Original text:
2. <del> For each new subscription that the User Agent generates for an application, it also generates an asymmetric key pair for use in Diffie-Hellman (DH) DH or elliptic-curve Diffie-Hellman (ECDH) ECDH. The public key for this key pair can then be distributed by the application to the Application Server along with the URI of the subscription.  The private key MUST remain secret. </del>  <del> This key pair is used with the Diffie-Hellman key exchange as described in Section 4.2 of I-D.ietf-httpbis-encryption-encoding. </del>  <del> A User Agent MUST generate and provide a public key for the scheme described in mti. </del>  <del> The public key MUST be accompanied by a key identifier that can be used in the "keyid" parameter to identify which key is in use.  Key identifiers need only be unique within the context of a subscription. </del>  2.1. <del> As described in I-D.ietf-httpbis-encryption-encoding, use of Diffie- Hellman for key agreement requires that the receiver provide clear information about its chosen group and the format for the "dh" parameter with each potential sender. </del>  <del> This document only describes a single ECDH group and point format, described in mti.  A specification that defines alternative groups or formats MUST provide a means of indicating precisely which group and format is in use for every public key that is provided. </del>  <del> 2.2. </del>  <del> The application using the subscription distributes the key identifier and public key along with other subscription information, such as the subscription URI and expiration time. </del>  <del> The communication medium by which an application distributes the key identifier and public key MUST be confidentiality protected for the reasons described in I-D.ietf-webpush-protocol.  Most applications that use push messaging have a pre-existing relationship with an Application Server.  Any existing communication mechanism that is authenticated and provides confidentiality and integrity, such as HTTPS RFC2818, is sufficient. </del>  <del> 2.3. </del>  To ensure that push messages are correctly authenticated, a symmetric authentication secret is added to the information generated by a User Agent.  The authentication secret is mixed into the key derivation process described in I-D.ietf-httpbis-encryption-encoding. <del> The authentication secret ensures that exposure or leakage of the DH public key - which, as a public key, is not necessarily treated as a secret - does not enable an adversary to generate valid push messages. A User Agent MUST generate and provide a hard to guess sequence of </del>  octets that is used for authentication of push messages.  This SHOULD be generated by a cryptographically strong random number generator <del> RFC4086 and be at least 16 octets long. </del>  <del> 3. </del>  <del> An Application Server that has the public key, group and format information plus the authentication secret can encrypt a message for the User Agent. </del>  <del> 3.1. </del>  <del> The Application Server generates a new DH or ECDH key pair in the same group as the value generated by the User Agent. </del>  <del> From the newly generated key pair, the Application Server performs a DH or ECDH computation with the public key provided by the User Agent to find the input keying material for key derivation.  The Application Server then generates 16 octets of salt that is unique to the message.  A random RFC4086 salt is acceptable. </del>  <del> Web push uses the authentication secret defined in Section 4.3 of I- D.ietf-httpbis-encryption-encoding.  This authentication secret (see auth) is generated by the user agent and shared with the application server. </del>  <del> 3.2. </del>  <del> The Application Server then encrypts the payload.  Header fields are populated with base64url encoded RFC7515 values: </del>  <del> the salt is added to the "salt" parameter of the Encryption header field; and </del>  <del> the public key for its DH or ECDH key pair is placed in the "dh" parameter of the Crypto-Key header field. </del>  <del> An application server MUST encrypt a push message with a single </del>  record.  This allows for a minimal receiver implementation that handles a single record.  If the message is 4096 octets or longer, the "rs" parameter MUST be set to a value that is longer than the encrypted push message length. <del> Note that a push service is not required to support more than 4096 octets of payload body, which equates to 4077 octets of cleartext, so the "rs" parameter can be omitted for messages that fit within this limit. </del>  <del> An application server MUST NOT use other content encodings for push </del>  messages.  In particular, content encodings that compress could result in leaking of push message contents.  The Content-Encoding <del> header field therefore has exactly one value, which is "aesgcm128". Multiple "aesgcm128" values are not permitted. </del>  <del> An application server MUST include exactly one entry in the </del>  Encryption field, and at most one entry having a "dh" parameter in the Crypto-Key field.  This allows the "keyid" parameter to be omitted from both header fields. <del> An application server MUST NOT include an "aesgcm128" parameter in the Encryption header field. </del>  <del> 4. </del>  <del> A User Agent decrypts messages are decrypted as described in I- D.ietf-httpbis-encryption-encoding.  The authentication secret described in derivation is used in key derivation. </del>  <del> Note that the value of the "keyid" parameter is used to identify the correct share, if there are multiple values for the Crypto-Key header field. </del>  <del> A receiver is not required to support multiple records.  Such a receiver MUST check that the record size is large enough to contain the entire payload body in a single record.  The "rs" parameter MUST NOT be exactly equal to the length of the payload body minus the length of the authentication tag (16 octets); that length indicates that the message has been truncated. </del>  <del> 5. </del>  <del> User Agents MUST expose an elliptic curve Diffie-Hellman share on the P-256 curve FIPS186. </del>  <del> Public keys, such as are encoded into the "dh" parameter, MUST be in the form of an uncompressed point as described in X.692 (that is, a 65 octet sequence that starts with a 0x04 octet). </del>  <del> The label for this curve is the string "P-256" encoded in ASCII (that is, the octet sequence 0x50, 0x2d, 0x32, 0x35, 0x36). </del>  <del> 6. </del>  <del> This document has no IANA actions. </del>  7.

Reviewer comments:
In httpwg/http-extensions I have proposed the removal of the Diffie-Hellman pieces. This PR adopts the parts that were removed from the base spec that webpush needs. In the process, I've had to make a bunch of changes. It loses a bit of generality, which makes things easier to follow, which I've tried to capitalize on by being a lot more explicit about the overall process. . NAME NAME NAME would you mind reviewing this change to see that I haven't messed something up?

Revised text:
2.  <ins> Encrypting a push message uses elliptic-curve Diffie-Hellman (ECDH) ECDH on the P-256 curve FIPS186 to establish a shared secret (see dh) and a symmetric secret for authentication (see auth). </ins>  <ins> A User Agent generates an ECDH key pair and authentication secret that it associates with each subscription it creates.  The ECDH public key and the authentication secret are sent to the Application Server with other details of the push subscription. </ins>  <ins> When sending a message, an Application Server generates an ECDH key pair and a random salt.  The ECDH public key is encoded into the "dh" parameter of the Crypto-Key header field; the salt is encoded into the "salt" parameter of the Encryption header field.  The ECDH key pair can be discarded after encrypting the message. </ins>  <ins> The content of the push message is encrypted or decrypted using a content encryption key and nonce that is derived using all of these inputs and the process described in encryption. </ins> 2.1.  <ins> The application using the subscription distributes the subscription public key and authentication secret to an authorized Application Server.  This could be sent along with other subscription information that is provided by the User Agent, such as the push subscription URI. An application MUST use an authenticated, confidentiality protected communications medium for this purpose.  In addition to the reasons described in I-D.ietf-webpush-protocol, this ensures that the authentication secret is not revealed to unauthorized entities, which can be used to generate push messages that will be accepted by the User Agent. Most applications that use push messaging have a pre-existing relationship with an Application Server.  Any existing communication mechanism that is authenticated and provides confidentiality and integrity, such as HTTPS RFC2818, is sufficient. 3. Push message encryption happens in four phases: The input keying material used for deriving the content encryption keys used for the push message is derived using elliptic-curve Diffie-Hellman ECDH (dh). This is then combined with the application secret to produce the input keying material used in I-D.ietf-httpbis-encryption-encoding (combine). </ins>  <ins> A content encryption key and nonce are derived using the process in I-D.ietf-httpbis-encryption-encoding with an expanded context string (context). </ins>  <ins> Encryption or decryption follows according to I-D.ietf-httpbis- encryption-encoding. </ins>  <ins> The key derivation process is summarized in summary.  Restrictions on the use of the encrypted content coding are described in restrict. </ins>  <ins> 3.1. For each new subscription that the User Agent generates for an Application, it also generates a P-256 FIPS186 key pair for use in elliptic-curve Diffie-Hellman (ECDH) ECDH. When sending a push message, the Application Server also generates a new ECDH key pair on the same P-256 curve. </ins>  <ins> The ECDH public key for the Application Server is included in the "dh" parameter of the Crypto-Key header field (see iana).  The uncompressed point form defined in X9.62 (that is, a 65 octet sequence that starts with a 0x04 octet) is encoded using base64url RFC7515 to produce the "dh" parameter value. An Application combines its ECDH private key with the public key provided by the User Agent using the process described in ECDH; on receipt of the push message, a User Agent combines its private key with the public key provided by the Application Server in the "dh" parameter in the same way.  These operations produce the same value for the ECDH shared secret. 3.2. </ins> To ensure that push messages are correctly authenticated, a symmetric authentication secret is added to the information generated by a User Agent.  The authentication secret is mixed into the key derivation process described in I-D.ietf-httpbis-encryption-encoding.  <ins> A User Agent MUST generate and provide a hard to guess sequence of 16 </ins> octets that is used for authentication of push messages.  This SHOULD be generated by a cryptographically strong random number generator  <ins> RFC4086. </ins>  <ins> 3.3. </ins>  <ins> The shared secret produced by ECDH is combined with the authentication secret using HMAC-based key derivation function (HKDF) described in RFC5869.  This produces the input keying material used by I-D.ietf-httpbis-encryption-encoding. </ins>  <ins> The HKDF function uses SHA-256 hash algorithm FIPS180-4 with the following inputs: </ins>  <ins> the authentication secret </ins>  <ins> the shared secret derived using ECDH </ins>  <ins> the ASCII-encoded string "Content-Encoding: auth" with a terminal zero octet </ins>  <ins> 32 octets (i.e., the output is the length of the underlying SHA-256 HMAC function output) </ins>  <ins> 3.4. </ins>  <ins> The derivation of the content encryption key and nonce uses an additional context string. </ins>  <ins> The context is comprised of a label of "P-256" encoded in ASCII (that is, the octet sequence 0x50, 0x2d, 0x32, 0x35, 0x36), a zero-valued octet, the length of the User Agent public key (65 octets) encoded as a two octet unsigned integer in network byte order, the User Agent public key, the length of the Application Server public key (65 octets), and the Application Server public key. </ins>  <ins> 3.5. This results in a the final content encryption key and nonce generation using the following sequence, which is shown here in pseudocode with HKDF expanded into separate discrete steps using HMAC with SHA-256: Note that this omits the exclusive OR of the final nonce with the record sequence number, since push messages contain only a single record (see restrict) and the sequence number of the first record is zero. 4. An Application Server MUST encrypt a push message with a single </ins> record.  This allows for a minimal receiver implementation that handles a single record.  If the message is 4096 octets or longer, the "rs" parameter MUST be set to a value that is longer than the encrypted push message length.  <ins> A push service is not required to support more than 4096 octets of payload body (see Section 7.2 of I-D.ietf-webpush-protocol), which equates to 4077 octets of cleartext, so the "rs" parameter can be omitted for messages that fit within this limit. </ins>  <ins> An Application Server MUST NOT use other content encodings for push </ins> messages.  In particular, content encodings that compress could result in leaking of push message contents.  The Content-Encoding  <ins> header field therefore has exactly one value, which is "aesgcm". Multiple "aesgcm" values are not permitted. </ins>  <ins> An Application Server MUST include exactly one entry in the </ins> Encryption field, and at most one entry having a "dh" parameter in the Crypto-Key field.  This allows the "keyid" parameter to be omitted from both header fields.  <ins> An Application Server MUST NOT include an "aesgcm" parameter in the Encryption header field. </ins>  <ins> A User Agent is not required to support multiple records.  A User Agent MAY ignore the "rs" parameter.  If a record size is size is present, but unchecked, decryption will fail with high probability for all valid cases.  Decryption will also succeed if the push message contains a single record from a longer truncated message. Given that an Application Server is prohibited from generating such a message, this is not considered a serious risk. </ins>  <ins> 5. </ins>  <ins> The following example shows a push message being sent to a push service. </ins>  <ins> This example shows the ASCII encoded string, "I am the walrus".  The content body is shown here encoded in URL-safe base64url for presentation reasons only.  Line wrapping of the "dh" parameter is added for presentation purposes. </ins>  <ins> Since there is no ambiguity about which keys are being used, the "keyid" parameter is omitted from both the Encryption and Crypto-Key header fields.  The keys shown below use uncompressed points X9.62 encoded using base64url. </ins>  <ins> The sender's private key used in this example is "nCScek-QpEjmOOlT- rQ38nZzvdPlqa00Zy0i6m2OJvY".  Intermediate values for this example are included in ex-intermediate. </ins>  <ins> 6. </ins>  <ins> This document defines the "dh" parameter for the Crypto-Key header field in the "Hypertext Transfer Protocol (HTTP) Crypto-Key Parameters" registry defined in I-D.ietf-httpbis-encryption-encoding. </ins>  <ins> Parameter Name: dh Purpose: The "dh" parameter contains a Diffie-Hellman share which is used to derive the input keying material used in "aesgcm" content coding. </ins>  <ins> Reference: this document. </ins> 7.