Can any random 64 digit hex be used as a private key for Bitcoin or Ethereum?

[CounterEntropy]

For example 0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF ?

[o_e_l_e_o]

In bitcoin, private keys range from 1 to n-1, with n being the order of the base point G for the secp256k1 curve. In simple terms, the largest valid private key (n-1) is:

Code:

FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4140

It is possible however to use private keys larger than this number, as all private keys are modulo n. The key you have given as an example leads to an address which has previously been used back in 2017: https://blockchair.com/bitcoin/address/1CLrrRUwXswyF2EVAtuXyqdk4qb8DSUHCX

Note that it's a terrible idea to create your own private key by manually choosing a 64 digit hexadecimal number.

[MrFreeDragon]

For example 0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF0123456789ABCDEF ?

Yes, it is possible. You mentioned "random 64 digit hex" - so be sure that they are really random and not manually selected. It is important for security reason.

For example you can use dice with 16 sides, roll it 64 times and receive your random private key.

https://www.thediceshoponline.com/cat/41/sides/13/d16