plan: libxcrypt-compatible sha512-crypt (16-char salt); KAT test

src/plan.rs

@@ -219,11 +219,19 @@ pub fn classify_secret(s: &str) -> SecretKind {
     SecretKind::Plaintext(s.to_string())
 }
 
-/// SHA512-crypt hash (what `mkpasswd -m sha-512` produces).
+/// SHA512-crypt hash compatible with glibc/libxcrypt (Dovecot, `mkpasswd -m sha-512`).
+///
+/// The salt is 12 random bytes = exactly 16 salt characters. sha-crypt 0.6's
+/// default (16 bytes -> 22 characters) hashes over all 22 characters, while
+/// libxcrypt only uses the first 16, so such hashes never verify in Dovecot
+/// (caught by the e2e test).
 pub fn sha512_crypt(password: &str) -> Result<String> {
+    use rand::RngExt;
     use sha_crypt::{PasswordHasher, ShaCrypt};
-    let h =
-        ShaCrypt::default().hash_password(password.as_bytes()).map_err(|e| anyhow::anyhow!("hashing password: {e}"))?;
+    let salt: [u8; 12] = rand::rng().random();
+    let h = ShaCrypt::default()
+        .hash_password_with_salt(password.as_bytes(), &salt)
+        .map_err(|e| anyhow::anyhow!("hashing password: {e}"))?;
     Ok(h.as_str().to_string())
 }
 
@@ -487,6 +495,14 @@ mod tests {
         assert!(h.starts_with("$6$"));
         assert!(verify_sha_crypt("correct horse", &h));
         assert!(!verify_sha_crypt("wrong", &h));
+        // glibc/libxcrypt only use 16 salt characters; longer salts never verify there.
+        let salt = h.trim_start_matches("$6$").split('$').find(|f| !f.starts_with("rounds=")).unwrap();
+        assert_eq!(salt.len(), 16, "{h}");
+        // Algorithm matches glibc (Drepper's test vector).
+        assert!(verify_sha_crypt(
+            "Hello world!",
+            "$6$saltstring$svn8UoSVapNtMuq1ukKS4tPQd8iKwSMHWjl/O817G3uBnIFNjnQJuesI68u4OTLiBFdcbYEdFCoEOfaS35inz1"
+        ));
     }
 
     #[test]