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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def validate_pwhash(_self, params):
 """ Validate password hash using YubiHSM. """

    pwhash, nonce, aead, key_handle = get_pwhash_bits(params)
    d_aead = aead.decode('hex')
    plaintext_len = len(d_aead) - pyhsm.defines.YSM_AEAD_MAC_SIZE
    pw = pwhash.ljust(plaintext_len, chr(0x0))
    if hsm.validate_aead(nonce.decode('hex'), key_handle, d_aead, pw):
        return "OK pwhash validated"
    return "ERR Could not validate pwhash"