The DNSCrypt protocol
draft-denis-dprive-dnscrypt-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Author | Frank Denis | ||
| Last updated | 2023-03-09 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-denis-dprive-dnscrypt-00
Network Working Group F. Denis
Internet-Draft Individual Contributor
Intended status: Informational 9 March 2023
Expires: 10 September 2023
The DNSCrypt protocol
draft-denis-dprive-dnscrypt-00
Abstract
The DNSCrypt protocol is designed to encrypt and authenticate DNS
traffic between clients and resolvers. This document specifies the
protocol and its implementation.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-denis-dprive-dnscrypt/.
Source for this draft and an issue tracker can be found at
https://github.com/DNSCrypt/dnscrypt-protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
Denis Expires 10 September 2023 [Page 1]
Internet-Draft DNSCrypt March 2023
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Protocol overview . . . . . . . . . . . . . . . . . . . . . . 5
4. Key management . . . . . . . . . . . . . . . . . . . . . . . 5
5. Session Establishment . . . . . . . . . . . . . . . . . . . . 6
6. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Padding for client queries over UDP . . . . . . . . . . . . . 6
8. Client queries over UDP . . . . . . . . . . . . . . . . . . . 7
9. Padding for client queries over TCP . . . . . . . . . . . . . 7
10. Client queries over TCP . . . . . . . . . . . . . . . . . . . 8
11. Authenticated encryption and key exchange algorithm . . . . . 8
12. Certificates . . . . . . . . . . . . . . . . . . . . . . . . 9
13. Security considerations . . . . . . . . . . . . . . . . . . . 12
14. Operational considerations . . . . . . . . . . . . . . . . . 12
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
16. Appendix 1: The Box-XChaChaPoly algorithm . . . . . . . . . . 13
16.1. HChaCha20 . . . . . . . . . . . . . . . . . . . . . . . 13
16.2. Test Vector for the HChaCha20 Block Function . . . . . . 14
16.3. ChaCha20_DJB . . . . . . . . . . . . . . . . . . . . . . 14
16.4. XChaCha20_DJB . . . . . . . . . . . . . . . . . . . . . 15
16.5. XChaCha20_DJB-Poly1305 . . . . . . . . . . . . . . . . . 15
16.6. The Box-XChaChaPoly algorithm . . . . . . . . . . . . . 16
17. Normative References . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The document defines a specific protocol, DNSCrypt, that encrypts and
authenticates DNS [RFC1035] queries and responses, improving
confidentiality, integrity, and resistance to attacks affecting the
original DNS protocol.
The protocol is designed to be lightweight, extensible, and simple to
implement securely on top of an existing DNS client, server or proxy.
DNS packets don't need to be parsed nor rewritten. DNSCrypt simply
wraps them in a secure, encrypted container. Encrypted packets are
then exchanged the same way as regular packets, using the standard
DNS transport mechanisms. Queries and responses are sent over UDP,
falling back to TCP for large responses only if necessary.
Denis Expires 10 September 2023 [Page 2]
Internet-Draft DNSCrypt March 2023
DNSCrypt is stateless. Every query can be processed independently
from other queries. There are no session identifiers. Clients can
replace their keys whenever they want, without extra interactions
with servers.
DNSCrypt packets can securely be proxied without having to be
decrypted, allowing client IP addresses to be hidden from resolvers
("Anonymized DNSCrypt").
A recursive DNS server can accept DNSCrypt queries on the same IP
address and port as regular DNS. Similarly, DNSCrypt and DoH can
also share the same IP address and TCP port.
Finally, DNSCrypt addresses two security issues inherent to regular
DNS over UDP: amplification and fragment attacks.
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.
Definitions for client queries:
* <dnscrypt-query>: <client-magic> <client-pk> <client-nonce>
<encrypted-query>
* <client-magic>: a 8 byte identifier for the resolver certificate
chosen by the client.
* <client-pk>: the client's public key, whose length depends on the
encryption algorithm defined in the chosen certificate.
* <client-sk>: the client's secret key.
* <resolver-pk>: the resolver's public key.
* <client-nonce>: a unique query identifier for a given (<client-
sk>, <resolver-pk>) tuple. The same query sent twice for the same
(<client-sk>, <resolver-pk>) tuple must use two distinct <client-
nonce> values. The length of <client-nonce> depends on the chosen
encryption algorithm.
* AE: the authenticated encryption function.
Denis Expires 10 September 2023 [Page 3]
Internet-Draft DNSCrypt March 2023
* <encrypted-query>: AE(<shared-key> <client-nonce> <client-nonce-
pad>, <client-query> <client-query-pad>)
* <shared-key>: the shared key derived from <resolver-pk> and
<client-sk>, using the key exchange algorithm defined in the
chosen certificate. -<client-query>: the unencrypted client query.
The query is not modified; in particular, the query flags are not
altered and the query length must be kept in queries prepared to
be sent over TCP.
* <client-nonce-pad>: <client-nonce> length is half the nonce length
required by the encryption algorithm. In client queries, the
other half, <client-nonce-pad> is filled with NUL bytes.
* <client-query-pad>: the variable-length padding.
Definitions for server responses:
* <dnscrypt-response>: <resolver-magic> <nonce> <encrypted-response>
* <resolver-magic>: the 0x72 0x36 0x66 0x6e 0x76 0x57 0x6a 0x38 byte
sequence
* <nonce>: <client-nonce> <resolver-nonce>
* <client-nonce>: the nonce sent by the client in the related query.
* <client-pk>: the client's public key.
* <resolver-sk>: the resolver's secret key.
* <resolver-nonce>: a unique response identifier for a given
(<client-pk>, <resolver-sk>) tuple. The length of <resolver-
nonce> depends on the chosen encryption algorithm.
* DE: the authenticated decryption function.
* <encrypted-response>: DE(<shared-key>, <nonce>, <resolver-
response> <resolver-response-pad>)
* <shared-key>: the shared key derived from <resolver-sk> and
<client-pk>, using the key exchange algorithm defined in the
chosen certificate.
* <resolver-response>: the unencrypted resolver response. The
response is not modified; in particular, the query flags are not
altered and the response length must be kept in responses prepared
to be sent over TCP.
Denis Expires 10 September 2023 [Page 4]
Internet-Draft DNSCrypt March 2023
* <resolver-response-pad>: the variable-length padding.
3. Protocol overview
The protocol operates as follows:
1. The DNSCrypt client sends a DNS query to a DNSCrypt server to
retrieve the server's public keys.
2. The client generates its own key pair.
3. The client encrypts unmodified DNS queries using a server's
public key, padding them as necessary, and concatenates them to a
nonce and a copy of the client's public key. The resulting
output is sent using standard DNS transport mechanisms.
4. Encrypted queries are decrypted by the server using the attached
client public key and the server's own secret key. The output is
a regular DNS packet that doesn't require any special processing.
5. To send an encrypted response, the server adds padding to the
unmodified response, encrypts the result using the client's
public key and the client's nonce, and truncates the response if
necessary. The resulting packet, truncated or not, is sent to
the client using standard DNS mechanisms.
6. The client authenticates and decrypts the response using its
secret key, the server's public key, the attached nonce, and its
own nonce. If the response was truncated, the client may adjust
internal parameters and retry over TCP. If not, the output is a
regular DNS response that can be directly forwarded to
applications and stub resolvers.
4. Key management
Both the client and the resolver initially generate a short-term key
pair for each supported encryption system.
The client generates a key pair for each resolver it communicates
with, and the resolver generates a key pair for each client it
communicates with. The resolver also generates a public key for each
supported encryption system.
Denis Expires 10 September 2023 [Page 5]
Internet-Draft DNSCrypt March 2023
5. Session Establishment
From a client perspective, a DNSCrypt session begins with the client
sending a non-authenticated DNS query to a DNSCrypt-enabled resolver.
This DNS query encodes the certificate versions supported by the
client, as well as a public identifier of the provider requested by
the client.
The resolver responds with a public set of signed certificates that
must be verified by the client using a previously distributed public
key, known as the provider public key. Each certificate includes a
validity period, a serial number, a version that defines a key
exchange mechanism, an authenticated encryption algorithm and its
parameters, as well as a short-term public key, known as the resolver
public key.
A resolver can support multiple algorithms and advertise multiple
resolver public keys simultaneously. The client picks the one with
the highest serial number among the currently valid ones that match a
supported protocol version.
Each certificate includes a magic number that the client must prefix
its queries with, in order for the resolver to know what certificate
was chosen by the client to construct a given query.
The encryption algorithm, resolver public key, and client magic
number from the chosen certificate are then used by the client to
send encrypted queries. These queries include the client public key.
Using this client public key, and knowing which certificate was
chosen by the client as well as the relevant secret key, the resolver
verifies and decrypts the query and encrypts the response using the
same parameters.
6. Transport
The DNSCrypt protocol can use the UDP and TCP transport protocols.
DNSCrypt Clients and resolvers should support the protocol over UDP
and must support it over TCP.
The default port for this protocol should be 443, both for TCP and
UDP.
7. Padding for client queries over UDP
Prior to encryption, queries are padded using the ISO/IEC 7816-4
format. The padding starts with a byte valued 0x80 followed by a
variable number of NUL bytes.
Denis Expires 10 September 2023 [Page 6]
Internet-Draft DNSCrypt March 2023
<client-query> <client-query-pad> must be at least <min-query-len>
bytes. If the length of the client query is less than <min-query-
len>, the padding length must be adjusted in order to satisfy this
requirement.
<min-query-len> is a variable length, initially set to 256 bytes, and
must be a multiple of 64 bytes.
8. Client queries over UDP
Client queries sent using UDP must be padded as described in section
3.
A UDP packet can contain a single query, whose entire content is the
<dnscrypt-query> construction documented in section 2.
UDP packets using the DNSCrypt protocol can be fragmented into
multiple IP packets and can use a single source port.
After having received a query, the resolver can either ignore the
query or reply with a DNSCrypt-encapsulated response.
The client must verify and decrypt the response using the resolver's
public key, the shared secret and the received nonce. If the
response cannot be verified, the response must be discarded.
If the response has the TC flag set, the client must:
1. send the query again using TCP
2. set the new minimum query length as:
<min-query-len> ::= min(<min-query-len> + 64, <max-query-len>)
<min-query-len> must be capped so that the full length of a DNSCrypt
packet doesn't exceed the maximum size required by the transport
layer.
The client may decrease <min-query-len>, but the length must remain a
multiple of 64 bytes.
9. Padding for client queries over TCP
Prior to encryption, queries are padded using the ISO/IEC 7816-4
format. The padding starts with a byte valued 0x80 followed by a
variable number of NUL bytes.
Denis Expires 10 September 2023 [Page 7]
Internet-Draft DNSCrypt March 2023
The length of <client-query-pad> is randomly chosen between 1 and 256
bytes (including the leading 0x80), but the total length of <client-
query> <client-query-pad> must be a multiple of 64 bytes.
For example, an originally unpadded 56-bytes DNS query can be padded
as:
<56-bytes-query> 0x80 0x00 0x00 0x00 0x00 0x00 0x00 0x00
or
<56-bytes-query> 0x80 (0x00 * 71)
or
<56-bytes-query> 0x80 (0x00 * 135)
or
<56-bytes-query> 0x80 (0x00 * 199)
10. Client queries over TCP
Encrypted client queries over TCP only differ from queries sent over
UDP by the padding length computation and by the fact that they are
prefixed with their length, encoded as two big-endian bytes.
Cleartext DNS query payloads are not prefixed by their length, even
when sent over TCP.
Unlike UDP queries, a query sent over TCP can be shorter than the
response.
After having received a response from the resolver, the client and
the resolver must close the TCP connection. Multiple transactions
over the same TCP connections are not allowed by this revision of the
protocol.
11. Authenticated encryption and key exchange algorithm
The Box-XChaChaPoly construction, and the way to use it described in
this section, must be referenced in certificates as version 2 of the
public-key authenticated encryption system.
Denis Expires 10 September 2023 [Page 8]
Internet-Draft DNSCrypt March 2023
The construction, originally implemented in the libsodium
cryptographic library and exposed under the name
"crypto_box_curve25519xchacha20poly1305", uses the Curve25119
elliptic curve in Montgomery form and the hchacha20 hash function for
key exchange, the XChaCha20 stream cipher, and Poly1305 for message
authentication.
The public and secret keys are 32 bytes long in storage. The MAC is
16 bytes long, and is prepended to the ciphertext.
When using Box-XChaChaPoly, this construction requires a 24 bytes
nonce, that must not be reused for a given shared secret.
With a 24 bytes nonce, a question sent by a DNSCrypt client must be
encrypted using the shared secret, and a nonce constructed as
follows: 12 bytes chosen by the client followed by 12 NUL (0x00)
bytes.
A response to this question must be encrypted using the shared
secret, and a nonce constructed as follows: the bytes originally
chosen by the client, followed by bytes chosen by the resolver.
The resolver's half of the nonce should be randomly chosen.
The client's half of the nonce can include a timestamp in addition to
a counter or to random bytes, so that when a response is received,
the client can use this timestamp to immediately discard responses to
queries that have been sent too long ago, or dated in the future.
12. Certificates
The client begins a DNSCrypt session by sending a regular unencrypted
TXT DNS query to the resolver IP address, on the DNSCrypt port, first
over UDP, then, in case of failure, timeout or truncation, over TCP.
Resolvers are not required to serve certificates both on UDP and TCP.
The name in the question (<provider name) must follow this scheme:
<protocol-major-version> . dnscrypt-cert . <zone>
A major protocol version has only one certificate format.
A DNSCrypt client implementing the second version of the protocol
must send a query with the TXT type and a name of the form:
2.dnscrypt-cert.example.com
Denis Expires 10 September 2023 [Page 9]
Internet-Draft DNSCrypt March 2023
The zone must be a valid DNS name, but may not be registered in the
DNS hierarchy.
A single provider name can be shared by multiple resolvers operated
by the same entity, and a resolver can respond to multiple provider
names, especially to support multiple protocol versions
simultaneously.
In order to use a DNSCrypt-enabled resolver, a client must know the
following information:
* The resolver IP address and port
* The provider name
* The provider public key
The provider public key is a long-term key whose sole purpose is to
verify the certificates. It is never used to encrypt or verify DNS
queries. A unique provider public key can be used to sign multiple
certificates.
For example, an organization operating multiple resolvers can use a
unique provider name and provider public key across all resolvers,
and just provide a list of IP addresses and ports. Each resolver may
have its unique set of certificates that can be signed with the same
key.
Certificates should be signed on dedicated hardware and not on the
resolvers. Resolvers must serve the certificates, provided that they
have already been signed.
A successful response to certificate request contains one or more TXT
records, each record containing a certificate encoded as follows:
* <cert>: <cert-magic> <es-version> <protocol-minor-version>
<signature> <resolver-pk> <client-magic> <serial> <ts-start> <ts-
end> <extensions>
* <cert-magic>: 0x44 0x4e 0x53 0x43
* <es-version>: the cryptographic construction to use with this
certificate.
For Box-XChaChaPoly, <es-version> must be 0x00 0x02.
* <protocol-minor-version>: 0x00 0x00
Denis Expires 10 September 2023 [Page 10]
Internet-Draft DNSCrypt March 2023
* <signature>: a 64-byte signature of (<resolver-pk> <client-magic>
<serial> <ts-start> <ts-end> <extensions>) using the Ed25519
algorithm and the provider secret key. Ed25519 must be used in
this version of the protocol.
* <resolver-pk>: the resolver short-term public key, which is 32
bytes when using X25519.
* <client-magic>: the first 8 bytes of a client query that was built
using the information from this certificate. It may be a
truncated public key. Two valid certificates cannot share the
same <client-magic>.
* <client-magic> must not start with 0x00 0x00 0x00 0x00 0x00 0x00
0x00 (seven all-zero bytes) in order to avoid a confusion with the
QUIC protocol.
* <serial>: a 4 byte serial number in big-endian format. If more
than one certificates are valid, the client must prefer the
certificate with a higher serial number.
* <ts-start>: the date the certificate is valid from, as a big-
endian 4-byte unsigned Unix timestamp.
* <ts-end>: the date the certificate is valid until (inclusive), as
a big-endian 4-byte unsigned Unix timestamp.
* <extensions>: empty in the current protocol version, but may
contain additional data in future revisions, including minor
versions. The computation and the verification of the signature
must include the extensions. An implementation not supporting
these extensions must ignore them.
Certificates made of these information, without extensions, are 116
bytes long. With the addition of the cert-magic, es-version and
protocol-minor-version, the record is 124 bytes long.
After having received a set of certificates, the client checks their
validity based on the current date, filters out the ones designed for
encryption systems that are not supported by the client, and chooses
the certificate with the higher serial number.
DNSCrypt queries sent by the client must use the <client-magic>
header of the chosen certificate, as well as the specified encryption
system and public key.
The client must check for new certificates every hour, and switch to
a new certificate if:
Denis Expires 10 September 2023 [Page 11]
Internet-Draft DNSCrypt March 2023
* the current certificate is not present or not valid any more
or
* a certificate with a higher serial number than the current one is
available.
13. Security considerations
DNSCrypt does not protect against attacks on DNS infrastructure.
14. Operational considerations
Special attention should be paid to the uniqueness of the generated
secret keys.
Client public keys can be used by resolvers to authenticate clients,
link queries to customer accounts, and unlock business-specific
features such as redirecting specific domain names to a sinkhole.
Resolvers accessible from any client IP address can also opt for only
responding to a set of whitelisted public keys.
Resolvers accepting queries from any client must accept any client
public key. In particular, an anonymous client can generate a new
key pair for every session, or even for every query.
his mitigates the ability for a resolver to group queries by client
public keys, and discover the set of IP addresses a user might have
been operating.
Resolvers must rotate the short-term key pair every 24 hours at most,
and must throw away the previous secret key.
After a key rotation, a resolver must still accept all the previous
keys that haven't expired.
Provider public keys may be published as a DNSSEC-signed TXT records,
in the same zone as the provider name.
For example, a query for the TXT type on the name
"2.pubkey.example.com" may return a signed record containing a
hexadecimal-encoded provider public key for the provider name
"2.dnscrypt-cert.example.com".
Denis Expires 10 September 2023 [Page 12]
Internet-Draft DNSCrypt March 2023
As a client is likely to reuse the same key pair many times, servers
are encouraged to cache shared keys instead of performing the X25519
operation for each query. This makes the computational overhead of
DNSCrypt negligible compared to plain DNS.
15. IANA Considerations
This document has no IANA actions.
16. Appendix 1: The Box-XChaChaPoly algorithm
The Box-XChaChaPoly algorithm combines the X25519 [RFC7748] key
exchange mechanism with a variant of the ChaCha20-Poly1305
constrution defined in [RFC8439].
16.1. HChaCha20
HChaCha20 is an intermediary step based on the construction and
security proof used to create XSalsa20, an extended-nonce Salsa20
variant.
HChaCha20 is initialized the same way as the ChaCha20 cipher defined
in [RFC8439], except that HChaCha20 uses a 128-bit nonce and has no
counter. Instead, the block counter is replaced by the first 32 bits
of the nonce.
Consider the two figures below, where each non-whitespace character
represents one nibble of information about the ChaCha states (all
numbers little-endian):
cccccccc cccccccc cccccccc cccccccc
kkkkkkkk kkkkkkkk kkkkkkkk kkkkkkkk
kkkkkkkk kkkkkkkk kkkkkkkk kkkkkkkk
bbbbbbbb nnnnnnnn nnnnnnnn nnnnnnnn
ChaCha20 State: c=constant k=key b=blockcount n=nonce
cccccccc cccccccc cccccccc cccccccc
kkkkkkkk kkkkkkkk kkkkkkkk kkkkkkkk
kkkkkkkk kkkkkkkk kkkkkkkk kkkkkkkk
nnnnnnnn nnnnnnnn nnnnnnnn nnnnnnnn
HChaCha20 State: c=constant k=key n=nonce
After initialization, proceed through the ChaCha rounds as usual.
Denis Expires 10 September 2023 [Page 13]
Internet-Draft DNSCrypt March 2023
Once the 20 ChaCha rounds have been completed, the first 128 bits and
last 128 bits of the ChaCha state (both little-endian) are
concatenated, and this 256-bit subkey is returned.
16.2. Test Vector for the HChaCha20 Block Function
o Key = 00:01:02:03:04:05:06:07:08:09:0a:0b:0c:0d:0e:0f:10:11:12:13:
14:15:16:17:18:19:1a:1b:1c:1d:1e:1f. The key is a sequence of
octets with no particular structure before we copy it into the
HChaCha state.
o Nonce = (00:00:00:09:00:00:00:4a:00:00:00:00:31:41:59:27)
After setting up the HChaCha state, it looks like this:
61707865 3320646e 79622d32 6b206574
03020100 07060504 0b0a0908 0f0e0d0c
13121110 17161514 1b1a1918 1f1e1d1c
09000000 4a000000 00000000 27594131
ChaCha state with the key setup.
After running 20 rounds (10 column rounds interleaved with 10
"diagonal rounds"), the HChaCha state looks like this:
423b4182 fe7bb227 50420ed3 737d878a
0aa76448 7954cdf3 846acd37 7b3c58ad
77e35583 83e77c12 e0076a2d bc6cd0e5
d5e4f9a0 53a8748a 13c42ec1 dcecd326
HChaCha state after 20 rounds
HChaCha20 will then return only the first and last rows, in little
endian, resulting in the following 256-bit key:
82413b42 27b27bfe d30e4250 8a877d73
a0f9e4d5 8a74a853 c12ec413 26d3ecdc
Resultant HChaCha20 subkey
16.3. ChaCha20_DJB
ChaCha20 was originally designed to have a 8 byte nonce.
For the needs of TLS, [RFC8439] changed this to set N_MIN and N_MAX
to 12, at the expense of a smaller internal counter.
Denis Expires 10 September 2023 [Page 14]
Internet-Draft DNSCrypt March 2023
DNSCrypt uses ChaCha20 as originally specified, with N_MIN = N_MAX =
8.
We refer to this variant as ChaCha20_DJB.
Common implementations may just refer to it as ChaCha20 and the IETF
version as ChaCha20-IETF.
The internal counter in ChaCha20_DJB is 4 bytes larger than ChaCha20.
There are no other differences between ChaCha20_DJB and ChaCha20.
16.4. XChaCha20_DJB
XChaCha20_DJB can be constructed from ChaCha20 implementation and
HChaCha20.
All one needs to do is:
1. Pass the key and the first 16 bytes of the 24-byte nonce to
HChaCha20 to obtain the subkey.
2. Use the subkey and remaining 8 byte nonce with ChaCha20_DJB.
16.5. XChaCha20_DJB-Poly1305
XChaCha20 is a stream cipher and offers no integrity guarantees
without being combined with a MAC algorithm (e.g. Poly1305).
XChaCha20_DJB-Poly1305 adds an authentication tag to ciphertext
encrypted with XChaCha20_DJB.
The Poly1305 key is computed as in [RFC8439], by encrypting an empty
block.
Finally, the output of the Poly1305 function is prepended to the
ciphertext:
* <k>: encryption key
* <m>: message to encrypt
* XChaCha20_DJB-Poly1305(<k>, <m>): Poly1305(XChaCha20_DJB(<k>,
<m>)) || XChaCha20_DJB(<k>, <m>)
Denis Expires 10 September 2023 [Page 15]
Internet-Draft DNSCrypt March 2023
16.6. The Box-XChaChaPoly algorithm
The Box-XChaChaPoly algorithm combines the key exchange mechanism
X25519 defined [RFC7748] with the XChaCha20_DJB-Poly1305
authenticated encryption algorithm.
* <k>: encryption key
* <m>: message to encrypt
* <pk>: recipent's public key
* <sk>: sender's secret key
* sk: HChaCha20(X25519(<pk>, <sk>))
* Box-XChaChaPoly(pk, sk, m): XChaCha20_DJB-Poly1305(<sk>, <m>)
17. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8439] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,
<https://www.rfc-editor.org/info/rfc8439>.
Author's Address
Frank Denis
Individual Contributor
Email: fde@00f.net
Denis Expires 10 September 2023 [Page 16]