Standards Publication 197 (FIPS
197): Announcing the advanced encryption standard (AES).
Retrieved
from https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.197.pdf
National Institute of Standards and Technology (NIST). (2019).
Cryptographic standards and guidelines: AES development.
Retrieved from https://csrc.nist.gov/projects/cryptographic-
standards-and-guidelines/archived-crypto-projects/aes-

3. development
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According to (“Advanced Encryption Standard”, 2002), the
National Institute of Standards and Technology (NIST) selected
Advance Encryption Standards (AES) in 2001 to replace Data
Encryption Standard (DES) because DES’s key size was too
small (56 bits). The initial name was Rijndael block cipher.
AES is a symmetric block cipher, which means that the secret
key is the same for encryption and decryption and the algorithm
are applied to a block of data. The key sizes are 128, 192, and
256 bits. The message (plaintext) is sent with the key and the
AES algorithm transform the message in a ciphertext. Simply
put AES breaks the data to encrypt in a table of four-by-four
sixteen bytes and shuffle it around by shifting the bytes in the
rows and mixing the columns. Each block is encrypted
separately. According to Lake (2018), AES consists of the
following steps:
1. AES key expansion: the initial key is used to produce new
keys using the Rijndael’s key schedule.
2. Initial round
Add Round key: the initial key is added to the message.
3. Rounds
Substitute Bytes: the algorithm looks at a table to substitute
each byte to the corresponding items in a prearranged table.
Shift Rows: The first row stays the same. Bytes on the second,
third, and fourth rows are shifted one, two or three position to
the left respectively.
Mix Columns: the columns are shifted around according to a
mathematical function.
Add Round Key: the key previously created are added
to the mixed columns.
Multiple rounds are performed; nine rounds if the key size is
128 bits; 11 and 13 if the 192- and 256-bit key are used
respectively.
4. Final Round

4. Substitute Bytes
Shift Rows
Add Round Key
Example: Data and keys can be arranged as below. The
algorithm will apply the byte substitution, row shifting, column
mixing and add round keys. The encrypted data is therefore
made difficult to crack.
t i a k i t
h s d n e a m a
i m a o y r p n
s y t w s e o t
AES is used in messaging apps, in programs such as WinZip and
and in file transfer protocols like FTPS and HTTPS (Lake,
2018).
References
Advanced Encryption Standard. 2002. Retrieved from
https://www.cs.mcgill.ca/~kaleigh/computers/crypto_rijndael.ht
ml
Lake, J. 2018. What is AES encryption and how does it work?
Retrieved from https://www.comparitech.com/blog/information-
security/what-is-aes-encryption/