Add Secp256k1.uncompressPubkey

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to
 ### Changed
 
 - all: The TypeScript compilation target is now ES2018.
+- @cosmjs/crypto: Add `Secp256k1.uncompressPubkey`.
 - @cosmjs/faucet: Set default value of `FAUCET_GAS_LIMIT` to 100_000 to better
   support Cosmos SDK 0.45 chains.
 - @cosmjs/stargate: The `AminoTypes` now always requires an argument of type

packages/crypto/src/secp256k1.spec.ts

@@ -570,6 +570,30 @@ describe("Secp256k1", () => {
     });
   });
 
+  describe("uncompressPubkey", () => {
+    it("throws for a pubkey with invalid length", () => {
+      const pubkey = fromHex("aa".repeat(32));
+      expect(() => Secp256k1.uncompressPubkey(pubkey)).toThrowError(/invalid pubkey length/i);
+    });
+
+    it("returns an uncompressPubkey pubkey unchanged", () => {
+      // Test data generated at https://iancoleman.io/bitcoin-key-compression/
+      const pubkey = fromHex(
+        "044f04181eeba35391b858633a765c4a0c189697b40d216354d50890d350c7029013b587a681e836cc187a8164b98a5848a2b89b3173315fdd0740d5032e259cd5",
+      );
+      expect(Secp256k1.uncompressPubkey(pubkey)).toEqual(pubkey);
+    });
+
+    it("uncompresses a compressed pubkey", () => {
+      // Test data generated at https://iancoleman.io/bitcoin-key-compression/
+      const uncompressed = fromHex(
+        "044f04181eeba35391b858633a765c4a0c189697b40d216354d50890d350c7029013b587a681e836cc187a8164b98a5848a2b89b3173315fdd0740d5032e259cd5",
+      );
+      const compressed = fromHex("034f04181eeba35391b858633a765c4a0c189697b40d216354d50890d350c70290");
+      expect(Secp256k1.uncompressPubkey(compressed)).toEqual(uncompressed);
+    });
+  });
+
   describe("trimRecoveryByte", () => {
     it("throws for a signature with invalid length", () => {
       const signature = fromHex("aa".repeat(66));

packages/crypto/src/secp256k1.ts

@@ -121,6 +121,22 @@ export class Secp256k1 {
     }
   }
 
+  /**
+   * Takes a compressed or uncompressed pubkey and returns an uncompressed one.
+   *
+   * This function is idempotent.
+   */
+  public static uncompressPubkey(pubkey: Uint8Array): Uint8Array {
+    switch (pubkey.length) {
+      case 33:
+        return Uint8Array.from(secp256k1.keyFromPublic(pubkey).getPublic(false, "array"));
+      case 65:
+        return pubkey;
+      default:
+        throw new Error("Invalid pubkey length");
+    }
+  }
+
   public static trimRecoveryByte(signature: Uint8Array): Uint8Array {
     switch (signature.length) {
       case 64: