A system, comprising: at least one computer; and at least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by the at least one computer, cause the at least one computer to perform: receiving, via at least one network, a first communication from a device different from the at least one computer; identifying, from the first communication, a request to access a web resource; generating a multi-channel image that encodes a challenge text, the multi-channel image comprising data stored in a plurality of channels including one or more color channels (520, 530, 540) and a transparency channel, the generating comprising: generating decoding information representing the challenge text; storing the decoding information in the transparency channel of the multi-channel image; and storing another image in the one or more color channels (520. 530, 540) of the multi-channel image; transmitting, via the at least one network, the generated multi-channel image to the device; receiving, via the at least one network, a second communication from the device; identifying, from the second communication, a response text; and providing the device with access to the web resource based on a comparison between the challenge text and the response text; wherein the image stored in the one or more color channels (520, 530, 540) of the multi-channel image stores a fake challenge text, wherein the fake challenge text is different from the challenge text, or wherein the image stored in the one or more color channels (520, 530, 540) of the multi-channel image stores no text. at least one computer; and at least one non-transitory computer-readable storage medium storing processor-executable instructions that, when executed by the at least one computer, cause the at least one computer to perform: receiving, via at least one network, a first communication from a device different from the at least one computer; identifying, from the first communication, a request to access a web resource; generating a multi-channel image that encodes a challenge text, the multi-channel image comprising data stored in a plurality of channels including one or more color channels (520, 530, 540) and a transparency channel, the generating comprising: generating decoding information representing the challenge text; storing the decoding information in the transparency channel of the multi-channel image; and storing another image in the one or more color channels (520. 530, 540) of the multi-channel image; transmitting, via the at least one network, the generated multi-channel image to the device; receiving, via the at least one network, a second communication from the device; identifying, from the second communication, a response text; and providing the device with access to the web resource based on a comparison between the challenge text and the response text; wherein the image stored in the one or more color channels (520, 530, 540) of receiving, via at least one network, a first communication from a device different from the at least one computer; identifying, from the first communication, a request to access a web resource; generating a multi-channel image that encodes a challenge text, the multi-channel image comprising data stored in a plurality of channels including one or more color channels (520, 530, 540) and a transparency channel, the generating comprising: generating decoding information representing the challenge text; storing the decoding information in the transparency channel of the multi-channel image; and storing another image in the one or more color channels (520. 530, 540) of the multi-channel image; generating decoding information representing the challenge text; storing the decoding information in the transparency channel of the multi-channel image; and storing another image in the one or more color channels (520. 530, 540) of the multi-channel image; transmitting, via the at least one network, the generated multi-channel image to the device; receiving, via the at least one network, a second communication from the device; identifying, from the second communication, a response text; and providing the device with access to the web resource based on a comparison between the challenge text and the response text; wherein the image stored in the one or more color channels (520, 530, 540) of the multi-channel image stores a fake challenge text, wherein the fake challenge text is different from the challenge text, or wherein the image stored in the one or more color channels (520, 530, 540) of the multi-channel image stores no text.