A Perfect Forward Secure Extension to Oblivious HTTP

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Table of Contents

1. Introduction

Oblivious HTTP ([OHTTP]) is a protocol for forwarding encrypted HTTP messages. It does not provide Perfect Forward Secrecy (PFS). Chunked OHTTP ([CHUNKED]) expands OHTTP to be suitable for longer-lived streams, but still does not offer PFS.¶

Unfortunately, providing a streaming abstraction over OHTTP makes it an attractive tool to provide privacy. This is leading application designers to build Remote Procedure Call (RPC) systems over this bidirectional stream, without realizing the security cost of losing PFS.¶

This document proposes a solution that offers PFS to all data sent over OHTTP apart from the client's first flight. This provides privacy and security properties similar to TLS 0-RTT (see Section 2.3 of [TLS]) run over HTTP CONNECT (see Section 9.3.6 of [HTTP]) without losing the performance nor request-correlation-prevention properties of OHTTP. This mechanism is designed to be backwards compatible with unextended OHTTP.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document uses SerializePublicKey(), DeserializePublicKey(), GenerateKeyPair(), SetupBaseS(), SetupBaseR(), Export(), Seal(), and Open() from [HPKE].¶

3. Mechanism

This mechanism relies on the generation of two more ephemeral key pairs per OHTTP request: one for the client (denoted skC, pkC) and one for the gateway (not denoted since it only exists inside of SetupBaseS()).¶

The client starts by generating a second ephemeral key pair, using the same KEM it has selected for this request:¶

skC, pkC = GenerateKeyPair()

The client then adds the serialized public key SerializePublicKey(pkC) to its Binary HTTP ([BHTTP]) request headers using the "OHTTP-PFS" header. That header is a Structured Header Field Item of type Byte Sequence as defined in Section 3.3.5 of [STRUCTURED]. For example:¶

ohttp-pfs: :dGhpcyBpcyBhIHB1YmxpYyBrZXk=:

The client then encrypts the Binary HTTP request following the procedure in Section 4.3 of [OHTTP]. The gateway follows the procedure in that same section to recover the Binary HTTP request.¶

The gateway then checks the request for the presence of the "ohttp-pfs" header to determine whether this extension is in use. If it is, it uses DeserializePublicKey() to parse the key then uses the HPKE receiver context (rctxt) from the OHTTP request as follows:¶

1. Export a secret, req_secret, from rctxt, using the string "OHTTP PFS Request Derivation" as the exporter_context parameter to context.Export; see Section 5.3 of [HPKE]. The length of this secret is Nsecret, the length of the shared secret for the KEM associated with context.¶

2. Build info2 by concatenating the ASCII-encoded string "OHTTP PFS Response", a zero byte, the values of key_id, kem_id, kdf_id, and aead_id, as one 8-bit integer and three 16-bit integers, respectively, each in network byte order, and req_secret.¶

3. Create a sending HPKE context by invoking SetupBaseS() (Section 5.1.1 of [HPKE]) with the public key of the client pkE and info2. This yields the context sctxt2 and an encapsulation key enc2.¶

4. Encrypt response by invoking the Seal() method on sctxt2 (Section 5.2 of [HPKE]) with empty associated data aad, yielding ciphertext ct2.¶

5. Concatenate enc2 and ct2, yielding an Encapsulated Response, enc_response.¶

In pseudocode, this procedure is as follows:¶

req_secret = rctxt.Export("OHTTP PFS Request Derivation", Nsecret)
info2 = concat(encode_str("OHTTP PFS Response"),
               encode(1, 0),
               encode(1, key_id),
               encode(2, kem_id),
               encode(2, kdf_id),
               encode(2, aead_id),
               req_secret)
enc2, sctxt2 = SetupBaseS(pkC, info2)
ct2 = sctxt2.Seal("", response)
enc_response = concat(enc2, ct2)

The client then reverses this process to extract the response. To decapsulate an Encapsulated Response, enc_response, it uses the HPKE sender context (sctxt) from the OHTTP request as follows:¶

1. Parses enc_request into enc2 and ct2 (indicated using the function parse() in pseudocode). Note that enc2 is of fixed-length, so there is no ambiguity in parsing this structure.¶

2. Export a secret, req_secret, from sctxt, using the string "OHTTP PFS Request Derivation" as the exporter_context parameter to context.Export; see Section 5.3 of [HPKE]. The length of this secret is Nsecret, the length of the shared secret for the KEM associated with context.¶

3. Build info2 by concatenating the ASCII-encoded string "OHTTP PFS Response", a zero byte, the values of key_id, kem_id, kdf_id, and aead_id, as one 8-bit integer and three 16-bit integers, respectively, each in network byte order, and req_secret.¶

4. Create a receiving HPKE context, rctxt2, by invoking SetupBaseR() (Section 5.1.1 of [HPKE]) with skE, enc2, and info2.¶

5. Decrypt ct2 by invoking the Open() method on rctxt2 (Section 5.2 of [HPKE]), with an empty associated data aad, yielding response or an error on failure.¶

In pseudocode, this procedure is as follows:¶

enc2, ct2 = parse(enc_response)
req_secret = sctxt.Export("OHTTP PFS Request Derivation", Nsecret)
info2 = concat(encode_str("OHTTP PFS Response"),
               encode(1, 0),
               encode(1, key_id),
               encode(2, kem_id),
               encode(2, kdf_id),
               encode(2, aead_id),
               req_secret)
rctxt2 = SetupBaseR(enc2, skE, info2)
response, error = rctxt2.Open("", ct2)

4. Media Type

This mechanism uses a media type to differentiate the response from OHTTP responses without this extension. The "message/ohttp-res-pfs" is used to indicate that this extension is in use for the response.¶

Gateways MAY respond with "message/ohttp-res" responses and use the response encoding described in Section 4.2 of [OHTTP] if it does not PFS to be useful for the response. This allows saving computational resources, for example, if the response is a generic response with an error status code where no part of the response is tied to information in the request.¶

If chunked OHTTP [CHUNKED] is in use, the response media type for this extension is "message/ohttp-chunked-res-pfs".¶

5. Chunked OHTTP

Chunked OHTTP ([CHUNKED]) is different in that the client can send more data after its first flight.¶

For the reponse, all chunks are encrypted by the gateway using the sctxt2 context and decrypted by the client using the rctxt2 context, and using the AAD construction described in [CHUNKED].¶

For the request, there is a distinction between the first flight and subsequent chunks. The first chunks are encrypted using sctxt as described in [CHUNKED]. The client keeps using sctxt to send subsequent chunks until is receives the first chunk from the gateway. Once the client receives that, it creates rctxt2 (see Section 3), and performs the following steps to switch to it for sending:¶

1. Encrypt an empty chunk, the encrypted last first flight chunk elff_chunk with sctxt using empty associated data aad. Send this chunk with a variable-length integer length prefix.¶

2. Use that as the associated data aad to encrypt the first PFS chunk sealed_pfs_chunk1 using sctxt2. Send that chunk with a variable-length integer length prefix.¶

3. Encrypt all subsequent non-final chunks with an empty AAD and a variable-length integer length prefix, using sctxt2.¶

4. Encrypt the final chunk with the AAD set to the ASCII-encoded string "final" and prefixed with a zero length, using sctxt2.¶

In pseudocode, this procedure is as follows:¶

elff_chunk = sctxt.Seal("", "")
elff_chunk_len = varint_encode(len(elff_chunk))
send(concat(elff_chunk_len, elff_chunk))
sealed_pfs_chunk1 = sctxt2.Seal(elff_chunk, pfs_chunk1)
sealed_pfs_chunk1_len = varint_encode(len(sealed_pfs_chunk1))
send(concat(sealed_pfs_chunk1_len, sealed_pfs_chunk1))
sealed_pfs_chunk2 = sctxt2.Seal("", pfs_chunk2)
sealed_pfs_chunk_len = varint_encode(len(sealed_pfs_chunk))
send(concat(sealed_pfs_chunk_len, sealed_pfs_chunk2))
...
sealed_final_chunk = sctxt2.Seal("final", chunk)
send(concat(varint_encode(0), sealed_final_chunk))

The gateway needs to handle this transition. It initially handles chunks using the procedure from [CHUNKED]. When the gateway decrypts a chunk with a zero-length plaintext, this means that the client is transitioning to the PFS context (zero-length plaintext non-final chunks are explicitly disallowed in chunked OHTTP without PFS, see Section 6 of [CHUNKED]).¶

When the gateway decrypts its first zero-length plaintext using the rctxt context, it acts on the ciphertext elff_chunk as follows:¶

1. Save elff_chunk until the next chunk is received.¶

2. Wait for and parse the next variable-length integer length-prefixed chunk, sealed_pfs_chunk1.¶

3. Decrypt sealed_pfs_chunk1 using rctxt2 with the associated data aad set to elff_chunk.¶

4. Keep decrypting variable-length integer length-prefixed chunks with the associated data aad empty.¶

5. When encoutering a variable-length integer length-prefix set to zero, decrypt the final chunk using the associated data aad set to the ASCII-encoded string "final".¶

6. Security Considerations

The security considerations described in Section 6 of [OHTTP] and Section 7 of [CHUNKED] apply to this document as well.¶

7. IANA Considerations

8. References

Acknowledgments

Thank you to Martin Thomson for asking me to write this draft, and to Chris Wood for supporting it more than two years before it was written.¶

Author's Address