Implementação de sha1 hmac

Oi galera.

Precisava de uma ajudinha para fazer um trabalho da universidade sobre o HMAC com SHA1 em java

Aqui esta com tudo ao promenor :

[color=red]
HMAC(K,m) is mathematically defined by
HMAC(K,m) = H((K XOR opad) || H((K XOR ipad) || m)).

H(·) be a cryptographic hash function
K be a secret key padded to the right with extra zeros to the block size of the hash function
m be the message to be authenticated
|| denote concatenation

opad be the outer padding (0x5c5c5c…5c5c, one-block-long hexadecimal constant)
ipad be the inner padding (0x363636…3636, one-block-long hexadecimal constant)

The following pseudocode demonstrates how HMAC may be implemented.

function hmac (key, message)

if (length(key) > blocksize) then
    key = hash(key) // keys longer than blocksize are shortened
end if
if (length(key) < blocksize) then
    key = key || zeroes(blocksize - length(key)) //keys shorter than blocksize are zero-padded
end if

o_key_pad = [0x5c * blocksize] XOR key // Where blocksize is that of the underlying hash function
i_key_pad = [0x36 * blocksize] XOR key // Where &#8853; is exclusive or (XOR)

return hash(o_key_pad || hash(i_key_pad || message)) // Where || is concatenation

end function [/color]

Eu não sei como implementar esse pseudocodigo.

Obrigado

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