15. A computer-implemented method comprising: accessing characters of a challenge phrase; determining first processing to apply to a first group of characters from the challenge phrase; determining second processing to apply to a second group of characters from the challenge phrase, the second processing being different than the first processing; generating, using a computer processor, a first image comprising the first group of characters using the first processing; generating, using the computer processor, a second image comprising the second group of characters using the second processing; and providing the first image and the second image for use in a challenge-response test to control access to protected content, wherein at least one of the first processing and the second processing comprises at least one of the following techniques: changing the position of one or more of the individual characters from image to image; rotating one or more of the individual characters; obscuring part of one or more of the individual characters; distorting one or more of the individual characters using an image processing technique; displaying one or more of the individual characters within predefined vertical segments, and altering the width of the vertical segments from image to image; or displaying the one or more individual characters on a background that moves from image to image, wherein the first group of characters and the second group of characters comprise one or more common characters, and the first group of characters includes at least one character not included in the second group of characters.
15. A computer-implemented method comprising: accessing characters of a challenge phrase; determining first processing to apply to a first group of characters from the challenge phrase; determining second processing to apply to a second group of characters from the challenge phrase, the second processing being different than the first processing; generating, using a computer processor, a first image comprising the first group of characters using the first processing; generating, using the computer processor, a second image comprising the second group of characters using the second processing; and providing the first image and the second image for use in a challenge-response test to control access to protected content, wherein at least one of the first processing and the second processing comprises at least one of the following techniques: changing the position of one or more of the individual characters from image to image; rotating one or more of the individual characters; obscuring part of one or more of the individual characters; distorting one or more of the individual characters using an image processing technique; displaying one or more of the individual characters within predefined vertical segments, and altering the width of the vertical segments from image to image; or displaying the one or more individual characters on a background that moves from image to image, wherein the first group of characters and the second group of characters comprise one or more common characters, and the first group of characters includes at least one character not included in the second group of characters. 18. The computer-implemented method according to claim 15 , wherein: at least one of the common characters is distorted or rotated by the first processing; and the at least one common character is distorted or rotated by the second processing to appear differently in the second image relative to the appearance of the at least one common character in the first image.