Add password module for hashing on the client side.

Hashing a password on the client side is useful so that you can set a
user's password without ever sending it in plain text to the
server. This avoids leaking passwords in the log or elsewhere.

postgres-protocol/src/authentication/sasl.rs

@@ -32,7 +32,7 @@ fn normalize(pass: &[u8]) -> Vec<u8> {
     }
 }
 
-fn hi(str: &[u8], salt: &[u8], i: u32) -> [u8; 32] {
+pub(crate) fn hi(str: &[u8], salt: &[u8], i: u32) -> [u8; 32] {
     let mut hmac = Hmac::<Sha256>::new_varkey(str).expect("HMAC is able to accept all key sizes");
     hmac.update(salt);
     hmac.update(&[0, 0, 0, 1]);

postgres-protocol/src/lib.rs

@@ -19,6 +19,7 @@ use std::io;
 pub mod authentication;
 pub mod escape;
 pub mod message;
+pub mod password;
 pub mod types;
 
 /// A Postgres OID.

postgres-protocol/src/password/mod.rs

@@ -0,0 +1,104 @@
+//! Functions to encrypt a password in the client.
+//!
+//! This is intended to be used by client applications that wish to
+//! send commands like `ALTER USER joe PASSWORD 'pwd'`. The password
+//! need not be sent in cleartext if it is encrypted on the client
+//! side. This is good because it ensures the cleartext password won't
+//! end up in logs pg_stat displays, etc.
+
+use crate::authentication::sasl;
+use hmac::{Hmac, Mac, NewMac};
+use md5::Md5;
+use rand::RngCore;
+use sha2::digest::FixedOutput;
+use sha2::{Digest, Sha256};
+
+#[cfg(test)]
+mod test;
+
+const SCRAM_DEFAULT_ITERATIONS: u32 = 4096;
+const SCRAM_DEFAULT_SALT_LEN: usize = 16;
+
+/// Hash password using SCRAM-SHA-256 with a randomly-generated
+/// salt.
+///
+/// The client may assume the returned string doesn't contain any
+/// special characters that would require escaping in an SQL command.
+pub fn scram_sha_256(password: &[u8]) -> String {
+    let mut salt: [u8; SCRAM_DEFAULT_SALT_LEN] = [0; SCRAM_DEFAULT_SALT_LEN];
+    let mut rng = rand::thread_rng();
+    rng.fill_bytes(&mut salt);
+    scram_sha_256_salt(password, salt)
+}
+
+// Internal implementation of scram_sha_256 with a caller-provided
+// salt. This is useful for testing.
+pub(crate) fn scram_sha_256_salt(password: &[u8], salt: [u8; SCRAM_DEFAULT_SALT_LEN]) -> String {
+    // Prepare the password, per [RFC
+    // 4013](https://tools.ietf.org/html/rfc4013), if possible.
+    //
+    // Postgres treats passwords as byte strings (without embedded NUL
+    // bytes), but SASL expects passwords to be valid UTF-8.
+    //
+    // Follow the behavior of libpq's PQencryptPasswordConn(), and
+    // also the backend. If the password is not valid UTF-8, or if it
+    // contains prohibited characters (such as non-ASCII whitespace),
+    // just skip the SASLprep step and use the original byte
+    // sequence.
+    let prepared: Vec<u8> = match std::str::from_utf8(password) {
+        Ok(password_str) => {
+            match stringprep::saslprep(password_str) {
+                Ok(p) => p.into_owned().into_bytes(),
+                // contains invalid characters; skip saslprep
+                Err(_) => Vec::from(password),
+            }
+        }
+        // not valid UTF-8; skip saslprep
+        Err(_) => Vec::from(password),
+    };
+
+    // salt password
+    let salted_password = sasl::hi(&prepared, &salt, SCRAM_DEFAULT_ITERATIONS);
+
+    // client key
+    let mut hmac =
+        Hmac::<Sha256>::new_varkey(&salted_password).expect("HMAC is able to accept all key sizes");
+    hmac.update(b"Client Key");
+    let client_key = hmac.finalize().into_bytes();
+
+    // stored key
+    let mut hash = Sha256::default();
+    hash.update(client_key.as_slice());
+    let stored_key = hash.finalize_fixed();
+
+    // server key
+    let mut hmac =
+        Hmac::<Sha256>::new_varkey(&salted_password).expect("HMAC is able to accept all key sizes");
+    hmac.update(b"Server Key");
+    let server_key = hmac.finalize().into_bytes();
+
+    format!(
+        "SCRAM-SHA-256${}:{}${}:{}",
+        SCRAM_DEFAULT_ITERATIONS,
+        base64::encode(salt),
+        base64::encode(stored_key),
+        base64::encode(server_key)
+    )
+}
+
+/// **Not recommended, as MD5 is not considered to be secure.**
+///
+/// Hash password using MD5 with the username as the salt.
+///
+/// The client may assume the returned string doesn't contain any
+/// special characters that would require escaping.
+pub fn md5(password: &[u8], username: &str) -> String {
+    // salt password with username
+    let mut salted_password = Vec::from(password);
+    salted_password.extend_from_slice(username.as_bytes());
+
+    let mut hash = Md5::new();
+    hash.update(&salted_password);
+    let digest = hash.finalize();
+    format!("md5{:x}", digest)
+}

postgres-protocol/src/password/test.rs

@@ -0,0 +1,19 @@
+use crate::password;
+
+#[test]
+fn test_encrypt_scram_sha_256() {
+    // Specify the salt to make the test deterministic. Any bytes will do.
+    let salt: [u8; 16] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16];
+    assert_eq!(
+        password::scram_sha_256_salt(b"secret", salt),
+        "SCRAM-SHA-256$4096:AQIDBAUGBwgJCgsMDQ4PEA==$8rrDg00OqaiWXJ7p+sCgHEIaBSHY89ZJl3mfIsf32oY=:05L1f+yZbiN8O0AnO40Og85NNRhvzTS57naKRWCcsIA="
+    );
+}
+
+#[test]
+fn test_encrypt_md5() {
+    assert_eq!(
+        password::md5(b"secret", "foo"),
+        "md54ab2c5d00339c4b2a4e921d2dc4edec7"
+    );
+}