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Generating Alternative Initial Value while wrapping keys with AES

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lulliezy

Am following the instructions on https://datatracker.ietf.org/doc/html/rfc5649#section-3 ("AES Key Wrap with Padding Algorithm") and I have gotten to a point where I need to generate the LSB(32,A) for the Alternative Initial Value (AIV). Am using NodeJS with buffers to implement the algorithm. My understanding is 32-bits === buffer.length == 4 or in other words, length 4 of buffer is the 32-bits referenced in the article. I have padded the key after converting it buffer then padding with the length value of 8 - (length % 8) with 0s as value as indicated in the article. Now the thing I have not been able to figure out is getting the value of 32-bit MLI. How do I get the MLI, I just know its Message Length Indicator but thats all I know about it.

Example:

const key = Buffer.from('base64 key', 'base64');
const kek = Buffer.from('A65959A6', 'hex');

Now here I have only MSB(32, A) but not LSB(32, A), how do I get the value, and is there anything I am doing wrong, please help I have already spent alot of time trying to figure this out.

Scenario: Let's say my key length is 75, now I have to pad the remaining 5 characters for it to be multiples of 8, now how do I generate LSB(32, A) in this case?

23
0 + 0
aes
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