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The DNSCrypt protocol

Document Type Active Internet-Draft (individual)
Author Frank Denis
Last updated 2024-02-07
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Network Working Group                                           F. Denis
Internet-Draft                                    Individual Contributor
Intended status: Informational                           7 February 2024
Expires: 10 August 2024

                         The DNSCrypt protocol


   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

   Source for this draft and an issue tracker can be found at

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

   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 August 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 . . . . . . . . . . . . .   7
   8.  Client Queries Over UDP . . . . . . . . . . . . . . . . . . .   7
   9.  Padding For Client Queries Over TCP . . . . . . . . . . . . .   8
   10. Client Queries Over TCP . . . . . . . . . . . . . . . . . . .   8
   11. Authenticated Encryption And Key Exchange Algorithm . . . . .   9
   12. Certificates  . . . . . . . . . . . . . . . . . . . . . . . .   9
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  12
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   15. Operational Considerations  . . . . . . . . . . . . . . . . .  12
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   17. Appendix 1: The Box-XChaChaPoly Algorithm . . . . . . . . . .  13
     17.1.  HChaCha20  . . . . . . . . . . . . . . . . . . . . . . .  13
     17.2.  Test Vector For The HChaCha20 Block Function . . . . . .  14
     17.3.  ChaCha20_DJB . . . . . . . . . . . . . . . . . . . . . .  15
     17.4.  XChaCha20_DJB  . . . . . . . . . . . . . . . . . . . . .  16
     17.5.  XChaCha20_DJB-Poly1305 . . . . . . . . . . . . . . . . .  16
     17.6.  The Box-XChaChaPoly Algorithm  . . . . . . . . . . . . .  16
   18. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The document defines the DNSCrypt protocol, which 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.

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   DNS packets do not need to be parsed or 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.

   DNSCrypt is stateless.  Every query can be processed independently
   from other queries.  There are no session identifiers.  In order to
   better defend against fingerprinting, 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").

   Recursive DNS servers can accept DNSCrypt queries on the same IP
   address and port used for regular DNS traffic.  Similarly, DNSCrypt
   and DoH can also share the same IP address and TCP port.

   Lastly, DNSCrypt mitigates two common security vulnerabilities in
   regular DNS over UDP: amplification and fragmentation attacks.

2.  Conventions And Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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>

   *  <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.

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   *  <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> is determined by the
      chosen encryption algorithm.

   *  AE: the authenticated encryption function.

   *  <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

   *  <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>)

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   *  <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.

   *  <resolver-response-pad>: the variable-length padding.

3.  Protocol Overview

   The DNSCrypt protocol operates through the following steps:

   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 transmitted to the server via standard DNS transport

   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 client's nonce included
       in the response, and the client's original 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

4.  Key Management

   Both clients and resolvers generate short-term key pairs for each
   encryption system they support.

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   Clients generate unique key pairs for each resolver they communicate
   with, while resolvers create individual key pairs for every client
   they interact with.  Additionally, the resolver creates a public key
   for each encryption system it supports.

5.  Session Establishment

   From the client's perspective, a DNSCrypt session is initiated when
   the client sends an unauthenticated DNS query to a DNSCrypt-capable
   resolver.  This DNS query contains encoded information about the
   certificate versions supported by the client and a public identifier
   of the desired provider.

   The resolver sends back a collection of signed certificates that the
   client MUST verify using the pre-distributed 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.

   Resolvers have the ability to support various algorithms and can
   concurrently advertise multiple short-term public keys (resolver
   public keys).  The client picks the one with the highest serial
   number among the currently valid ones that match a supported protocol

   Every certificate contains a unique magic number that the client MUST
   include at the beginning of their queries.  This allows the resolver
   to identify which certificate the client selected for crafting a
   particular 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.

   With the knowledge of the chosen certificate and corresponding secret
   key, along with the client's public key, the resolver is able to
   verify, decrypt the query, and then encrypt the response utilizing
   identical parameters.

6.  Transport

   The DNSCrypt protocol can use the UDP and TCP transport protocols.
   DNSCrypt clients and resolvers SHOULD support the protocol via UDP,
   and MUST support it over TCP.

   Both TCP and UDP connections using DNSCrypt SHOULD employ port 443 by

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7.  Padding For Client Queries Over UDP

   Before encryption takes place, queries are padded according to the
   ISO/IEC 7816-4 standard.  Padding begins with a single byte holding
   the value 0x80, succeeded by any number of NUL bytes.

   <client-query> <client-query-pad> MUST be at least <min-query-len>
   bytes.  In this context, <client-query> represents the original
   client query, while <client-query-pad> denotes the added padding.

   Should the client query's length fall short of <min-query-len> bytes,
   the padding length MUST be adjusted in order to satisfy the length

   <min-query-len> is a variable length, initially set to 256 bytes, and
   MUST be a multiple of 64 bytes.  It represents the minimum permitted
   length for a client query, inclusive of padding.

8.  Client Queries Over UDP

   UDP-based client queries need to follow the padding guidelines
   outlined in section 3.

   Each UDP packet MUST hold one query, with the complete content
   comprising the <dnscrypt-query> structure specified in section 2.

   UDP packets employing the DNSCrypt protocol have the capability to be
   split into distinct IP packets sharing the same source port.

   Upon receiving a query, the resolver may choose to either disregard
   it or send back a response encrypted using DNSCrypt.

   The client MUST authenticate and, if authentication succeeds, decrypt
   the response with the help of the resolver's public key, the shared
   secret, and the obtained nonce.  In case the response fails
   verification, it MUST be disregarded by the client.

   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>)

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   <min-query-len> denotes the minimum permitted length for a client
   query, including padding.  That value 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

   Queries MUST undergo padding using the ISO/IEC 7816-4 format before
   being encrypted.  The padding starts with a byte valued 0x80 followed
   by a variable number of NUL bytes.

   The length of <client-query-pad> is selected randomly, ranging from 1
   to 256 bytes, including the initial byte valued at 0x80.  The total
   length of <client-query> <client-query-pad> MUST be a multiple of 64

   For example, an originally unpadded 56-bytes DNS query can be padded

   <56-bytes-query> 0x80 0x00 0x00 0x00 0x00 0x00 0x00 0x00


   <56-bytes-query> 0x80 (0x00 * 71)


   <56-bytes-query> 0x80 (0x00 * 135)


   <56-bytes-query> 0x80 (0x00 * 199)

10.  Client Queries Over TCP

   The sole differences between encrypted client queries transmitted via
   TCP and those sent using UDP lie in the padding length calculation
   and the inclusion of a length prefix, represented as two big-endian

   In contrast, cleartext DNS query payloads do not necessitate a length
   prefix, regardless of whether they are transmitted via TCP.

   Unlike UDP queries, a query sent over TCP can be shorter than the

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   After having received a response from the resolver, the client and
   the resolver MUST close the TCP connection to ensure security and
   comply with this revision of the protocol, which prohibits multiple
   transactions over the same TCP connection.

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.

   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

   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 necessitates the use of
   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)

   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.

   Randomly selecting the resolver's portion of the nonce is

   The client's half of the nonce MAY include a timestamp in addition to
   a counter or to random bytes.  Incorporating a timestamp allows for
   prompt elimination of responses to queries that were sent too long
   ago or are dated in the future.  This practice enhances security and
   prevents potential replay attacks.

12.  Certificates

   To initiate a DNSCrypt session, a client transmits an ordinary
   unencrypted TXT DNS query to the resolver's IP address and DNSCrypt
   port.  The attempt is first made using UDP; if unsuccessful due to
   failure, timeout, or truncation, the client then proceeds with TCP.

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   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:

   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

   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 single provider public key can be employed 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

   It is RECOMMENDED that certificates are signed using specialized
   hardware rather than directly on the resolvers themselves.  Once
   signed, resolvers SHOULD make these certificates available to
   clients.  Signing certificates on dedicated hardware helps ensure
   security and integrity, as it isolates the process from potential
   vulnerabilities present in the resolver's system.

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   A successful response to a 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

   *  <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
      confusion with the QUIC protocol.

   *  <serial>: a 4-byte serial number in big-endian format.  If more
      than one certificate is 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 verification of the signature MUST
      include the extensions.  An implementation not supporting these
      extensions MUST ignore them.

   Certificates made of this information, without extensions, are 116
   bytes long.  With the addition of <cert-magic>, <es-version>, and
   <protocol-minor-version>, the record is 124 bytes long.

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   After receiving 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:

   *  The current certificate is not present or not valid anymore,


   *  A certificate with a higher serial number than the current one is

13.  Implementation Status

   _This note is to be removed before publishing as an RFC._

   Multiple implementations of the protocol described in this document
   have been developed and verified for interoperability.

   A comprehensive list of known implementations can be found at

14.  Security Considerations

   DNSCrypt does not protect against attacks on DNS infrastructure.

15.  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.

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   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.  This 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

   Provider public keys MAY be published as DNSSEC-signed TXT records,
   in the same zone as the provider name.  For example, a query for the
   TXT type on the name "" may return a signed
   record containing a hexadecimal-encoded provider public key for the
   provider name "".

   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.

16.  IANA Considerations

   This document has no IANA actions.

17.  Appendix 1: The Box-XChaChaPoly Algorithm

   The Box-XChaChaPoly algorithm combines the X25519 [RFC7748] key
   exchange mechanism with a variant of the ChaCha20-Poly1305
   construction specified in [RFC8439].

17.1.  HChaCha20

   HChaCha20 is an intermediate step based on the construction and
   security proof used to create XSalsa20, an extended-nonce Salsa20

   HChaCha20 is initialized in 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):

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                     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.
   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.

17.2.  Test Vector For The HChaCha20 Block Function

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   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

17.3.  ChaCha20_DJB

   ChaCha20 was originally designed to have a 8 byte nonce.

   For the needs of TLS, [RFC8439] changed this by setting N_MIN and
   N_MAX to 12, at the expense of a smaller internal counter.

   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.

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   The internal counter in ChaCha20_DJB is 4 bytes larger than ChaCha20.
   There are no other differences between ChaCha20_DJB and ChaCha20.

17.4.  XChaCha20_DJB

   XChaCha20_DJB can be constructed from an existing ChaCha20
   implementation and the HChaCha20 function.

   All that needs to be done 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.

17.5.  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 the ciphertext
   encrypted with XChaCha20_DJB.

   The Poly1305 key is computed as in [RFC8439], by encrypting an empty

   Finally, the output of the Poly1305 function is prepended to the

   *  <k>: encryption key

   *  <m>: message to encrypt

   *  <ct>: XChaCha20_DJB(<k>, <m>)

   *  XChaCha20_DJB-Poly1305(<k>, <m>): Poly1305(<ct>) || <ct>

17.6.  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

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   *  <sk>: sender's secret key

   *  <sk'>: HChaCha20(X25519(<pk>, <sk>))

   *  Box-XChaChaPoly(pk, sk, m): XChaCha20_DJB-Poly1305(<sk'>, <m>)

18.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <>.

   [RFC8439]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 8439, DOI 10.17487/RFC8439, June 2018,

Author's Address

   Frank Denis
   Individual Contributor

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