A method for detection of modification of an item of content, the method comprising: obtaining, for the item of content, a respective first value of each attribute in a set of one or more attributes of the item of content, the set of one or more attributes selected such that, for each of one or more predetermined types of modification, said type of modification affects the value of at least one attribute in the set of one or more attributes; performing a watermark decoding operation on the item of content; and in response to the watermark decoding operation producing payload data from the item of content: determining that the one or more predetermined types of modification have not been applied to the item of content if, for each attribute in the set of one or more attributes, the respective first value for that attribute matches a respective second value for that attribute determined using the payload; or determining that a modification has been applied to the item of content if, for at least one attribute in the set of one or more attributes, the respective first value for that attribute does not match a respective second value for that attribute determined using the payload.