Selective encryption to enable trick play

A method of decoding selectively encrypted digital video content comprising:

• receiving the selectively encrypted digital video content, wherein the selectively encrypted digital video content has certain unencrypted packets containing start of frame (SOF) headers, wherein the certain unencrypted packets were selected based upon a selection criterion that is explicitly based at least in part upon the presence or absence of a start of an SOF header in the selected packets;
• calculating offsets for at least one of I frames and P frames from the SOF headers;
• and
• storing the calculated offsets as an offset table.

A decoder for decoding selectively encrypted digital video content, comprising:

• a receiver that receives the selectively encrypted digital video content, wherein the selectively encrypted digital video content has unencrypted packets containing start of frame (SOF) headers;
• wherein the certain unencrypted packets were selected based upon a selection criterion that is explicitly based at least in part upon the presence or absence of a start of an SOF header in the selected packets;
• an offset calculator tat calculates offsets for at least one of I frames and P frames from the SOF headers;
• and
• means for storing the calculated offsets as an offset table.

A selective encryption encoder for selective encryption of digital video content, comprising;

• means for receiving a plurality of packets containing the digital video content;
• a programmed processor that selects packets for encryption according to a selection criterion, wherein the selected packets exclude certain packets containing start of frame (SOF) headers;
• wherein the selection criterion is explicitly based at least in part upon the presence or absence of a SOF header in the selected packets;
• and
• an encrypter for encrypting the selected packets while retaining the certain packets containing the SOF headers unencrypted to form selectively encrypted digital video content.