deps(crypto): upgrade argon2 to 0.6 without breaking stored password hashes

crates/crypto/Cargo.toml

@@ -17,7 +17,7 @@ blake3 = "1.8.5"
 hex = "0.4.3"
 thiserror = "2.0.18"
 ed25519-dalek = "3.0.0"
-argon2 = "0.5"
+argon2 = "0.6"
 
 [dev-dependencies]
 rand_chacha = "0.3"

crates/crypto/src/credentials.rs

@@ -2,7 +2,8 @@
 //! generation and hashing, constant-time comparison. Lives here because
 //! RustCrypto (argon2, blake3) and the RNG stay quarantined in this crate.
 
-use argon2::password_hash::{PasswordHash, PasswordHasher, PasswordVerifier, SaltString};
+use argon2::password_hash::phc::PasswordHash;
+use argon2::password_hash::{PasswordHasher, PasswordVerifier};
 use argon2::Argon2;
 use std::sync::OnceLock;
 
@@ -11,11 +12,10 @@
     use rand_core::{OsRng, RngCore};
     let mut salt = [0u8; 16];
     OsRng.fill_bytes(&mut salt);
-    let salt = SaltString::encode_b64(&salt).expect("16 bytes is a valid salt length");
-    Argon2::default()
-        .hash_password(password.as_bytes(), &salt)
-        .expect("argon2 hashing with default parameters cannot fail")
-        .to_string()
+    let hash: PasswordHash = Argon2::default()
+        .hash_password_with_salt(password.as_bytes(), &salt)
+        .expect("argon2 hashing with default parameters cannot fail");
+    hash.to_string()
 }
 
 static DUMMY: OnceLock<String> = OnceLock::new();
@@ -102,6 +102,18 @@
         );
     }
 
+    #[test]
+    fn hash_from_argon2_0_5_still_verifies() {
+        // Made by argon2 0.5.3 (the version before 0.6): rows stored by older
+        // sc-forge builds must keep verifying after the upgrade.
+        let legacy = concat!(
+            "$argon2id$v=19$m=19456,t=2,p=1$12Jq/QaZmfBGlN6TfZu3Rg",
+            "$nVEPZmM6n6sSG8yZfXvFF5E00aLDJTCVu7bpGnEhDyU"
+        );
+        assert!(verify_password("legacy-0.5-fixture-pw", Some(legacy)));
+        assert!(!verify_password("legacy-0.5-fixture-px", Some(legacy)));
+    }
+
     #[test]
     fn same_password_hashes_differently() {
         let p = pw();