To recover the private key for an encryption algorithm is an attack on the algorithm, since knowing the private key allows one to decrypt anything that has been encrypted with the key, which defeats the confidentiality property of encryption.
The algorithm is always assumed to be known as it is often public knowledge. For public-key algorithms, the public key is considered public knowledge too, as its name indicates. To recover the private key knowing some plaintext and its corresponding ciphertext, or even many (plaintext, ciphertext) pairs, is called a known-plaintext attack. The subcase where the attacker can submit many plaintexts for encryption and obtain the corresponding ciphertexts is called a chosen-plaintext attack.
If an encryption algorithm is vulnerable to such an attack, it is considered deeply broken. Any encryption algorithm that is considered safe for use must have no attack that is significantly better than brute force. Brute force is of course possible in theory, but not in practice: for any reasonable algorithm, it would take longer than the present age of the universe using all the computing power now available to mankind to find the key by brute force.
Therefore, assuming the algorithm is not completely broken, no practical tool can exist to find the private key given the algorithm, the public key, and any number of (plaintext, ciphertext) pairs.