The invention relates to embedding a spread spectrum watermark in a data signal as well as to detection of the embedded watermark. A data signal (4) is encrypted (15) or received in the form of an encrypted data signal (9), the signal being encrypted by modifying (3) it in accordance with one or more entries of a look-up-table (2) in which an encryption table (6) is stored. The encryption is carried out by a first computing device (15) such as a server device. The watermark is actually embedded while decrypting (13) the signal. This takes place in a second computing device (16), possibly in a client device, in a similar manner The client device employs a decryption table (8), which is a modified (i.e. watermarked) version of the encryption table (6). The decryption table may generated by the server and securely communicated to the client. The data signal is decrypted in accordance with entries of a look-up-table (12). The look-up-tables (2,12) in server and client are addressed by the same sequence of pointers generated by respective index generators (1). The mismatch between encryption table (6) and decryption table (8) causes the de-signal (14) to have an embedded watermark.