fix: make TOTP issuer field optional (#240)

Both per:
- google authenticator wiki: https://github.com/google/google-authenticator/wiki/Key-Uri-Format#issuer
- yubikey website: https://docs.yubico.com/yesdk/users-manual/application-oath/uri-string-format.html

`issuer` parameter in `otpauth://` URI scheme is optional, even though
it's highly recommended to be present.

This parameter was actually optional in this repo initially, but was
changed to non optional. I'm changing it back to optional.

Co-authored-by: Stefan Seemayer <stefan@seemayer.de>

src/db/otp.rs

@@ -33,7 +33,7 @@ impl std::str::FromStr for TOTPAlgorithm {
 #[derive(Debug, PartialEq, Eq, Zeroize, ZeroizeOnDrop)]
 pub struct TOTP {
     pub label: String,
-    pub issuer: String,
+    pub issuer: Option<String>,
     pub period: u64,
     pub digits: u32,
     pub algorithm: TOTPAlgorithm,
@@ -119,7 +119,6 @@ impl std::str::FromStr for TOTP {
         }
 
         let secret = secret.ok_or(TOTPError::MissingField("secret"))?;
-        let issuer = issuer.ok_or(TOTPError::MissingField("issuer"))?;
 
         let secret =
             base32::decode(base32::Alphabet::Rfc4648 { padding: true }, &secret).ok_or(TOTPError::Base32)?;
@@ -201,7 +200,7 @@ mod kdbx4_otp_tests {
         let expected = TOTP {
             label: "KeePassXC:none".to_string(),
             secret: b"Hello!\xDE\xAD\xBE\xEF".to_vec(),
-            issuer: "KeePassXC".to_string(),
+            issuer: Some("KeePassXC".to_string()),
             period: 30,
             digits: 6,
             algorithm: TOTPAlgorithm::Sha1,
@@ -231,7 +230,7 @@ mod kdbx4_otp_tests {
         let expected = TOTP {
             label: "sha512%20totp:none".to_string(),
             secret: b"123456".to_vec(),
-            issuer: "sha512 totp".to_string(),
+            issuer: Some("sha512 totp".to_string()),
             period: 30,
             digits: 6,
             algorithm: TOTPAlgorithm::Sha512,
@@ -247,7 +246,7 @@ mod kdbx4_otp_tests {
         let totp = TOTP {
             label: "KeePassXC:none".to_string(),
             secret: b"Hello!\xDE\xAD\xBE\xEF".to_vec(),
-            issuer: "KeePassXC".to_string(),
+            issuer: Some("KeePassXC".to_string()),
             period: 30,
             digits: 6,
             algorithm: TOTPAlgorithm::Sha1,
@@ -278,4 +277,22 @@ mod kdbx4_otp_tests {
             Err(TOTPError::MissingField("secret"))
         ));
     }
+
+    #[test]
+    fn totp_minimal() -> Result<(), TOTPError> {
+        let otp_str = "otpauth://totp/KeePassXC:none?secret=JBSWY3DPEHPK3PXP&period=30&digits=6";
+
+        let expected = TOTP {
+            label: "KeePassXC:none".to_string(),
+            secret: b"Hello!\xDE\xAD\xBE\xEF".to_vec(),
+            issuer: None,
+            period: 30,
+            digits: 6,
+            algorithm: TOTPAlgorithm::Sha1,
+        };
+
+        assert_eq!(otp_str.parse::<TOTP>()?, expected);
+
+        Ok(())
+    }
 }