
DNSCrypt version 2 protocol specification
=========================================

1. Protocol overview
--------------------

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.

Both the client and the resolver initially generate a short-term key
pair for each supported encryption system.

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.

2. Common 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.

<encrypted-query> ::= AE(<shared-key> <client-nonce> <client-nonce-pad>,
                         <client-query> <client-query-pad>)

AE ::= the authenticated encryption algorithm.

<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> ::= variable-length padding.

3. 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.

<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.

4. 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.

5. 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.

The length of <client-query-pad> is randomly chosen between 0 and 256
bytes, 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)

6. Client queries over TCP
--------------------------

Client queries over TCP only differ from queries sent over UDP by the
padding length computation.

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.

7. Common definitions for resolver responses
--------------------------------------------

<dnscrypt-response> ::= <resolver-magic> <nonce> <encrypted-response>

<resolver-magic> ::= 0x72 0x36 0x66 0x6e 0x76 0x57 0x6a 0x38

<nonce> ::= <client-nonce> <resolver-nonce>

<client-nonce> ::= the nonce sent by the client in the related query.

<client-pk> ::= the client's secret key.

<resolver-sk> ::= the resolver's public 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.

<encrypted-response> ::= AE(<shared-key>, <nonce>,
                            <resolver-response> <resolver-response-pad>)

AE ::= the authenticated encryption algorithm.

<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> ::= variable-length padding.

8. Padding for resolver responses
---------------------------------

Prior to encryption, responses 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.

The total length of <resolver-response> <resolver-response-pad> must
be a multiple of 64 bytes.

The length of <resolver-response-pad> must be between 0 and 256 bytes,
and must be constant for a given (<resolver-sk>, <client-nonce>) tuple.

A pseudo random function can be used to satisfy this requirement.

9. Resolver responses over UDP
------------------------------

The resolver must verify and decrypt client queries. Queries that
cannot be verified must be ignored.

Any client-supplied nonce must be accepted. However, a resolver can
ignore or refuse queries encrypted using untrusted public keys.

Responses must be padded using the algorithm described in section 8
and encrypted as described in section 7.

If the full client query length is shorter than 256 bytes, or shorter
than the full response length, the resolver may truncate the response
and set the TC flag prior to encrypting it. The response length should
always be equal to or shorter than the initial client query length.

10. Resolver responses over TCP
-------------------------------

The resolver must verify and decrypt client queries. Queries that
cannot be verified must be ignored.

Any client-supplied nonce must be accepted. However, a resolver can
ignore or refuse queries encrypted using untrusted public keys.

Responses must be padded using the algorithm described in section 8
and encrypted as described in section 7.

Responses must be send unmodified even if their length exceeds the
length of the client query.

11. Authenticated encryption and key exchange algorithm
-------------------------------------------------------

The X25519-XSalsa20Poly1305 construction, and the way to use it
described in this section, must be referenced in certificates as
version 1 of the public-key authenticated encryption system.

In the current version, the construction, originally implemented in
the NaCl cryptographic library and exposed under the name
"crypto_box", uses the Curve25119 elliptic curve in Montgomery form
and the hsalsa20 hash function for key exchange, the XSalsa20 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 X25519-XSalsa20Poly1305, 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 (0) 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 must follow this scheme:

<provider name> ::= <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

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 X25519-XSalsa20Poly1305, <es-version> must be 0x00 0x01.
For X25519-Chacha20Poly1305, <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>.

<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 4-byte
unsigned Unix timestamp.

<ts-end> ::= the date the certificate is valid until (inclusive), as a
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 the current certificate is not present or not valid
any more.

The client shouldn't switch to a new certificate if the current one is
still valid and still present in the set of returned certificates,
even if the new set includes certificates with a higher serial number
than the current one.

13. Operational perspective
---------------------------

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. 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 that do not keep query logs should rotate the short-term key pair
every 24 hours, and must throw away the previous secret key.

During a key rotation, and provided that the old key hasn't been
compromised, a resolver shoud accept both the old and the new key for at
least 4 hours, and public them as different certificates.

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".

While authenticated and unauthenticated queries can share the same
resolver TCP and/or UDP port, this should be avoided. Client magic
numbers do not completely prevent collisions with legitimate unauthenticated
DNS queries. In addition, DNSCrypt offers some mitigation against
abusing resolvers to conduct DDoS attacks. Accepting unauthenticated
queries on the same port would defeat this mechanism.

As this version of the protocol may require many short-lived TCP
sessions, the TCP Fast Open mechanism should be used where available.

14. Implementations
-------------------

Known open source implementations of the DNSCrypt version 2 protocol are:

- dnscrypt-wrapper - server-side implementation in C
- powerdns dnsdist - a DNS loadbalancer that provides server-side DNSCrypt
- dnscrypt-proxy - client-side implementation in C
- pcap_dnsproxy - client-side implementation in C++
- dnscrypt-python - client-side implementation in Python

Known closed source implementations of the protocol :

- OpenNIC Fusl servers - server-side implementation as a Linux kernel module
