• Aes Round Key
• Round Key Generation In Aes Examples
• Crypto++ Aes Example

I want to use encryption algorithm available in.Net Security namespace, however I am trying to understand how to generate the key, for example AES algorithm needs 256 bits, that 16 bytes key, and some initialization vector, which is also few bytes. Can I use any combination of values in my Key and IV? All zeros in Key and IV are valid or not? Jan 10, 2018  AES algorithm is the Block Cipher Symmetric Algorithm Block Size is 128 bits Key Size is 128 bits ( 4 words or 16 Bytes ) Sub Key Size is 1 Word (32 bit) Number of Sub keys 44 Words Number of. Key Generation. The round-key generator creates sixteen 48-bit keys out of a 56-bit cipher key. The process of key generation is depicted in the following illustration − The logic for Parity drop, shifting, and Compression P-box is given in the DES description. DES Analysis. The DES satisfies both the desired properties of block cipher.

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AES uses a key schedule to expand a short key into a number of separate round keys. The three AES variants have a different number of rounds. Each variant requires a separate 128-bit round key for each round plus one more.^{[note 1]} The key schedule produces the needed round keys from the initial key.

Steps in the AES Encryption Process. But first we need to look in more detail at the generation of the Round Keys, so called because there is a different one for each round in the process. Of the corresponding word in the previous round key and the preceding word in the current round key. For example, using XOR for addition: 1 Using.

Round constants[edit]

The round constant rcon_i for round i of the key expansion is the 32-bit word:

where rc_i is an eight-bit value defined as:

where ${\displaystyle \oplus }$ is the bitwise XOR operator and constants such as 00₁₆ and 1B₁₆ are given in hexadecimal. Equivalently:

${\displaystyle r{c}_i=x^{i-1}}$

where the bits of rc_i are treated as the coefficients of an element of the finite field${\displaystyle \mathrm{G}\mathrm{F}(2)[\mathrm{x}]/({\mathrm{x}}^8+{\mathrm{x}}^4+{\mathrm{x}}^3+\mathrm{x}+1)}$, so that e.g. ${\displaystyle r{c}_{10}={36}_{16}={00110110}_2}$ represents the polynomial ${\displaystyle x^5+x^4+x^2+x}$.

AES uses up to rcon₁₀ for AES-128 (as 11 round keys are needed), up to rcon₈ for AES-192, and up to rcon₇ for AES-256.^{[note 2]}

The key schedule[edit]

Define:

• N as the length of the key in 32-bit words: 4 words for AES-128, 6 words for AES-192, and 8 words for AES-256
• K₀, K₁, .. K_N-1 as the 32-bit words of the original key
• R as the number of round keys needed: 11 round keys for AES-128, 13 keys for AES-192, and 15 keys for AES-256^{[note 3]}
• W₀, W₁, .. W_4R-1 as the 32-bit words of the expanded key^{[note 4]}

Aes Round Key

Also define RotWord as a one-byte left circular shift:

and SubWord as an application of the AES S-box to each of the four bytes of the word:

Round Key Generation In Aes Examples

Then for ${\displaystyle i=0\dots 4R-1}$:

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Notes[edit]

1. ^Non-AES Rijndael variants require up to 256 bits of expanded key per round
2. ^The Rijndael variants with larger block sizes use more of these constants, up to rcon₂₉ for Rijndael with 128-bit keys and 256 bit blocks (needs 15 round keys of each 256 bit, which means 30 full rounds of key expansion, which means 29 calls to the key schedule core using the round constants). The remaining constants for i ≥ 11 are: 6C, D8, AB, 4D, 9A, 2F, 5E, BC, 63, C6, 97, 35, 6A, D4, B3, 7D, FA, EF and C5
3. ^Other Rijndael variants require max(N, B) + 7 round keys, where B is the block size in words
4. ^Other Rijndael variants require BR words of expanded key, where B is the block size in words

References[edit]

• FIPS PUB 197: the official AES standard (PDF file)

External links[edit]

• schematic view of the key schedule for 128 and 256 bit keysfor 160-bit keys on Cryptography Stack Exchange

Crypto++ Aes Example