# What is Derivation: Definition and 999 Discussions

In cryptography, a key derivation function (KDF) is a cryptographic hash function that derives one or more secret keys from a secret value such as a main key, a password, or a passphrase using a pseudorandom function. KDFs can be used to stretch keys into longer keys or to obtain keys of a required format, such as converting a group element that is the result of a Diffie–Hellman key exchange into a symmetric key for use with AES. Keyed cryptographic hash functions are popular examples of pseudorandom functions used for key derivation.

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1. ### A Derivation of energy-momentum tensor in "QFT and the SM" by Schwartz

My question is about this step in the derivation: When the ##\partial_\nu \mathcal L## in 3.33 moves under the ##\partial_\mu## in 3.34 and gets contracted, I'd expect it to become ##\delta_{\mu \nu} \mathcal L##. Why is it rather ##g_{\mu \nu} \mathcal L## in the 3.34? (In this text, ##g_{\mu...

43. ### A Landau's Maximum Mass Limit Derivation in Shapiro & Teukolsky (1983)

In Section 3.4 of Shapiro & Teukolsky (1983), a simple derivation, due to Landau, of the maximum mass limit for white dwarfs and neutron stars is given. I will briefly describe it here and then pose my question. The basic method is to derive an expression for the total energy (excluding rest...
44. ### I Best Derivation of E=mc^2 - Fdx = V^2 dm

F= V dm/dt = V (dm/dx)(dx/dt) = V^2 (dm/dx) Fdx = V^2 dm E = m v^2. ,mass flowing with constant velocity If velocity is changing rather than mass, then E = 1/2 m V^2 Ok, joking aside, what is the best derivation of E= mc^2 ?