refactor: Make Fingerprint not implement Display (#8177)

Currently, the Fingerprint type implements Display, but this doesn't get
you the canonical fingerprint representation, but something
human-readable. This is confusing, and back when I first used
`Fingerprint`, I immediately wrote a bug because of this. So, instead,
make a function `human_readable()` on Fingerprint.

This comes from the discussion at
https://github.com/chatmail/core/pull/8174#discussion_r3143130722.

deltachat-jsonrpc/src/api/types/qr.rs

@@ -238,7 +238,7 @@ impl From<Qr> for QrObject {
                 is_v3,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::AskVerifyContact {
                     contact_id,
                     fingerprint,
@@ -257,7 +257,7 @@ impl From<Qr> for QrObject {
                 is_v3,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::AskVerifyGroup {
                     grpname,
                     grpid,
@@ -278,7 +278,7 @@ impl From<Qr> for QrObject {
                 is_v3,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::AskJoinBroadcast {
                     name,
                     grpid,
@@ -321,7 +321,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::WithdrawVerifyContact {
                     contact_id,
                     fingerprint,
@@ -338,7 +338,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::WithdrawVerifyGroup {
                     grpname,
                     grpid,
@@ -357,7 +357,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::WithdrawJoinBroadcast {
                     name,
                     grpid,
@@ -374,7 +374,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::ReviveVerifyContact {
                     contact_id,
                     fingerprint,
@@ -391,7 +391,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::ReviveVerifyGroup {
                     grpname,
                     grpid,
@@ -410,7 +410,7 @@ impl From<Qr> for QrObject {
                 authcode,
             } => {
                 let contact_id = contact_id.to_u32();
-                let fingerprint = fingerprint.to_string();
+                let fingerprint = fingerprint.human_readable();
                 QrObject::ReviveJoinBroadcast {
                     name,
                     grpid,

src/chat.rs

@@ -1210,7 +1210,8 @@ SELECT id, rfc724_mid, pre_rfc724_mid, timestamp, ?, 1 FROM msgs WHERE chat_id=?
             );
             let fingerprint = contact
                 .fingerprint()
-                .context("Contact does not have a fingerprint in encrypted chat")?;
+                .context("Contact does not have a fingerprint in encrypted chat")?
+                .human_readable();
             if let Some(public_key) = contact.public_key(context).await? {
                 if let Some(relay_addrs) = addresses_from_public_key(&public_key) {
                     let relays = relay_addrs.join(",");

src/contact.rs

@@ -1396,7 +1396,7 @@ WHERE addr=?
         let Some(fingerprint_other) = contact.fingerprint() else {
             return Ok(stock_str::encr_none(context));
         };
-        let fingerprint_other = fingerprint_other.to_string();
+        let fingerprint_other = fingerprint_other.human_readable();
 
         let stock_message = if contact.public_key(context).await?.is_some() {
             stock_str::messages_are_e2ee(context)
@@ -1410,7 +1410,7 @@ WHERE addr=?
         let fingerprint_self = load_self_public_key(context)
             .await?
             .dc_fingerprint()
-            .to_string();
+            .human_readable();
         if addr < contact.addr {
             cat_fingerprint(
                 &mut ret,

src/key.rs

@@ -1,7 +1,7 @@
 //! Cryptographic key module.
 
 use std::collections::BTreeMap;
-use std::fmt;
+use std::fmt::{self, Write as _};
 use std::io::Cursor;
 
 use anyhow::{Context as _, Result, bail, ensure};
@@ -583,6 +583,21 @@ impl Fingerprint {
     pub fn hex(&self) -> String {
         hex::encode_upper(&self.0)
     }
+
+    /// Make a human-readable fingerprint.
+    pub fn human_readable(&self) -> String {
+        let mut f = String::new();
+        // Split key into chunks of 4 with space and newline at 20 chars
+        for (i, c) in self.hex().chars().enumerate() {
+            if i > 0 && i % 20 == 0 {
+                writeln!(&mut f).ok();
+            } else if i > 0 && i % 4 == 0 {
+                write!(&mut f, " ").ok();
+            }
+            write!(&mut f, "{c}").ok();
+        }
+        f
+    }
 }
 
 impl From<pgp::types::Fingerprint> for Fingerprint {
@@ -599,22 +614,6 @@ impl fmt::Debug for Fingerprint {
     }
 }
 
-/// Make a human-readable fingerprint.
-impl fmt::Display for Fingerprint {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Split key into chunks of 4 with space and newline at 20 chars
-        for (i, c) in self.hex().chars().enumerate() {
-            if i > 0 && i % 20 == 0 {
-                writeln!(f)?;
-            } else if i > 0 && i % 4 == 0 {
-                write!(f, " ")?;
-            }
-            write!(f, "{c}")?;
-        }
-        Ok(())
-    }
-}
-
 /// Parse a human-readable or otherwise formatted fingerprint.
 impl std::str::FromStr for Fingerprint {
     type Err = anyhow::Error;
@@ -890,7 +889,7 @@ i8pcjGO+IZffvyZJVRWfVooBJmWWbPB1pueo3tx8w3+fcuzpxz+RLFKaPyqXO+dD
             1, 2, 4, 8, 16, 32, 64, 128, 255, 1, 2, 4, 8, 16, 32, 64, 128, 255, 19, 20,
         ]);
         assert_eq!(
-            fp.to_string(),
+            fp.human_readable(),
             "0102 0408 1020 4080 FF01\n0204 0810 2040 80FF 1314"
         );
     }

src/qr.rs

@@ -645,7 +645,7 @@ async fn decode_openpgp(context: &Context, qr: &str) -> Result<Qr> {
         }
     } else {
         Ok(Qr::FprWithoutAddr {
-            fingerprint: fingerprint.to_string(),
+            fingerprint: fingerprint.human_readable(),
         })
     }
 }

src/qr/qr_tests.rs

@@ -388,7 +388,7 @@ async fn test_decode_openpgp_fingerprint() -> Result<()> {
         bob,
         &format!(
             "OPENPGP4FPR:{}#a=alice@example.org",
-            alice_contact.fingerprint().unwrap()
+            alice_contact.fingerprint().unwrap().hex()
         ),
     )
     .await?;

src/securejoin.rs

@@ -861,7 +861,8 @@ fn encrypted_and_signed(
         } else {
             warn!(
                 context,
-                "Message does not match expected fingerprint {expected_fingerprint}.",
+                "Message does not match expected fingerprint {}.",
+                expected_fingerprint.human_readable()
             );
             false
         }