Methods and computer program products for protecting interpreted-language source code from unauthorized copying, alteration, distribution, or reverse engineering, without regard to the choice of programming language. A language identifier and the source code are input to an encoding program, which provides an encrypted file that includes the language identifier and source code. The encrypted file is input to a decoding program. The decoding program decrypts the language identifier and opens a suitable interpreter. The decoder then decrypts an encrypted source code instruction and inputs the result to the interpreter, looping through the instructions until reaching the end of the encrypted file, passing decrypted instructions to the interpreter at a rate suitable for execution by the interpreter substantially in real time. Thus a single decoding program may be used with a number of different interpreted languages, and with a number of existing interpreters.