Switch to prefixed IDs and tell agents which Mac a message came from

mac/Sources/Byre/IDs.swift

@@ -0,0 +1,22 @@
+import CryptoKit
+import Foundation
+
+private let idAlphabet = Array("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz")
+
+func newID(_ prefix: String) -> String {
+    prefix + String((0..<11).map { _ in idAlphabet.randomElement()! })
+}
+
+// IDs from before prefixed IDs were 12 lowercase hex characters; the server renamed each to this same derivation.
+func migratedID(_ prefix: String, _ old: String) -> String {
+    guard old.count == 12, old.allSatisfy({ "0123456789abcdef".contains($0) }) else {
+        return old
+    }
+    var n = SHA256.hash(data: Data(old.utf8)).prefix(8).reduce(UInt64(0)) { $0 << 8 | UInt64($1) }
+    var out = ""
+    for _ in 0..<11 {
+        out.append(idAlphabet[Int(n % UInt64(idAlphabet.count))])
+        n /= UInt64(idAlphabet.count)
+    }
+    return prefix + out
+}