Change:
A server encodes one of its assigned server IDs in any CID it generates using the relevant configuration. <del> 4.4. All connection IDs use the following format: Each configuration specifies the length of the Server ID and Nonce fields, with limits defined for each algorithm.  When using a given configuration, the server MUST generate CIDs of length equal to the lengths of these three fields. The Server ID is assigned to each server in accordance with sid- allocation.  Dynamically allocated SIDs are limited to seven octets or fewer.  Statically allocated ones have different limits for each algorithm. The configuration agent assigns a server ID to every server in its pool, and determines a server ID length (in octets) sufficiently large to encode all server IDs, including potential future servers. The Nonce is selected by the server when it generates a CID.  As the name implies, a server MUST use a nonce no more than once when generating a CID for a given server ID and unique set of configuration parameters. The nonce length MUST be at least 4 octets.  Additional limits on its length are different for each algorithm.  See cid-entropy for limits on nonce generation. As QUIC version 1 limits connection IDs to 20 octets, the server ID and nonce lengths MUST sum to 19 octets or less. </del> 5. <del> Encryption in the algorithms below uses the AES-128-ECB cipher. Future standards could add new algorithms that use other ciphers to provide cryptographic agility in accordance with RFC7696.  QUIC-LB implementations SHOULD be extensible to support new algorithms. </del> 5.1. <del> The Plaintext CID Algorithm makes no attempt to obscure the mapping of connections to servers, significantly increasing linkability. </del> <ins> All connection IDs use the following format: </ins> <del> 5.1.1. </del> <ins> 5.2. </ins> <del> See cid-format. </del> <ins> The configuration agent assigns a server ID to every server in its pool in accordance with sid-allocation, and determines a server ID length (in octets) sufficiently large to encode all server IDs, including potential future servers. </ins> <del> 5.1.2. </del> <ins> Each configuration specifies the length of the Server ID and Nonce fields, with limits defined for each algorithm. </ins> <del> On each incoming packet, the load balancer extracts consecutive octets, beginning with the second octet.  These bytes represent the server ID.  It ignores the nonce. </del> <ins> Optionally, it also defines a 16-octet key.  Note that failure to define a key means that observers can determine the assigned server of any connection, significantly increasing the linkability of QUIC address migration. </ins> <del> 5.1.3. </del> <ins> The nonce length MUST be at least 4 octets.  The server ID length MUST be at least 1 octet. </ins> <del> When a server needs a new connection ID, it encodes one of its assigned server IDs in consecutive octets beginning with the second and chooses a nonce.  This nonce MUST appear to be random (see cid- entropy). </del> <ins> As QUIC version 1 limits connection IDs to 20 octets, the server ID and nonce lengths MUST sum to 19 octets or less. </ins> <del> 5.2. </del> <ins> 5.3. </ins> <del> The Encrypted Short CID algorithm provides cryptographic protection at the cost of additional per-packet processing at the load balancer to decrypt every incoming connection ID, unless the load balancer maintains state for the routing information of any given 4-tuple. </del> <ins> The server writes the first octet and its server ID into their respective fields. If there is no key in the configuration, the server MUST fill the Nonce field with bytes that appear to be random.  If there is a key, the server fills the nonce field with a nonce of its choosing.  See cid-entropy for details. The server MAY append additional bytes to the connection ID, up to the limit specified in that version of QUIC, for its own use.  These bytes MUST NOT provide observers with any information that could link two connection IDs to the same connection, client, or server.  In particular, all servers using a configuration MUST consistently add the same length to each connection ID, to preserve the linkability objectives of QUIC-LB.  Any additional bytes SHOULD appear random unless individual servers are not distinguishable (e.g. any server using that configuration appends identical bytes to every connection ID). If there is no key in the configuration, the Connection ID is complete.  Otherwise, there are further steps, as described in the two following subsections. Encryption below uses the AES-128-ECB cipher.  Future standards could add new algorithms that use other ciphers to provide cryptographic agility in accordance with RFC7696.  QUIC-LB implementations SHOULD be extensible to support new algorithms. </ins> <del> 5.2.1. </del> <ins> 5.3.1. </ins> <del> The nonce length MUST be no fewer than 4 octets.  The nonce SHOULD be at least as long as the server ID in order to save the load balancer an encryption pass; see below. </del> <ins> When the nonce length and server ID length sum to exactly 16 octets, the server MUST use a single-pass encryption algorithm.  All connection ID octets except the first form an AES-ECB block.  This block is encrypted once, and the result forms the second through seventeenth most significant bytes of the connection ID. </ins> <del> The configuration agent also selects an 16-octet AES-ECB key to use for connection ID decryption. </del> <ins> 5.3.2. </ins> <del> 5.2.2. </del> <ins> Any other field length requires four passes for encryption and at least three for decryption.  To understand this algorithm, it is useful to define four functions that minimize the amount of bit- shifting necessary in the event that there are an odd number of octets. </ins> <del> When generating a routable connection ID, the server writes arbitrary bits into its nonce octets, and its provided server ID into the server ID octets.  See cid-entropy for nonce generation considerations. </del> <ins> The expand_left() function outputs 16 octets, with its first argument in the most significant bits, its second argument in the least significant byte, and zeros in all other positions.  Thus, </ins> <del> The server encrypts the server ID using the following four pass algorithm, which leverages 128-bit AES Electronic Codebook (ECB) mode, much like QUIC header protection. </del> <ins> expand_right() is similar, except that the second argument is in the most significant byte, and the first argument is in the least significant bits.  Therefore, </ins> <del> In the text below, ^ is the XOR function and || is concatenation. The truncate() function takes the most significant octets of its argument, so that the XOR function operates on fields of the same length.  The expand() function outputs 16 octets, with its first argument in the most significant bits, its second argument in the least significant bits, and zeros in all other positions.  Thus, </del> <ins> Similarly, truncate_left() and truncate_right() take the most significant and least significant bits, respectively, from a ciphertext.  For example, to take 28 bits of a ciphertext: </ins> The example at the end of this section helps to clarify the steps described below.

Discussion:
narrows single-pass encryption to a special-case optimization, and simplifies presentation of the design.
There's a lot of needless bitshifting in the current algorithm. When reading from the right, each byte has to be shifted 4 bits and ORed with the neighboring byte, as currently written. There's an easier way. When encrypting side X (= { left, right }) and XORing with side Y. Set Y to zero. Set the index value (= 1..4) in the furthest byte that has been zeroed. Encrypt to get ciphertext XOR Y with the bits in the same position as Y.
There has quietly been some work on tightening up the QUIC-LB specification. At the moment, we are still short on implementations but I am hearing something might happen soon. Anyway, Christian Huitema has made substantial contributions to the security properties of Stream Cipher CID, which allows smallish CIDs, by making it a three-pass algorithm. We still have the "Block Cipher CID option" which requires CIDs of at least 17 bytes; AFAICT the only advantage at this point is that it can be decoded with 1 block encryption operation instead of three. In principle, QUIC-LB load balancers can be run with no per-connection state, in which case this would be a per-packet operation. I strongly suspect that real LBs will keep some per-4tuple state, as they do today; if so, this crypto operation only needs to occur once per packet where the 4-tuple is new. If so, the CPU impact is vanishingly small except in a storm of garbage packets. So AFAICT, the use case for Block Cipher is as follows: Willing to run one crypto operation per packet/new 4-tuple Not OK with doing three crypto operations satisfied with 17B + CIDs I strongly suspect this does not describe a real implementer, and am inclined to simply delete this in my effort to simplify the design. Nevertheless, I'm taking this to the list in case someone thinks this is an important use case.
I am pretty confident about the properties of the stream cipher, but I am not a cryptographer Should we ask a review from the CFRG?
I implemented block cipher cid in my proxy server.

It is a bit sad to hear that it is going to be removed from the spec, but I have no strong opinion to against that. If it is removed I'll just remove the reference to quic lb spec and call it our own. The good thing of block cipher cid is that its implementation is quite easy. 17 bytes cid has good entropy for random bits to manage several cid per connection.
NAME It's not gone yet! At the very least, we're going to do the crypto review.
FWIW, h2o also uses 17-byte CID backed by AES-ECB. Our server-id is 64-bit. While that might sound a bit large, it is good to have some additional space that can be used for detecting broken packets carrying bogus CIDs. Also, we do not retain 4-tuple state for each connection, because we do not want to retain connection state for all the server cluster. Instead, we decrypt CID of each packet. Our format is different from what is in QUIC-LB (our design predates the first revision of the draft), so we do not mind support for block ciphers getting removed from the document. I'm just pointing out that there are use cases where block ciphers are sufficient.
NAME thanks for the field report. While you're not conforming to the standard today, I hope you'll consider h2o servers eventually supporting the standard (at last optionally) so they can be used with migration with commodity load balancers. Would the lack of a block cipher make a meaningful difference in your willingness to do so?
if you've implemented this, please join the #quic-lb channel and we should do interop once the spec settles down
NAME Implementing the Stream Cipher CID algorithm is easy for the backend servers. IMO the practical concerns are related to load balancers. Each of the three-pass AES-ECB operation has dependency on earlier passes. That means that it is impossible to run these passes in parallel. My napkin calculation tells me that the CPU cycles required for running Stream Cipher CID algorithm on each full-sized packet would cost like 30% of decrypting a full-sized packet. There would be at least two ways to mitigate this problem. One is process multiple packets in parallel. The other is cache information, as you've suggested in this issue.

If the load balancer developers are happy to accept this complexity (for saving few bytes of CID), then I do not think I have a reason to argue against.
Please don't make decisions now about the cost of three passes. The CFRG review might very well ask us to increase the number of passes, especially if the nonce is something like a counter instead of a random number. Consider for example the usage pattern in which after a key rotation all servers in the cluster set their nonce to exactly 1. In effect, that means we are almost down to two passes, and that may arguably be too few. Or maybe not. Review will tell. If I look at the literature, 3 passes is the absolute minimum for achieving an acceptable level of mixing, but there are papers recommending 5, 6 or even 12 passes.
NAME thank you for the caution. Then I wonder if we need to decide the fate of block cipher CIDs, before the design of stream cipher CIDs become more stable.
I am not resolving this issue until the crypto review is complete.
It's unfortunately that routing by will meet some problems, image this scenario: The right way SHOULD be that quic-lb route all packets through CID, then the cpu cost can't be avoid, but I still approve the usage of stream cipher, from a safety point of view.
Good point, I'll consider this when I
reduced the Block CID (now "single-pass encryption") to the special case where noncelen + sidlen == 16.
Given the work in , I'm disinclined to remove this special case. Closing.
That works. The macros are clever, and you may want to explain the cleverness somewhere. It is obvious to me that you want to align "right" and "left" to an even number of bytes, adding a null nibble at the left or right or at the right of left. That will indeed save a lot of bit twiddling. Found an unrelated typo.