20. A computer-implemented method comprising: receiving a request for protected content from a client, the protected content comprising data; determining a challenge phrase comprising a plurality of characters; dividing, using a computer processor, the challenge phrase into at least two character subsets selected from the plurality of characters comprising the challenge phrase, each of the at least two character subsets comprising less than all of the characters comprising the challenge phrase; sending, in response to the request, the at least two character subsets for display on the client in a series of images, wherein at least one of the following techniques is applied to the character subsets: 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 either (i) within predefined vertical segments, and altering the width of the vertical segments from image to image or (ii) on a background that moves from image to image; and receiving from the client and in response to the at least two character subsets, an answer to the challenge phrase, wherein access to the protected content is limited based on whether the answer correctly solves the challenge phrase.
20. A computer-implemented method comprising: receiving a request for protected content from a client, the protected content comprising data; determining a challenge phrase comprising a plurality of characters; dividing, using a computer processor, the challenge phrase into at least two character subsets selected from the plurality of characters comprising the challenge phrase, each of the at least two character subsets comprising less than all of the characters comprising the challenge phrase; sending, in response to the request, the at least two character subsets for display on the client in a series of images, wherein at least one of the following techniques is applied to the character subsets: 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 either (i) within predefined vertical segments, and altering the width of the vertical segments from image to image or (ii) on a background that moves from image to image; and receiving from the client and in response to the at least two character subsets, an answer to the challenge phrase, wherein access to the protected content is limited based on whether the answer correctly solves the challenge phrase. 21. The computer-implemented method according to claim 20 , wherein distorting the characters comprises applying shearing, arc, wave, or swirl image processing techniques to the characters.