Yescrypt Parameters

In the /etc/shadow file passwords hashed with yescrypt, usually have this j9T parameter as shown in the following example shadow entry, but information about this parameter is hard to come by.

test_user_1:$6$j9T$xyz$VKswtvLoVpOLcpjDMIFXhxa8ukqqKSKHjcPBLZUk9NxWldmlFQY4stUGo.QjEhav7mp86ih2PRqYPqjkhWi5y.:19796:0:99999:7:::

When you create a user using adduser test_user_1, you are might never see this j9T change, but now you might question what does this parameter even do?

Yescrypt Parameter j9T

Let's look at creating a yescrypt hash using python with the following line hasher = Yescrypt(n=2 ** 16, r=1, p=1, mode=Mode.MCF)

Code for this example can be found here GitHub Repo

The important parameters in this case are

• n amount of blocks
• r size of the blocks in 128-byte units
• p amount of paraller threads run

Recomended paremeters for different setups can be found here

So j9T Means What?

• J Seems to be the same no matter the parameters.
• The second parameter is the amount of blocks, the value of the second parameter can be calculated using $8\cdot 2^n$, where $n$ is the block parameter value so in this case $9$, which results in $8\cdot 2^9=4096\ blocks$. Note!! If this parameters is bigger than 9 rather than showing numbers 10 etc, letters are shown in alphabetical order, so 10 would be A.
• Third parameter is the size of the blocks the formula for calculating the size of the blocks is $3+n$, so in this case, we know that $T$ is the $20th$ alphabet, and the alphabetical counting starts after $9$ so, $block_{size}=3+9+20=32 \Rightarrow T$.