Elements of this invention are based upon different properties of a serial link.  Various serial links for transmitting data and clock signals are well known. 
One conventional serial link, used primarily for high-speed transmission of video data from a host processor (e.g., a personal computer) to a monitor, is known as a transition minimized differential signaling interface (“TMDS” link).  The characteristics of a TMDS link include the following:
1.  video data are encoded and then transmitted as encoded words (each 8-bit word of digital video data is converted to an encoded 10-bit word before transmission);                a. the encoding determines a set of “in-band” words and a set of “out-of-band” words (the encoder can generate only “in-band” words in response to video data, although it can generate “out-of-band” words in response to control or sync signals.  Each in-band word is an encoded word resulting from encoding of one input video data word.  All words transmitted over the link that are not in-band words are “out-of-band” words);        b. the encoding of video data is performed such that the in-band words are transition minimized (a sequence of in-band words has a reduced or minimized number of transitions);        c. the encoding of video data is performed such that the in-band words are DC balanced (the encoding prevents each transmitted voltage waveform that is employed to transmit a sequence of in-band words from deviating by more than a predetermined threshold value from a reference potential.  Specifically, the tenth bit of each “in-band” word indicates whether eight of the other nine bits thereof have been inverted during the encoding process to correct for an imbalance between running counts of ones and zeroes in the stream of previously encoded data bits);        
2.  the encoded video data and a video clock signal are transmitted as differential signals (the video clock and encoded video data are transmitted as differential signals over conductor pairs without the presence of a ground line);
3.  three conductor pairs are employed to transmit the encoded video, and a fourth conductor pair is employed to transmit the video clock signal; and
4.  signal transmission occurs in one direction, from a transmitter (typically associated with a desktop or portable computer, or other host) to a receiver (typically an element of a monitor or other display device). 
A use of the TMDS serial link is the “Digital Video Interface” interface (“DVI” link) adopted by the Digital Display Working Group.  It will be described with reference to FIG.  1.  A DVI link can be implemented to include two TMDS links (which share a common conductor pair for transmitting a video clock signal) or one TMDS link, as well as additional control lines between the transmitter and receiver.  The DVI link of FIG.  1 includes transmitter 1, receiver 3, and the following conductors between the transmitter and receiver: four conductor pairs (Channel 0, Channel 1, and Channel 2 for video data, and Channel C for a video clock signal), Display Data Channel (“DDC”) lines for bidirectional communication between the transmitter and a monitor associated with the receiver in accordance with the conventional Display Data Channel standard (the Video Electronics Standard Association's “Display Data Channel Standard,” Version 2, Rev. 0, dated Apr. 9, 1996), a Hot Plug Detect (HPD) line (on which the monitor transmits a signal that enables a processor associated with the transmitter to identify the monitor's presence), Analog lines (for transmitting analog video to the receiver), and Power lines (for providing DC power to the receiver and a monitor associated with the receiver).  The Display Data Channel standard specifies a protocol for bidirectional communication between a transmitter and a monitor associated with a receiver, including transmission by the monitor of an Extended Display Identification (“EDID”) message that specifies various characteristics of the monitor, and transmission by the transmitter of control signals for the monitor.  Transmitter 1 includes three identical encoder/serializer units (units 2, 4, and 5) and additional circuitry (not shown).  Receiver 3 includes three identical recovery/decoder units (units 8, 10, and 12) and inter-channel alignment circuitry 14 connected as shown, and additional circuitry (not shown). 
As shown in FIG.  1, circuit 2 encodes the data to be transmitted over Channel 0, and serializes the encoded bits.  Similarly, circuit 4 encodes the data to be transmitted over Channel 1 (and serializes the encoded bits), and circuit 6 encodes the data to be transmitted over Channel 2 (and serializes the encoded bits).  Each of circuits 2, 4, and 6 responds to a control signal (an active high binary control signal referred to as a “data enable” or “DE” signal) by selectively encoding either digital video words (in response to DE having a high value) or a control or synchronization signal pair (in response to DE having a low value).  Each of encoders 2, 4, and 6 receives a different pair of control or synchronization signals: encoder 2 receives horizontal and vertical synchronization signals (HSYNC and VSYNC); encoder 4 receives control bits CTL0 and CTL1; and encoder 6 receives control bits CTL2 and CTL3.  Thus, each of encoders 2, 4, and 6 generates in-band words indicative of video data (in response to DE having a high value), encoder 2 generates out-of-band words indicative of the values of HSYNC and VSYNC (in response to DE having a low value), encoder 4 generates out-of-band words indicative of the values of CTL0 and CTL1 (in response to DE having a low value), and encoder 6 generates out-of-band words indicative of the values of CTL2 and CTL3 (in response to DE having a low value).  In response to DE having a low value, each of encoders 4 and 6 generates one of four specific out-of-band words indicative of the values 00, 01, 10, or 11, respectively, of control bits CTL0 and CTL1 (or CTL2 and CTL3). 
It has been proposed to encrypt video data transmitted over a TMDS-like link.  For example, it has been proposed to use a cryptographic protocol known as “High-bandwidth Digital Content Protection” (“HDCP”) to encrypt digital video to be transmitted over a DVI link and to decrypt the data at the DVI receiver.  A DVI transmitter implementing HDCP outputs a 24-bit bus, known as cout[23:0], during the video active period (i.e. when DE is high).  This 24-bit cout data is “Exclusive Ored” (in logic circuitry in the transmitter) with the 24-bit RGB video data input to the transmitter in order to encrypt the video data.  The encrypted data is then encoded (according to the TMDS standard) for transmission.  The same cout data is also generated in the receiver.  After the encoded and encrypted data received at the receiver undergoes TMDS decoding, the cout data is processed together with the decoded video in logic circuitry in order to decrypt the decoded data and recover the original input video data. 
Before the transmitter begins to transmit HDCP encrypted, encoded video data, the transmitter and receiver communicate bidirectionally with each other to execute an authentication protocol (to verify that the receiver is authorized to receive protected content, and to establish shared secret values for use in encryption of input data and decryption of transmitted encrypted data).  After the receiver has been authenticated, the transmitter calculates the initial set of encryption keys (for encrypting the first line of input video data) in response to a control signal (identified as “ctl3” in FIG.  2) and sends the control signal ctl3 to the receiver (during each vertical blanking period, when DE is low) to cause the receiver to calculate an initial set of decryption keys (for decrypting the first received and decoded line of transmitted video data).  As shown in the timing diagram of FIG.  2, the control signal “ctl3” is a single high-going pulse.  The FIG.  3 circuit responds to each rising edge of control signal “ctl3” (which occurs during each vertical blanking interval).  As also shown in the timing diagram of FIG.  2, following generation of the initial set of encryption/decryption keys, each of the transmitter and receiver performs a re-keying operation during each blanking (vertical or horizontal) interval to generate a new set of keys for encrypting (or decrypting) the next line of video data, and actual encryption of input video data (or decryption of received, decoded video data) is performed using the latest set of keys only when DE is high (not during the blanking intervals). 
Each of the transmitter and receiver includes an HDCP cipher circuit (sometimes referred to herein as an “HDCP cipher”) of the type shown in FIG.  3.  The HDCP cipher includes linear feedback shift register (LFSR) module 80, block module 81 coupled to the output of LFSR module 80, and output module 82 coupled to an output of block module 81.  LFSR module 80 is employed to re-key block module 81 in response to each assertion of an enable signal (the signal “ReKey” shown in FIG.  3), using a session key (Ks) and frame key (Ki).  Block module 81 generates (and provides to module 80) the key Ks at the start of a session and generates (and applies to module 80) a new value of key Ki at the start of each frame of video data (in response to a rising edge of the control signal “ctl3,” which occurs in the first vertical blanking interval of a frame) The signal “ReKey” is asserted to module 81 at each falling edge of the DE signal (i.e., at the start of each vertical and each horizontal blanking interval), and at the end of a brief initialization period (during which module 81 generates updated values of keys Ks and Ki) after each rising edge of signal “ctl3.” 
Module 80 consists of four linear feedback shift registers (having different lengths) and combining circuitry coupled to the shift registers and configured to assert a single output bit per clock interval to block module 81 during each of a fixed number of clock cycles (e.g., 56 cycles) commencing on each assertion of the signal “ReKey” when DE is high (i.e., at the start of each line of video data).  This output bit stream is employed by block module 81 to re-key itself just prior to the start of each line of video data. 
Block module 81 comprises two halves, “Round Function K” and “Round Function B,” as shown in FIG.  4.  Round Function K includes 28-bit registers Kx, Ky, and Kz, seven S-Boxes (each a 4 input bit by 4 output bit S-Box including a look-up table) collectively labeled “S-Box K” in FIG.  4, and linear transformation unit K, connected as shown.  Round Function B includes 28-bit registers Bx, By, and Bz, seven S-Boxes (each a 4 input bit by 4 output bit S-Box including a look-up table) collectively labeled “S-Box B” in FIG.  4, and linear transformation unit B, connected as shown.  Round Function K and Round Function B are similar in design, but Round Function K performs one round of a block cipher per clock cycle to assert a different pair of 28-bit round keys (Ky and Kz) each clock cycle in response to the output of LFSR module 80, and Round Function B performs one round of a block cipher per clock cycle, in response to each 28-bit round key Ky from Round Function K and the output of LFSR module 80, to assert a different pair of 28-bit round keys (By and Bz) each clock cycle.  The transmitter generates value An at the start of the authentication protocol and the receiver responds to it during the authentication procedure.  The value An is used to randomize the session key.  Block module 81 operates in response to the authentication value (An) and an initialization value (Mi, also referred to as an integrity verification key) which is updated by output module 82 at the start of each frame (at each rising edge of the control signal “ctl3”). 
Each of linear transformation units K and B outputs 56 bits per clock cycle.  These output bits are the combined outputs of eight diffusion networks in each transformation unit.  Each diffusion network of linear transformation unit K produces seven output bits in response to seven of the current output bits of registers Ky and Kz.  Each of four of the diffusion networks of linear transformation unit B produces seven output bits in response to seven of the current output bits of registers By, Bz, and Ky, and each of the four other diffusion networks of linear transformation unit B produces seven output bits in response to seven of the current output bits of registers By and Bz. 
In Round Function K, one bit of register Ky takes its input from the bit stream asserted by module 80 when the ReKey signal is asserted.  In Round Function B, one bit of register By takes its input from the bit stream asserted by module 80 when the ReKey signal is asserted. 
Output module 82 performs a compression operation on the 28-bit keys (By, Bz, Ky and Kz) asserted to it (a total of 112 bits) by module 81 during each clock cycle, to generate one 24-bit block of pseudo-random bits cout[23:0] per clock cycle.  Each of the 24 output bits of module 82 consists of the exclusive OR (“XOR”) of nine terms as follows: (B0*K0)+(B1*K1)+(B2*K2)+(B3*K3)+(B4*K4)+(B5*K5)+(B6*K6)+(B7)+(K7), where “*” denotes a logical AND operation and “+” denotes a logical XOR operation.  FIG.  5 specifies the input values B0-B7 and K0-K7 in the preceding expression for generating each of the 24 output bits of module 82.  For example, FIG.  5 indicates that in order to generate output bit 0 (i.e., cout(0)), B0 is the seventeenth bit of register Bz, K0 is the third bit of register Kz, B1 is the twenty-sixth bit of register Bz, and so on. 
In the transmitter, logic circuitry 83 (shown in FIG.  3) receives each 24-bit block of cout data and each input 24-bit RGB video data word, and performs a bitwise XOR operation thereon in order to encrypt the video data, thereby generating a word of the “data_encrypted” data indicated in FIG.  3.  Typically, the encrypted data subsequently undergoes TMDS encoding before it is transmitted to a receiver.  In the receiver, logic circuitry 83 (shown in FIG.  3) receives each 24-bit block of cout data and each recovered 24-bit RGB video data word (after the recovered data has undergone TMDS decoding), and performs a bitwise XOR operation thereon in order to decrypt the recovered video data. 
Throughout the specification and in the claims the expression “TMDS-like link” will be used to denote a serial link, capable of transmitting digital video data (and a clock for the digital video data) from a transmitter to a receiver, and optionally also transmitting one or more additional signals (bidirectionally or unidirectionally) between the transmitter and receiver, that is or includes either a TMDS link or a link having some but not all of the characteristics of a TMDS link. 
There are several conventional TMDS-like links.  One type of TMDS-like link is the set of serial links known as Low Voltage Differential Signaling (“LVDS”) links (e.g., “LDI,” the LVDS Display Interface), each of which satisfies the TIA/EIA-644 standard or the IEEE-1596.3 standard.  In each system including an LVDS link, the data are sent on a high speed differential link with a synchronous clock.  There is a single clock line with a four to three duty cycle and several different combinations of data lines depending on the data rate and bit depth.  An LVDS link is a serial and differential video link, but the video data transmitted over an LVDS link is not encoded. 
Other TMDS-like links encode input video data and other data to be transmitted into encoded words comprising more bits than the incoming data using a coding algorithm other than the specific algorithm used in a TMDS link, and transmit the encoded video data as in-band characters and the other encoded data as out-of-band characters.  The characters need not be classified as in-band or out-of-band characters based according to whether they satisfy transition minimization and DC balance criteria.  Rather, other classification criteria could be used.  An example of an encoding algorithm, other than that used in a TMDS link but which could be used in a TMDS-like link, is IBM 8b10b coding.  The classification (between in-band and out-of-band characters) need not be based on just a high or low number of transitions.  For example, the number of transitions of each of the in-band and out-of-band characters could (in some embodiments) be in a single range (e.g., a middle range defined by a minimum and a maximum number of transitions). 
The data transmitted between the transmitter and receiver of a TMDS-like link can, but need not, be transmitted differentially (over a pair of conductors).  Although the differential nature of TMDS is important in some applications, it is contemplated that some TMDS-like links will transmit data other than differential data.  Also, although a TMDS link has four differential pairs (in the single pixel version), three for video data and the other for a video clock, a TMDS-like link could have a different number of conductors or conductor pairs. 
The primary data transmitted by a TMDS link is video data.  What is often significant about this is that the video data is not continuous, and instead has blanking intervals.  These blanking intervals provide an opportunity (exploited in some embodiments of the present invention) for auxiliary data to be transported, and they represent unused bandwidth.  However, many TMDS-like serial links do not transmit data having blanking intervals, and thus do not encode input data (for transmission) in response to a data enable signal.  For example, the audio serial links known as 12S and S/PDIF transmit continuous data. 
The expression “auxiliary data” is used in a broad sense herein to denote digital audio data or any other type of data other than video data and timing information for video data (e.g., a video clock).  For example, timing information for audio data (e.g., a clock for recovering transmitted audio data) falls within the scope of “auxiliary data.”  Other examples of “auxiliary data” transmitted in accordance with the invention include computer keyboard signals, still image data (generated by a camera, for example), text data, control signals for a power supply, picture in picture data, monitor control information (audio volume, brightness, power state), control signals for indicator lights on a monitor or keyboard, non-audio or video control information, etc. 
The term “stream” of data, as used herein, denotes that all the data are of the same type and is transmitted with the same clock frequency.  The term “channel,” as used herein, refers to that portion of a TMDS-like link that is employed to transmit data (e.g., a particular conductor or conductor pair between the transmitter and receiver over which the data are transmitted, and specific circuitry within the transmitter and/or receiver used for transmitting and/or recovery of the data) and to the technique employed to transmit the data over the link.  Because it is desirable to transmit many different streams of auxiliary data in important applications of the invention, preferred embodiments of the invention provide multiple channels for transmission of auxiliary data, including channels for transmission of auxiliary data in both directions over the link (that is, with and against the direction of the video data).  In some implementations, a channel is employed to transmit one stream of auxiliary data.  In other implementations, a channel is employed to transmit more than one stream of auxiliary data.