crypto: Add Secp256k1Signature.toFixedLength method

packages/crypto/src/secp256k1signature.spec.ts

@@ -76,6 +76,15 @@ describe("Secp256k1Signature", () => {
     ).toThrowError(/unsigned integer s must be encoded as unpadded big endian./i);
   });
 
+  it("can be encoded as fixed length", () => {
+    const signature = new Secp256k1Signature(new Uint8Array([0x22, 0x33]), new Uint8Array([0xaa]));
+    expect(signature.toFixedLength()).toEqual(
+      fromHex(
+        "000000000000000000000000000000000000000000000000000000000000223300000000000000000000000000000000000000000000000000000000000000aa",
+      ),
+    );
+  });
+
   it("can encode to DER", () => {
     // Signature 3045022100f25b86e1d8a11d72475b3ed273b0781c7d7f6f9e1dae0dd5d3ee9b84f3fab891022063d9c4e1391de077244583e9a6e3d8e8e1f236a3bf5963735353b93b1a3ba935
     // decoded by http://asn1-playground.oss.com/

packages/crypto/src/secp256k1signature.ts

@@ -119,6 +119,10 @@ export class Secp256k1Signature {
     }
   }
 
+  public toFixedLength(): Uint8Array {
+    return new Uint8Array([...this.r(32), ...this.s(32)]);
+  }
+
   public toDer(): Uint8Array {
     // DER supports negative integers but our data is unsigned. Thus we need to prepend
     // a leading 0 byte when the higest bit is set to differentiate nagative values

packages/crypto/types/secp256k1signature.d.ts

@@ -12,6 +12,7 @@ export declare class Secp256k1Signature {
   constructor(r: Uint8Array, s: Uint8Array);
   r(length?: number): Uint8Array;
   s(length?: number): Uint8Array;
+  toFixedLength(): Uint8Array;
   toDer(): Uint8Array;
 }
 /**