What is the encryption method called when pseudo-random numbers are used instead of true random numbers?
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3$\begingroup$ I like this question. It's simple, but seeing the number of questions on SO that confuse a OTP and stream cipher, it can be a good reference to use within comments and answers. $\endgroup$– Maarten Bodewes ♦Commented Oct 25, 2014 at 11:48
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Synchronous stream cipher, or just stream cipher.
In a synchronous stream cipher a stream of pseudo-random digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form, binary digits are used (bits), and the keystream is combined with the plaintext using the exclusive or operation (XOR). This is termed a binary additive stream cipher."
http://en.wikipedia.org/wiki/Stream_cipher#Synchronous_stream_ciphers
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1$\begingroup$ Basically, this makes the PRNG the key, no? (Or rather, the PRNG's seed, following the Kerckhoff Principle.) $\endgroup$ Commented Oct 25, 2014 at 18:36
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5$\begingroup$ Sure. It basically makes the PRNG the cipher and its seed the key. $\endgroup$ Commented Oct 25, 2014 at 20:55