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Cargo.lock

@@ -1193,8 +1193,8 @@ dependencies = [
  "once_cell",
  "parking_lot",
  "postgres",
- "postgres-protocol",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
- "postgres-types",
+ "postgres-types 0.2.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
  "postgres_ffi",
  "rand",
  "regex",
@@ -1208,7 +1208,7 @@ dependencies = [
  "tempfile",
  "thiserror",
  "tokio",
- "tokio-postgres",
+ "tokio-postgres 0.7.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
  "tokio-stream",
  "toml_edit",
  "tracing",
@@ -1326,9 +1326,9 @@ dependencies = [
  "fallible-iterator",
  "futures",
  "log",
- "postgres-protocol",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
  "tokio",
- "tokio-postgres",
+ "tokio-postgres 0.7.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
 ]
 
 [[package]]
@@ -1349,6 +1349,24 @@ dependencies = [
  "stringprep",
 ]
 
+[[package]]
+name = "postgres-protocol"
+version = "0.6.1"
+source = "git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d#f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d"
+dependencies = [
+ "base64 0.13.0",
+ "byteorder",
+ "bytes",
+ "fallible-iterator",
+ "hmac 0.10.1",
+ "lazy_static",
+ "md-5",
+ "memchr",
+ "rand",
+ "sha2",
+ "stringprep",
+]
+
 [[package]]
 name = "postgres-types"
 version = "0.2.1"
@@ -1356,7 +1374,17 @@ source = "git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b
 dependencies = [
  "bytes",
  "fallible-iterator",
- "postgres-protocol",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
+]
+
+[[package]]
+name = "postgres-types"
+version = "0.2.1"
+source = "git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d#f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d"
+dependencies = [
+ "bytes",
+ "fallible-iterator",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d)",
 ]
 
 [[package]]
@@ -1421,9 +1449,11 @@ name = "proxy"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "base64 0.13.0",
  "bytes",
  "clap",
  "hex",
+ "hmac 0.10.1",
  "lazy_static",
  "md5",
  "parking_lot",
@@ -1432,8 +1462,10 @@ dependencies = [
  "rustls 0.19.1",
  "serde",
  "serde_json",
+ "sha2",
+ "thiserror",
  "tokio",
- "tokio-postgres",
+ "tokio-postgres 0.7.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d)",
  "zenith_utils",
 ]
 
@@ -2094,8 +2126,30 @@ dependencies = [
  "percent-encoding",
  "phf",
  "pin-project-lite",
- "postgres-protocol",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
- "postgres-types",
+ "postgres-types 0.2.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
+ "socket2",
+ "tokio",
+ "tokio-util",
+]
+
+[[package]]
+name = "tokio-postgres"
+version = "0.7.1"
+source = "git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d#f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d"
+dependencies = [
+ "async-trait",
+ "byteorder",
+ "bytes",
+ "fallible-iterator",
+ "futures",
+ "log",
+ "parking_lot",
+ "percent-encoding",
+ "phf",
+ "pin-project-lite",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d)",
+ "postgres-types 0.2.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d)",
  "socket2",
  "tokio",
  "tokio-util",
@@ -2333,7 +2387,7 @@ dependencies = [
  "hyper",
  "lazy_static",
  "postgres",
- "postgres-protocol",
+ "postgres-protocol 0.6.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
  "postgres_ffi",
  "regex",
  "routerify",
@@ -2343,7 +2397,7 @@ dependencies = [
  "signal-hook",
  "tempfile",
  "tokio",
- "tokio-postgres",
+ "tokio-postgres 0.7.1 (git+https://github.com/zenithdb/rust-postgres.git?rev=9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858)",
  "tracing",
  "walkdir",
  "workspace_hack",

proxy/Cargo.toml

@@ -8,18 +8,23 @@ edition = "2018"
 
 [dependencies]
 anyhow = "1.0"
+base64 = "0.13.0"
 bytes = { version = "1.0.1", features = ['serde'] }
+clap = "2.33.0"
+hex = "0.4.3"
+hmac = "0.10.1"
 lazy_static = "1.4.0"
 md5 = "0.7.0"
-rand = "0.8.3"
-hex = "0.4.3"
 parking_lot = "0.11.2"
+rand = "0.8.3"
+reqwest = { version = "0.11", default-features = false, features = ["blocking", "json", "rustls-tls"] }
+rustls = "0.19.1"
 serde = "1"
 serde_json = "1"
-tokio = { version = "1.11", features = ["macros"] }
+sha2 = "0.9.8"
-tokio-postgres = { git = "https://github.com/zenithdb/rust-postgres.git", rev="9eb0dbfbeb6a6c1b79099b9f7ae4a8c021877858" }
+thiserror = "1.0.30"
-clap = "2.33.0"
+tokio = { version = "1.11", features = ['macros'] }
-rustls = "0.19.1"
+# tokio-postgres = { path = "../../rust-postgres/tokio-postgres" } TODO remove this
-reqwest = { version = "0.11", default-features = false, features = ["blocking", "json", "rustls-tls"] }
+tokio-postgres = { git = "https://github.com/zenithdb/rust-postgres.git", rev = "f1f16657aaebe2b9b4b16ef7abf6dc42301bad5d"}
 
 zenith_utils = { path = "../zenith_utils" }

proxy/src/auth.rs

@@ -0,0 +1,140 @@
+//! Authentication machinery.
+
+use crate::sasl::{SaslFirstMessage, SaslMechanism, SaslMessage, SaslStream};
+use crate::scram::key::ScramKey;
+use crate::scram::{ScramExchangeServer, ScramSecret};
+use anyhow::{bail, Context};
+use zenith_utils::postgres_backend::{PostgresBackend, ProtoState};
+use zenith_utils::pq_proto::{
+    BeAuthenticationSaslMessage as BeSaslMessage, BeMessage as Be, FeMessage as Fe, *,
+};
+
+// TODO: add SCRAM-SHA-256-PLUS
+/// A list of supported SCRAM methods.
+const SCRAM_METHODS: &[&str] = &["SCRAM-SHA-256"];
+
+/// Initial state of [`AuthStream`].
+pub struct Begin;
+
+/// Use [SCRAM](crate::scram)-based auth in [`AuthStream`].
+pub struct Scram<'a>(pub &'a ScramSecret);
+
+/// Use password-based auth in [`AuthStream`].
+pub struct Md5(
+    /// Salt for client.
+    pub [u8; 4],
+);
+
+/// Every authentication selector is supposed to implement this trait.
+pub trait AuthMethod {
+    /// Any authentication selector should provide initial backend message
+    /// containing auth method name and parameters, e.g. md5 salt.
+    fn first_message(&self) -> BeMessage<'_>;
+}
+
+impl AuthMethod for Scram<'_> {
+    #[inline(always)]
+    fn first_message(&self) -> BeMessage<'_> {
+        Be::AuthenticationSasl(BeSaslMessage::Methods(SCRAM_METHODS))
+    }
+}
+
+impl AuthMethod for Md5 {
+    #[inline(always)]
+    fn first_message(&self) -> BeMessage<'_> {
+        Be::AuthenticationMD5Password(self.0)
+    }
+}
+
+/// This wrapper for [`PostgresBackend`] performs client authentication.
+#[must_use]
+pub struct AuthStream<'a, State> {
+    /// The underlying stream which implements libpq's protocol.
+    pgb: &'a mut PostgresBackend,
+    /// State might contain ancillary data (see [`AuthStream::begin`]).
+    state: State,
+}
+
+/// Initial state of the stream wrapper.
+impl<'a> AuthStream<'a, Begin> {
+    /// Create a new wrapper for client authentication.
+    pub fn new(pgb: &'a mut PostgresBackend) -> Self {
+        Self { pgb, state: Begin }
+    }
+
+    /// Move to the next step by sending auth method's name & params to client.
+    pub fn begin<M: AuthMethod>(self, method: M) -> anyhow::Result<AuthStream<'a, M>> {
+        self.pgb.write_message(&method.first_message())?;
+        self.pgb.state = ProtoState::Authentication;
+
+        Ok(AuthStream {
+            pgb: self.pgb,
+            state: method,
+        })
+    }
+}
+
+/// Stream wrapper for handling simple MD5 password auth.
+impl AuthStream<'_, Md5> {
+    /// Perform user authentication; Raise an error in case authentication failed.
+    pub fn authenticate(mut self) -> anyhow::Result<()> {
+        let msg = self.read_password_message()?;
+        let (_trailing_null, _md5_response) = msg.split_last().context("bad password message")?;
+
+        Ok(())
+    }
+}
+
+/// Stream wrapper for handling [SCRAM](crate::scram) auth.
+impl AuthStream<'_, Scram<'_>> {
+    /// Perform user authentication; Raise an error in case authentication failed.
+    pub fn authenticate(mut self) -> anyhow::Result<ScramKey> {
+        // Initial client message contains the chosen auth method's name
+        let msg = self.read_password_message()?;
+        let sasl = SaslFirstMessage::parse(&msg).context("bad SASL message")?;
+
+        // Currently, the only supported SASL method is SCRAM
+        if !SCRAM_METHODS.contains(&sasl.method) {
+            bail!("unsupported SASL method: {}", sasl.method);
+        }
+
+        let secret = self.state.0;
+        let stream = (Some(msg.slice_ref(sasl.message)), &mut self);
+        let client_key = ScramExchangeServer::new(secret).authenticate(stream)?;
+
+        Ok(client_key)
+    }
+}
+
+/// Only [`AuthMethod`] states should receive password messages.
+impl<M: AuthMethod> AuthStream<'_, M> {
+    /// Receive a new [`PasswordMessage`](FeMessage::PasswordMessage) and extract its payload.
+    fn read_password_message(&mut self) -> anyhow::Result<bytes::Bytes> {
+        match self.pgb.read_message()? {
+            Some(Fe::PasswordMessage(msg)) => Ok(msg),
+            None => bail!("connection is lost"),
+            bad => bail!("unexpected message type: {:?}", bad),
+        }
+    }
+}
+
+/// Abstract away all intricacies of [`PostgresBackend`],
+/// since [SASL](crate::sasl) protocols are text-based.
+impl SaslStream for AuthStream<'_, Scram<'_>> {
+    type In = bytes::Bytes;
+
+    fn recv(&mut self) -> anyhow::Result<Self::In> {
+        self.read_password_message()
+    }
+
+    fn send(&mut self, data: &SaslMessage<impl AsRef<[u8]>>) -> anyhow::Result<()> {
+        let reply = match data {
+            SaslMessage::Continue(reply) => BeSaslMessage::Continue(reply.as_ref()),
+            SaslMessage::Final(reply) => BeSaslMessage::Final(reply.as_ref()),
+        };
+
+        self.pgb.write_message(&Be::AuthenticationSasl(reply))?;
+
+        Ok(())
+    }
+}

proxy/src/cplane_api.rs

@@ -21,35 +21,6 @@ enum ProxyAuthResponse {
     NotReady { ready: bool }, // TODO: get rid of `ready`
 }
 
-impl DatabaseInfo {
-    pub fn socket_addr(&self) -> anyhow::Result<SocketAddr> {
-        let host_port = format!("{}:{}", self.host, self.port);
-        host_port
-            .to_socket_addrs()
-            .with_context(|| format!("cannot resolve {} to SocketAddr", host_port))?
-            .next()
-            .ok_or_else(|| anyhow!("cannot resolve at least one SocketAddr"))
-    }
-}
-
-impl From<DatabaseInfo> for tokio_postgres::Config {
-    fn from(db_info: DatabaseInfo) -> Self {
-        let mut config = tokio_postgres::Config::new();
-
-        config
-            .host(&db_info.host)
-            .port(db_info.port)
-            .dbname(&db_info.dbname)
-            .user(&db_info.user);
-
-        if let Some(password) = db_info.password {
-            config.password(password);
-        }
-
-        config
-    }
-}
-
 pub struct CPlaneApi<'a> {
     auth_endpoint: &'a str,
     waiters: &'a ProxyWaiters,

proxy/src/db.rs

@@ -0,0 +1,69 @@
+///
+/// Utils for connecting with the postgres dataabase.
+///
+
+use crate::scram::key::ScramKey;
+use std::net::{SocketAddr, ToSocketAddrs};
+use anyhow::{Context, anyhow};
+
+/// Sufficient information to authenticate as client.
+pub struct ScramAuthSecret {
+    pub iterations: u32,
+    pub salt_base64: String,
+    pub client_key: ScramKey,
+    pub server_key: ScramKey,
+}
+
+#[non_exhaustive]
+pub enum AuthSecret {
+    Scram(ScramAuthSecret),
+    Password(String),
+}
+
+
+pub struct DatabaseAuthInfo {
+    pub host: String,
+    pub port: u16,
+    pub dbname: String,
+    pub user: String,
+    pub auth_secret: AuthSecret,
+}
+
+impl From<DatabaseAuthInfo> for tokio_postgres::Config {
+    fn from(auth_info: DatabaseAuthInfo) -> Self {
+        let mut config = tokio_postgres::Config::new();
+
+        config
+            .host(&auth_info.host)
+            .port(auth_info.port)
+            .dbname(&auth_info.dbname)
+            .user(&auth_info.user);
+
+        match auth_info.auth_secret {
+            AuthSecret::Scram(scram_secret) => {
+                config.add_scram_key(
+                    base64::decode(scram_secret.salt_base64).unwrap(),
+                    scram_secret.iterations,
+                    scram_secret.client_key.bytes.to_vec(),
+                    scram_secret.server_key.bytes.to_vec(),
+                );
+            },
+            AuthSecret::Password(password) => {
+                config.password(password);
+            }
+        }
+
+        config
+    }
+}
+
+impl DatabaseAuthInfo {
+    pub fn socket_addr(&self) -> anyhow::Result<SocketAddr> {
+        let host_port = format!("{}:{}", self.host, self.port);
+        host_port
+            .to_socket_addrs()
+            .with_context(|| format!("cannot resolve {} to SocketAddr", host_port))?
+            .next()
+            .ok_or_else(|| anyhow!("cannot resolve at least one SocketAddr"))
+    }
+}

proxy/src/main.rs

@@ -11,9 +11,14 @@ use state::{ProxyConfig, ProxyState};
 use std::thread;
 use zenith_utils::{tcp_listener, GIT_VERSION};
 
+mod db;
+mod auth;
 mod cplane_api;
 mod mgmt;
+mod parse;
 mod proxy;
+mod sasl;
+mod scram;
 mod state;
 mod waiters;
 

proxy/src/parse.rs

@@ -0,0 +1,18 @@
+//! Small parsing helpers.
+
+use std::convert::TryInto;
+use std::ffi::CStr;
+
+pub fn split_cstr(bytes: &[u8]) -> Option<(&CStr, &[u8])> {
+    let pos = bytes.iter().position(|&x| x == 0)?;
+    let (cstr, other) = bytes.split_at(pos + 1);
+    // SAFETY: we've already checked that there's a terminator
+    Some((unsafe { CStr::from_bytes_with_nul_unchecked(cstr) }, other))
+}
+
+pub fn split_at_const<const N: usize>(bytes: &[u8]) -> Option<(&[u8; N], &[u8])> {
+    (bytes.len() >= N).then(|| {
+        let (head, tail) = bytes.split_at(N);
+        (head.try_into().unwrap(), tail)
+    })
+}

proxy/src/proxy.rs

@@ -1,5 +1,7 @@
-use crate::cplane_api::{CPlaneApi, DatabaseInfo};
+use crate::auth::{self, AuthStream};
+use crate::cplane_api::CPlaneApi;
 use crate::ProxyState;
+use crate::db::{AuthSecret, DatabaseAuthInfo, ScramAuthSecret};
 use anyhow::{anyhow, bail};
 use lazy_static::lazy_static;
 use parking_lot::Mutex;
@@ -10,7 +12,7 @@ use std::collections::HashMap;
 use std::net::{SocketAddr, TcpStream};
 use std::{io, thread};
 use tokio_postgres::NoTls;
-use zenith_utils::postgres_backend::{self, PostgresBackend, ProtoState, Stream};
+use zenith_utils::postgres_backend::{self, PostgresBackend, Stream};
 use zenith_utils::pq_proto::{BeMessage as Be, FeMessage as Fe, *};
 use zenith_utils::sock_split::{ReadStream, WriteStream};
 
@@ -117,8 +119,9 @@ impl ProxyConnection {
                 None => return Ok(None),
             };
 
+            // HACK for local testing only
             // Both scenarios here should end up producing database credentials
-            if username.ends_with("@zenith") {
+            if true || username.ends_with("@zenith") {
                 self.handle_existing_user(&username, &dbname).map(Some)
             } else {
                 self.handle_new_user().map(Some)
@@ -126,7 +129,7 @@ impl ProxyConnection {
         };
 
         let conn = match authenticate() {
-            Ok(Some(db_info)) => connect_to_db(db_info),
+            Ok(Some(db_auth_info)) => connect_to_db(db_auth_info),
             Ok(None) => return Ok(None),
             Err(e) => {
                 // Report the error to the client
@@ -211,43 +214,43 @@ impl ProxyConnection {
         }
     }
 
-    fn handle_existing_user(&mut self, user: &str, db: &str) -> anyhow::Result<DatabaseInfo> {
+    fn handle_existing_user(&mut self, user: &str, db: &str) -> anyhow::Result<DatabaseAuthInfo> {
-        let md5_salt = rand::random::<[u8; 4]>();
+        let _cplane = CPlaneApi::new(&self.state.conf.auth_endpoint, &self.state.waiters);
 
-        // Ask password
+        // TODO read from console
-        self.pgb
+        // I got this by running `select rolname, rolpassword from pg_authid;`
-            .write_message(&Be::AuthenticationMD5Password(&md5_salt))?;
+        let secret = crate::scram::ScramSecret::parse("SCRAM-SHA-256$4096:tExym9TW7MBl7OsE1FcZVQ==$Ao3nb0bStHOVIqOEUSfXdlvF9XIynqIGzSmDCs2O4p8=:WV6Eenyz5FGwuuVfKh0AXQVnzz4NLnKVV7FpVz1/1zY=").unwrap();
-        self.pgb.state = ProtoState::Authentication; // XXX
 
-        // Check password
+        let client_key = AuthStream::new(&mut self.pgb)
-        let msg = match self.pgb.read_message()? {
+            .begin(auth::Scram(&secret))?
-            Some(Fe::PasswordMessage(msg)) => msg,
+            .authenticate()?;
-            None => bail!("connection is lost"),
-            bad => bail!("unexpected message type: {:?}", bad),
-        };
-        println!("got message: {:?}", msg);
-
-        let (_trailing_null, md5_response) = msg
-            .split_last()
-            .ok_or_else(|| anyhow!("unexpected password message"))?;
-
-        let cplane = CPlaneApi::new(&self.state.conf.auth_endpoint, &self.state.waiters);
-        let db_info = cplane.authenticate_proxy_request(
-            user,
-            db,
-            md5_response,
-            &md5_salt,
-            &self.psql_session_id,
-        )?;
 
         self.pgb
             .write_message_noflush(&Be::AuthenticationOk)?
             .write_message_noflush(&BeParameterStatusMessage::encoding())?;
 
-        Ok(db_info)
+        // TODO get this info from console and tell it to start the db
+        let host = "127.0.0.1";
+        let port = 5432;
+
+        let scram_auth_secret = ScramAuthSecret {
+            iterations: secret.iterations,
+            salt_base64: secret.salt_base64,
+            client_key,
+            server_key: secret.server_key,
+        };
+        let auth_info = DatabaseAuthInfo {
+            host: host.into(),
+            port,
+            dbname: db.into(),
+            user: user.into(),
+            auth_secret: AuthSecret::Scram(scram_auth_secret)
+        };
+
+        Ok(auth_info)
     }
 
-    fn handle_new_user(&mut self) -> anyhow::Result<DatabaseInfo> {
+    fn handle_new_user(&mut self) -> anyhow::Result<DatabaseAuthInfo> {
         let greeting = hello_message(&self.state.conf.redirect_uri, &self.psql_session_id);
 
         // First, register this session
@@ -265,7 +268,15 @@ impl ProxyConnection {
         self.pgb
             .write_message_noflush(&Be::NoticeResponse("Connecting to database.".into()))?;
 
-        Ok(db_info)
+        let db_auth_info = DatabaseAuthInfo {
+            host: db_info.host,
+            port: db_info.port,
+            dbname: db_info.dbname,
+            user: db_info.user,
+            auth_secret: AuthSecret::Password(db_info.password.unwrap())
+        };
+
+        Ok(db_auth_info)
     }
 }
 
@@ -286,7 +297,7 @@ fn hello_message(redirect_uri: &str, session_id: &str) -> String {
 
 /// Create a TCP connection to a postgres database, authenticate with it, and receive the ReadyForQuery message
 async fn connect_to_db(
-    db_info: DatabaseInfo,
+    db_info: DatabaseAuthInfo,
 ) -> anyhow::Result<(String, tokio::net::TcpStream, CancelKeyData)> {
     // Make raw connection. When connect_raw finishes we've received ReadyForQuery.
     let socket_addr = db_info.socket_addr()?;

proxy/src/sasl.rs

@@ -0,0 +1,161 @@
+//! Simple Authentication and Security Layer.
+//!
+//! RFC: <https://datatracker.ietf.org/doc/html/rfc4422>.
+//!
+//! Reference implementation:
+//! * <https://github.com/postgres/postgres/blob/94226d4506e66d6e7cbf4b391f1e7393c1962841/src/backend/libpq/auth-sasl.c>
+//! * <https://github.com/postgres/postgres/blob/94226d4506e66d6e7cbf4b391f1e7393c1962841/src/interfaces/libpq/fe-auth.c>
+
+use crate::parse::{split_at_const, split_cstr};
+use anyhow::Context;
+use thiserror::Error;
+
+/// SASL-specific payload of [`PasswordMessage`](zenith_utils::pq_proto::FeMessage::PasswordMessage).
+#[derive(Debug)]
+pub struct SaslFirstMessage<'a> {
+    /// Authentication method, e.g. `"SCRAM-SHA-256"`.
+    pub method: &'a str,
+    /// Initial client message.
+    pub message: &'a [u8],
+}
+
+impl<'a> SaslFirstMessage<'a> {
+    // NB: FromStr doesn't work with lifetimes
+    pub fn parse(bytes: &'a [u8]) -> Option<Self> {
+        let (method_cstr, tail) = split_cstr(bytes)?;
+        let method = method_cstr.to_str().ok()?;
+
+        let (len_bytes, message) = split_at_const(tail)?;
+        let len = u32::from_be_bytes(*len_bytes) as usize;
+        if len != message.len() {
+            return None;
+        }
+
+        Some(Self { method, message })
+    }
+}
+
+/// A single SASL message.
+/// This struct is deliberately decoupled from lower-level
+/// [`BeAuthenticationSaslMessage`](zenith_utils::pq_proto::BeAuthenticationSaslMessage).
+#[derive(Debug)]
+pub enum SaslMessage<T> {
+    /// We expect to see more steps.
+    Continue(T),
+    /// This is the final step.
+    Final(T),
+}
+
+/// This specialized trait provides capabilities akin to
+/// [`std::io::Read`]+[`std::io::Write`] in oder to
+/// abstract away underlying stream implementations.
+pub trait SaslStream {
+    /// We'd like to use `AsRef<[str]>` here, but afaik there's
+    /// no cheap way to make [`String`] out of [`bytes::Bytes`];
+    /// On the other hand, byte slices are a decent middle ground.
+    type In: AsRef<[u8]>;
+
+    /// Receive a [SASL](crate::sasl) message from a client.
+    fn recv(&mut self) -> anyhow::Result<Self::In>;
+
+    /// Send a [SASL](crate::sasl) message to a client.
+    fn send(&mut self, data: &SaslMessage<impl AsRef<[u8]>>) -> anyhow::Result<()>;
+}
+
+impl<S: SaslStream> SaslStream for &mut S {
+    type In = S::In;
+
+    #[inline(always)]
+    fn recv(&mut self) -> anyhow::Result<Self::In> {
+        S::recv(self)
+    }
+
+    #[inline(always)]
+    fn send(&mut self, data: &SaslMessage<impl AsRef<[u8]>>) -> anyhow::Result<()> {
+        S::send(self, data)
+    }
+}
+
+/// Sometimes it's necessary to mix in a message we got from somewhere else.
+impl<'a, V: AsRef<[u8]>, S: SaslStream<In = V>> SaslStream for (Option<V>, S) {
+    type In = S::In;
+
+    #[inline(always)]
+    fn recv(&mut self) -> anyhow::Result<Self::In> {
+        // Try returning a stashed message first
+        match self.0.take() {
+            Some(value) => Ok(value),
+            None => self.1.recv(),
+        }
+    }
+
+    #[inline(always)]
+    fn send(&mut self, data: &SaslMessage<impl AsRef<[u8]>>) -> anyhow::Result<()> {
+        self.1.send(data)
+    }
+}
+
+/// Fine-grained auth errors help in writing tests.
+#[derive(Error, Debug)]
+pub enum SaslError {
+    #[error("failed to authenticate client: {0}")]
+    AuthenticationFailed(&'static str),
+    #[error("bad client message")]
+    BadClientMessage,
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
+/// A convenient result type for SASL exchange.
+pub type Result<T> = std::result::Result<T, SaslError>;
+
+pub enum SaslStep<T, R> {
+    Transition(T),
+    Authenticated(R),
+}
+
+/// Every SASL mechanism (e.g. [SCRAM](crate::scram)) is expected to implement this trait.
+pub trait SaslMechanism<T>: Sized {
+    /// Produce a server challenge to be sent to the client.
+    /// This is how this method is called in PostgreSQL (libpq/sasl.h).
+    fn exchange(self, input: &str) -> Result<(SaslStep<Self, T>, String)>;
+
+    /// Perform SASL message exchange according to the underlying algorithm
+    /// until user is either authenticated or denied access.
+    fn authenticate(mut self, mut stream: impl SaslStream) -> Result<T> {
+        loop {
+            let msg = stream.recv()?;
+            let input = std::str::from_utf8(msg.as_ref()).context("bad encoding")?;
+
+            let (this, reply) = self.exchange(input)?;
+            match this {
+                SaslStep::Transition(this) => {
+                    stream.send(&SaslMessage::Continue(reply))?;
+                    self = this;
+                }
+                SaslStep::Authenticated(outcome) => {
+                    stream.send(&SaslMessage::Final(reply))?;
+                    return Ok(outcome);
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::ffi::CStr;
+
+    #[test]
+    fn parse_sasl_first_message() {
+        let proto = CStr::from_bytes_with_nul(b"SCRAM-SHA-256\0").unwrap();
+        let sasl = "n,,n=,r=KHQ2Gjc7NptyB8aov5/TnUy4".as_bytes();
+        let sasl_len = (sasl.len() as u32).to_be_bytes();
+        let bytes = [proto.to_bytes_with_nul(), sasl_len.as_ref(), sasl].concat();
+
+        let password = SaslFirstMessage::parse(&bytes).unwrap();
+        assert_eq!(password.method, proto.to_str().unwrap());
+        assert_eq!(password.message, sasl);
+    }
+}

proxy/src/scram.rs

@@ -0,0 +1,132 @@
+//! Salted Challenge Response Authentication Mechanism.
+//!
+//! RFC: <https://datatracker.ietf.org/doc/html/rfc5802>.
+//!
+//! Reference implementation:
+//! * <https://github.com/postgres/postgres/blob/94226d4506e66d6e7cbf4b391f1e7393c1962841/src/backend/libpq/auth-scram.c>
+//! * <https://github.com/postgres/postgres/blob/94226d4506e66d6e7cbf4b391f1e7393c1962841/src/interfaces/libpq/fe-auth-scram.c>
+
+mod channel_binding;
+pub mod key;  // TODO do I have to make it pub?
+mod messages;
+mod secret;
+mod signature;
+
+pub use channel_binding::*;
+pub use secret::*;
+
+use crate::sasl::{self, SaslError, SaslMechanism, SaslStep};
+use messages::{ClientFinalMessage, ClientFirstMessage, OwnedServerFirstMessage};
+use signature::SignatureBuilder;
+
+pub use self::secret::ScramSecret;
+
+/// Decode base64 into array without any heap allocations
+fn base64_decode_array<const N: usize>(input: impl AsRef<[u8]>) -> Option<[u8; N]> {
+    let mut bytes = [0u8; N];
+
+    let size = base64::decode_config_slice(input, base64::STANDARD, &mut bytes).ok()?;
+    if size != N {
+        return None;
+    }
+
+    Some(bytes)
+}
+
+#[derive(Debug)]
+enum ScramExchangeServerState {
+    /// Waiting for [`ClientFirstMessage`].
+    Initial,
+    /// Waiting for [`ClientFinalMessage`].
+    SaltSent {
+        cbind_flag: ChannelBinding<String>,
+        client_first_message_bare: String,
+        server_first_message: OwnedServerFirstMessage,
+    },
+}
+
+/// Server's side of SCRAM auth algorithm.
+#[derive(Debug)]
+pub struct ScramExchangeServer<'a> {
+    state: ScramExchangeServerState,
+    secret: &'a ScramSecret,
+}
+
+impl<'a> ScramExchangeServer<'a> {
+    pub fn new(secret: &'a ScramSecret) -> Self {
+        Self {
+            state: ScramExchangeServerState::Initial,
+            secret,
+        }
+    }
+}
+
+impl SaslMechanism<key::ScramKey> for ScramExchangeServer<'_> {
+    fn exchange(mut self, input: &str) -> sasl::Result<(sasl::SaslStep<Self, key::ScramKey>, String)> {
+        use ScramExchangeServerState::*;
+        use sasl::SaslStep::*;
+        match &self.state {
+            Initial => {
+                let client_first_message =
+                    ClientFirstMessage::parse(input).ok_or(SaslError::BadClientMessage)?;
+
+                let server_first_message = client_first_message.build_server_first_message(
+                    // TODO: use secure random
+                    &rand::random(),
+                    &self.secret.salt_base64,
+                    self.secret.iterations,
+                );
+                let msg = server_first_message.as_str().to_owned();
+
+                self.state = SaltSent {
+                    cbind_flag: client_first_message.cbind_flag.map(str::to_owned),
+                    client_first_message_bare: client_first_message.bare.to_owned(),
+                    server_first_message,
+                };
+
+                Ok((Transition(self), msg))
+            }
+            SaltSent {
+                cbind_flag,
+                client_first_message_bare,
+                server_first_message,
+            } => {
+                let client_final_message =
+                    ClientFinalMessage::parse(input).ok_or(SaslError::BadClientMessage)?;
+
+                let channel_binding = cbind_flag.encode(|_| {
+                    // TODO: make global design decision regarding the certificate
+                    todo!("fetch TLS certificate data")
+                });
+
+                // This might've been caused by a MITM attack
+                if client_final_message.channel_binding != channel_binding {
+                    return Err(SaslError::AuthenticationFailed("channel binding failed"));
+                }
+
+                if client_final_message.nonce != server_first_message.nonce() {
+                    return Err(SaslError::AuthenticationFailed("bad nonce"));
+                }
+
+                let signature_builder = SignatureBuilder {
+                    client_first_message_bare,
+                    server_first_message: server_first_message.as_str(),
+                    client_final_message_without_proof: client_final_message.without_proof,
+                };
+
+                let client_key = signature_builder
+                    .build(&self.secret.stored_key)
+                    .derive_client_key(&client_final_message.proof);
+
+                if client_key.sha256() != self.secret.stored_key {
+                    return Err(SaslError::AuthenticationFailed("keys don't match"));
+                }
+
+                let msg = client_final_message
+                    .build_server_final_message(signature_builder, &self.secret.server_key);
+
+                Ok((Authenticated(client_key), msg))
+            }
+        }
+    }
+}

proxy/src/scram/channel_binding.rs

@@ -0,0 +1,77 @@
+//! Definition and parser for channel binding flag (a part of GS2 header).
+
+/// Channel binding flag (possibly with params).
+#[derive(Debug, PartialEq, Eq)]
+pub enum ChannelBinding<T> {
+    /// Client doesn't support channel binding.
+    NotSupportedClient,
+    /// Client thinks server doesn't support channel binding.
+    NotSupportedServer,
+    /// Client wants to use this type of channel binding.
+    Required(T),
+}
+
+impl<T> ChannelBinding<T> {
+    pub fn map<R>(self, f: impl FnOnce(T) -> R) -> ChannelBinding<R> {
+        use ChannelBinding::*;
+        match self {
+            NotSupportedClient => NotSupportedClient,
+            NotSupportedServer => NotSupportedServer,
+            Required(x) => Required(f(x)),
+        }
+    }
+}
+
+impl<'a> ChannelBinding<&'a str> {
+    // NB: FromStr doesn't work with lifetimes
+    pub fn parse(input: &'a str) -> Option<Self> {
+        use ChannelBinding::*;
+        Some(match input {
+            "n" => NotSupportedClient,
+            "y" => NotSupportedServer,
+            other => Required(other.strip_prefix("p=")?),
+        })
+    }
+}
+
+impl<T: AsRef<str>> ChannelBinding<T> {
+    /// Encode channel binding data as base64 for subsequent checks.
+    pub fn encode(&self, get_cbind_data: impl FnOnce(&str) -> String) -> String {
+        use ChannelBinding::*;
+        match self {
+            NotSupportedClient => {
+                // base64::encode("n,,")
+                "biws".into()
+            }
+            NotSupportedServer => {
+                // base64::encode("y,,")
+                "eSws".into()
+            }
+            Required(s) => {
+                let s = s.as_ref();
+                let msg = format!("p={mode},,{data}", mode = s, data = get_cbind_data(s));
+                base64::encode(msg)
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn channel_binding_encode() {
+        use ChannelBinding::*;
+
+        let cases = [
+            (NotSupportedClient, base64::encode("n,,")),
+            (NotSupportedServer, base64::encode("y,,")),
+            (Required("foo"), base64::encode("p=foo,,bar")),
+        ];
+
+        for (cb, input) in cases {
+            assert_eq!(cb.encode(|_| "bar".to_owned()), input);
+        }
+    }
+}

proxy/src/scram/key.rs

@@ -0,0 +1,40 @@
+//! Tools for client/server/stored keys management.
+
+use sha2::{Digest, Sha256};
+
+/// Faithfully taken from PostgreSQL.
+pub const SCRAM_KEY_LEN: usize = 32;
+
+/// Thin wrapper for byte array.
+#[derive(Debug, PartialEq, Eq)]  // TODO maybe no debug? Avoid accidental logging.
+#[repr(transparent)]
+pub struct ScramKey {
+    pub bytes: [u8; SCRAM_KEY_LEN],  // TODO does it have to be public?
+}
+
+impl ScramKey {
+    pub fn sha256(&self) -> ScramKey {
+        let mut bytes = [0u8; SCRAM_KEY_LEN];
+        bytes.copy_from_slice({
+            let mut hash = Sha256::new();
+            hash.update(&self.bytes);
+            hash.finalize().as_slice()
+        });
+
+        bytes.into()
+    }
+}
+
+impl From<[u8; SCRAM_KEY_LEN]> for ScramKey {
+    #[inline(always)]
+    fn from(bytes: [u8; SCRAM_KEY_LEN]) -> Self {
+        Self { bytes }
+    }
+}
+
+impl AsRef<[u8]> for ScramKey {
+    #[inline(always)]
+    fn as_ref(&self) -> &[u8] {
+        &self.bytes
+    }
+}

proxy/src/scram/messages.rs

@@ -0,0 +1,228 @@
+//! Definitions for SCRAM messages.
+
+use super::base64_decode_array;
+use super::channel_binding::ChannelBinding;
+use super::key::{ScramKey, SCRAM_KEY_LEN};
+use super::signature::SignatureBuilder;
+use std::fmt;
+use std::ops::Range;
+
+/// Faithfully taken from PostgreSQL.
+const SCRAM_RAW_NONCE_LEN: usize = 18;
+
+/// Although we ignore all extensions, we still have to validate the message.
+fn validate_sasl_extensions<'a>(parts: impl Iterator<Item = &'a str>) -> Option<()> {
+    for mut chars in parts.map(|s| s.chars()) {
+        let attr = chars.next()?;
+        if !('a'..'z').contains(&attr) && !('A'..'Z').contains(&attr) {
+            return None;
+        }
+        let eq = chars.next()?;
+        if eq != '=' {
+            return None;
+        }
+    }
+
+    Some(())
+}
+
+#[derive(Debug)]
+pub struct ClientFirstMessage<'a> {
+    /// `client-first-message-bare`.
+    pub bare: &'a str,
+    /// Channel binding mode.
+    pub cbind_flag: ChannelBinding<&'a str>,
+    /// (Client username)[<https://github.com/postgres/postgres/blob/94226d4506e66d6e7cbf/src/backend/libpq/auth-scram.c#L13>].
+    pub username: &'a str,
+    /// Client nonce.
+    pub nonce: &'a str,
+}
+
+impl<'a> ClientFirstMessage<'a> {
+    // NB: FromStr doesn't work with lifetimes
+    pub fn parse(input: &'a str) -> Option<Self> {
+        let mut parts = input.split(',');
+
+        let cbind_flag = ChannelBinding::parse(parts.next()?)?;
+
+        // PG doesn't support authorization identity,
+        // so we don't bother defining GS2 header type
+        let authzid = parts.next()?;
+        if !authzid.is_empty() {
+            return None;
+        }
+
+        // Unfortunately, `parts.as_str()` is unstable
+        let pos = authzid.as_ptr() as usize - input.as_ptr() as usize + 1;
+        let (_, bare) = input.split_at(pos);
+
+        // In theory, these might be preceded by "reserved-mext" (i.e. "m=")
+        let username = parts.next()?.strip_prefix("n=")?;
+        let nonce = parts.next()?.strip_prefix("r=")?;
+
+        // Validate but ignore auth extensions
+        validate_sasl_extensions(parts)?;
+
+        Some(Self {
+            bare,
+            cbind_flag,
+            username,
+            nonce,
+        })
+    }
+
+    pub fn build_server_first_message(
+        &self,
+        nonce: &[u8; SCRAM_RAW_NONCE_LEN],
+        salt_base64: &str,
+        iterations: u32,
+    ) -> OwnedServerFirstMessage {
+        use std::fmt::Write;
+
+        let mut message = String::new();
+        write!(&mut message, "r={}", self.nonce).unwrap();
+        base64::encode_config_buf(nonce, base64::STANDARD, &mut message);
+        let combined_nonce = 2..message.len();
+        write!(&mut message, ",s={},i={}", salt_base64, iterations).unwrap();
+
+        // This design guarantees that it's impossible to create a
+        // server-first-message without receiving a client-first-message
+        OwnedServerFirstMessage {
+            message,
+            nonce: combined_nonce,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct ClientFinalMessage<'a> {
+    /// `client-final-message-without-proof`.
+    pub without_proof: &'a str,
+    /// Channel binding data (base64).
+    pub channel_binding: &'a str,
+    /// Combined client & server nonce.
+    pub nonce: &'a str,
+    /// Client auth proof.
+    pub proof: [u8; SCRAM_KEY_LEN],
+}
+
+impl<'a> ClientFinalMessage<'a> {
+    // NB: FromStr doesn't work with lifetimes
+    pub fn parse(input: &'a str) -> Option<Self> {
+        let (without_proof, proof) = input.rsplit_once(',')?;
+
+        let mut parts = without_proof.split(',');
+        let channel_binding = parts.next()?.strip_prefix("c=")?;
+        let nonce = parts.next()?.strip_prefix("r=")?;
+
+        // Validate but ignore auth extensions
+        validate_sasl_extensions(parts)?;
+
+        let proof = base64_decode_array(proof.strip_prefix("p=")?)?;
+
+        Some(Self {
+            without_proof,
+            channel_binding,
+            nonce,
+            proof,
+        })
+    }
+
+    pub fn build_server_final_message(
+        &self,
+        signature_builder: SignatureBuilder,
+        server_key: &ScramKey,
+    ) -> String {
+        let mut buf = String::from("v=");
+        base64::encode_config_buf(
+            signature_builder.build(server_key),
+            base64::STANDARD,
+            &mut buf,
+        );
+
+        buf
+    }
+}
+
+pub struct OwnedServerFirstMessage {
+    /// Owned `server-first-message`.
+    message: String,
+    /// Slice into `message`.
+    nonce: Range<usize>,
+}
+
+impl OwnedServerFirstMessage {
+    /// Extract combined nonce from the message.
+    #[inline(always)]
+    pub fn nonce(&self) -> &str {
+        &self.message[self.nonce.clone()]
+    }
+
+    /// Get reference to a text representation of the message.
+    #[inline(always)]
+    pub fn as_str(&self) -> &str {
+        &self.message
+    }
+}
+
+impl fmt::Debug for OwnedServerFirstMessage {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ServerFirstMessage")
+            .field("message", &self.as_str())
+            .field("nonce", &self.nonce())
+            .finish()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_client_first_message() {
+        use ChannelBinding::*;
+
+        // (Almost) real strings captured during debug sessions
+        let cases = [
+            (NotSupportedClient, "n,,n=pepe,r=t8JwklwKecDLwSsA72rHmVju"),
+            (NotSupportedServer, "y,,n=pepe,r=t8JwklwKecDLwSsA72rHmVju"),
+            (
+                Required("tls-server-end-point"),
+                "p=tls-server-end-point,,n=pepe,r=t8JwklwKecDLwSsA72rHmVju",
+            ),
+        ];
+
+        for (cb, input) in cases {
+            let msg = ClientFirstMessage::parse(input).unwrap();
+
+            assert_eq!(msg.bare, "n=pepe,r=t8JwklwKecDLwSsA72rHmVju");
+            assert_eq!(msg.username, "pepe");
+            assert_eq!(msg.nonce, "t8JwklwKecDLwSsA72rHmVju");
+            assert_eq!(msg.cbind_flag, cb);
+        }
+    }
+
+    #[test]
+    fn parse_client_final_message() {
+        let input = [
+            "c=eSws",
+            "r=iiYEfS3rOgn8S3rtpSdrOsHtPLWvIkdgmHxA0hf3JNOAG4dU",
+            "p=SRpfsIVS4Gk11w1LqQ4QvCUBZYQmqXNSDEcHqbQ3CHI=",
+        ]
+        .join(",");
+
+        let msg = ClientFinalMessage::parse(&input).unwrap();
+        assert_eq!(
+            msg.without_proof,
+            "c=eSws,r=iiYEfS3rOgn8S3rtpSdrOsHtPLWvIkdgmHxA0hf3JNOAG4dU"
+        );
+        assert_eq!(
+            msg.nonce,
+            "iiYEfS3rOgn8S3rtpSdrOsHtPLWvIkdgmHxA0hf3JNOAG4dU"
+        );
+        assert_eq!(
+            base64::encode(msg.proof),
+            "SRpfsIVS4Gk11w1LqQ4QvCUBZYQmqXNSDEcHqbQ3CHI="
+        );
+    }
+}

proxy/src/scram/secret.rs

@@ -0,0 +1,65 @@
+//! Tools for SCRAM server secret management.
+
+use super::base64_decode_array;
+use super::key::ScramKey;
+
+#[derive(Debug)]
+pub struct ScramSecret {
+    pub iterations: u32,
+    pub salt_base64: String,
+    pub stored_key: ScramKey,
+    pub server_key: ScramKey,
+}
+
+impl ScramSecret {
+    pub fn parse(input: &str) -> Option<Self> {
+        // SCRAM-SHA-256$<iterations>:<salt>$<storedkey>:<serverkey>
+        let s = input.strip_prefix("SCRAM-SHA-256$")?;
+        let (params, keys) = s.split_once('$')?;
+
+        let ((iterations, salt), (stored_key, server_key)) =
+            params.split_once(':').zip(keys.split_once(':'))?;
+
+        let secret = ScramSecret {
+            iterations: iterations.parse().ok()?,
+            salt_base64: salt.to_owned(),
+            stored_key: base64_decode_array(stored_key)?.into(),
+            server_key: base64_decode_array(server_key)?.into(),
+        };
+
+        Some(secret)
+    }
+
+    pub fn mock() -> Self {
+        todo!("see auth-scram.c : mock_scram_secret")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_scram_secret() {
+        let iterations = 4096;
+        let salt = "+/tQQax7twvwTj64mjBsxQ==";
+        let stored_key = "D5h6KTMBlUvDJk2Y8ELfC1Sjtc6k9YHjRyuRZyBNJns=";
+        let server_key = "Pi3QHbcluX//NDfVkKlFl88GGzlJ5LkyPwcdlN/QBvI=";
+
+        let secret = format!(
+            "SCRAM-SHA-256${iterations}:{salt}${stored_key}:{server_key}",
+            iterations = iterations,
+            salt = salt,
+            stored_key = stored_key,
+            server_key = server_key,
+        );
+
+        let parsed = ScramSecret::parse(&secret).unwrap();
+        assert_eq!(parsed.iterations, iterations);
+        assert_eq!(parsed.salt_base64, salt);
+
+        // TODO: derive from 'password'
+        assert_eq!(base64::encode(parsed.stored_key), stored_key);
+        assert_eq!(base64::encode(parsed.server_key), server_key);
+    }
+}

proxy/src/scram/signature.rs

@@ -0,0 +1,70 @@
+//! Tools for client/server signature management.
+
+use super::key::{ScramKey, SCRAM_KEY_LEN};
+use hmac::{Hmac, Mac, NewMac};
+use sha2::Sha256;
+
+#[derive(Debug)]
+pub struct SignatureBuilder<'a> {
+    pub client_first_message_bare: &'a str,
+    pub server_first_message: &'a str,
+    pub client_final_message_without_proof: &'a str,
+}
+
+impl SignatureBuilder<'_> {
+    pub fn build(&self, key: &ScramKey) -> Signature {
+        let mut mac = Hmac::<Sha256>::new_varkey(key.as_ref()).expect("bad key size");
+
+        mac.update(self.client_first_message_bare.as_bytes());
+        mac.update(b",");
+        mac.update(self.server_first_message.as_bytes());
+        mac.update(b",");
+        mac.update(self.client_final_message_without_proof.as_bytes());
+
+        // TODO: maybe newer `hmac` et al already migrated to regular arrays?
+        let mut signature = [0u8; SCRAM_KEY_LEN];
+        signature.copy_from_slice(mac.finalize().into_bytes().as_slice());
+        signature.into()
+    }
+}
+
+#[derive(Debug)]
+#[repr(transparent)]
+pub struct Signature {
+    bytes: [u8; SCRAM_KEY_LEN],
+}
+
+impl Signature {
+    /// Derive ClientKey from client's signature and proof
+    pub fn derive_client_key(&self, proof: &[u8; SCRAM_KEY_LEN]) -> ScramKey {
+        let signature = self.as_ref().iter();
+
+        // This is how the proof is calculated:
+        //
+        // 1. sha256(ClientKey) -> StoredKey
+        // 2. hmac_sha256(StoredKey, [messages...]) -> ClientSignature
+        // 3. ClientKey ^ ClientSignature -> ClientProof
+        //
+        // Step 3 implies that we can restore ClientKey from the proof
+        // by xoring the latter with the ClientSignature again. Afterwards
+        // we can check that the presumed ClientKey meets our expectations.
+        let mut bytes = [0u8; SCRAM_KEY_LEN];
+        for (i, value) in signature.zip(proof).map(|(x, y)| x ^ y).enumerate() {
+            bytes[i] = value;
+        }
+
+        bytes.into()
+    }
+}
+
+impl From<[u8; SCRAM_KEY_LEN]> for Signature {
+    fn from(bytes: [u8; SCRAM_KEY_LEN]) -> Self {
+        Self { bytes }
+    }
+}
+
+impl AsRef<[u8]> for Signature {
+    fn as_ref(&self) -> &[u8] {
+        &self.bytes
+    }
+}

zenith_utils/src/postgres_backend.rs

@@ -373,9 +373,8 @@ impl PostgresBackend {
                             }
                             AuthType::MD5 => {
                                 rand::thread_rng().fill(&mut self.md5_salt);
-                                let md5_salt = self.md5_salt;
                                 self.write_message(&BeMessage::AuthenticationMD5Password(
-                                    &md5_salt,
+                                    self.md5_salt,
                                 ))?;
                                 self.state = ProtoState::Authentication;
                             }

zenith_utils/src/pq_proto.rs

@@ -353,7 +353,8 @@ fn read_null_terminated(buf: &mut Bytes) -> anyhow::Result<Bytes> {
 #[derive(Debug)]
 pub enum BeMessage<'a> {
     AuthenticationOk,
-    AuthenticationMD5Password(&'a [u8; 4]),
+    AuthenticationMD5Password([u8; 4]),
+    AuthenticationSasl(BeAuthenticationSaslMessage<'a>),
     AuthenticationCleartextPassword,
     BackendKeyData(CancelKeyData),
     BindComplete,
@@ -381,6 +382,13 @@ pub enum BeMessage<'a> {
     KeepAlive(WalSndKeepAlive),
 }
 
+#[derive(Debug)]
+pub enum BeAuthenticationSaslMessage<'a> {
+    Methods(&'a [&'a str]),
+    Continue(&'a [u8]),
+    Final(&'a [u8]),
+}
+
 #[derive(Debug)]
 pub enum BeParameterStatusMessage<'a> {
     Encoding(&'a str),
@@ -552,6 +560,32 @@ impl<'a> BeMessage<'a> {
                 .unwrap(); // write into BytesMut can't fail
             }
 
+            BeMessage::AuthenticationSasl(msg) => {
+                buf.put_u8(b'R');
+                write_body(buf, |buf| {
+                    use BeAuthenticationSaslMessage::*;
+                    match msg {
+                        Methods(methods) => {
+                            buf.put_i32(10); // Specifies that SASL auth method is used.
+                            for method in methods.iter() {
+                                write_cstr(method.as_bytes(), buf)?;
+                            }
+                            buf.put_u8(0); // zero terminator for the list
+                        }
+                        Continue(extra) => {
+                            buf.put_i32(11); // Continue SASL auth.
+                            buf.put_slice(extra);
+                        }
+                        Final(extra) => {
+                            buf.put_i32(12); // Send final SASL message.
+                            buf.put_slice(extra);
+                        }
+                    }
+                    Ok::<_, io::Error>(())
+                })
+                .unwrap()
+            }
+
             BeMessage::BackendKeyData(key_data) => {
                 buf.put_u8(b'K');
                 write_body(buf, |buf| {