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The $\operatorname{SHA-224}$ is defined in the exact same manner as $\operatorname{SHA-256}$ with different initial values and the digest is obtained truncating the hash value, FIPS PUB 180-4 Page 23.
The different initial value provides domain separation. With domain separation $$\operatorname{SHA-224}(m) \neq \operatorname{SHA-256}(m)|_{224}$$ where $|_{...
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Sponge Construction
In sponge construction, the block size $b$ has two parts, $r$ is the rate and it is the written part and $c$ capacity is the untouched part by the input/output, $b= c+r$
The output is taken from $r$ in the squeezing stage. Therefore not all of the $b$ is the output. This prevents the length extension attack like in the truncations of ...
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