Fix MSB 1 leading to negative modulus in bun

src/asn1.ts

@@ -68,13 +68,17 @@ function encodeLength(length: number) {
 }
 
 /**
- * Encode a buffer (that represent an integer) as integer per ASN.1 spec (DER-encoding)
+ * Encode a buffer (that represents a positive integer) as integer per ASN.1 spec (DER-encoding)
  * See https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf chapter 8.3
  *
- * @param buffer - The buffer that represent an integer to encode
+ * @param buffer - The buffer that represents a positive integer
  * @returns The buffer
  */
-function encodeBufferAsInteger(buffer: Buffer) {
+function encodeBufferAsPositiveInteger(buffer: Buffer) {
+  if (buffer[0] >> 7) {
+    // MSB is 1, add leading zero to indicate positive number
+    buffer = Buffer.concat([Buffer.from([0]), buffer]);
+  }
   return Buffer.concat([
     encodeIdentifier({
       class: Asn1Class.Universal,
@@ -213,7 +217,10 @@ export function constructPublicKeyInDerFormat(n: Buffer, e: Buffer): Buffer {
   return encodeSequence([
     ALGORITHM_RSA_ENCRYPTION,
     encodeBufferAsBitString(
-      encodeSequence([encodeBufferAsInteger(n), encodeBufferAsInteger(e)])
+      encodeSequence([
+        encodeBufferAsPositiveInteger(n),
+        encodeBufferAsPositiveInteger(e),
+      ])
     ),
   ]);
 }

tests/unit/asn1.test.ts

@@ -30,6 +30,45 @@ describe("unit tests asn1", () => {
     expect(publicKey).toEqual(priorGeneratedPublicKey);
   });
 
+  test("Prepends 0 byte only if MSB is 1", () => {
+    const n = Buffer.from([0b10000000]); // MSB 1
+    const e = Buffer.from([0b01000000]); // MSB 0
+    const publicKeyDer = constructPublicKeyInDerFormat(n, e);
+    expect(publicKeyDer).toEqual(
+      Buffer.from([
+        48,
+        27,
+        48,
+        13,
+        6,
+        9,
+        42,
+        134,
+        72,
+        134,
+        247,
+        13,
+        1,
+        1,
+        1,
+        5,
+        0,
+        3,
+        10,
+        0,
+        48,
+        7,
+        2,
+        2,
+        0, // here is the leading 0
+        128, // prepended with leading 0
+        2,
+        1,
+        64, // not prepended with leading 0
+      ])
+    );
+  });
+
   test("Deconstructing public key works", () => {
     const priorGeneratedPublicKey = readFileSync(
       join(__dirname, "pubkey-test.der")

tests/unit/https.test.ts

@@ -19,9 +19,7 @@ describe("unit tests https", () => {
     const payload = JSON.stringify({ hello: "world" }) + "}";
     mockHttpsUri(uri, { responsePayload: payload });
     expect.assertions(1);
-    return expect(fetchJson(uri)).rejects.toThrow(
-      "Unexpected token } in JSON at position 17"
-    );
+    return expect(fetchJson(uri)).rejects.toThrow("JSON at position 17");
   });
 
   test("Fetch JSON error flow works: 404", () => {