fix: accept 32 or 64 bytes as keys for EdDSASigner (#299)

Only the first 32 bytes are used as the actual key

fixes #289

src/__tests__/EdDSASigner.test.ts

@@ -54,12 +54,12 @@ describe('EdDSASigner', () => {
     )
   })
 
-  it('refuses wrong key size (half)', async () => {
+  it('accepts 32 byte keys size (github #289)', async () => {
     expect.assertions(1)
     const privateKey = '278a5de700e29faae8e40e366ec5012b5ec63d36ec77e8a2417154cc1d25383f'
     expect(() => {
       EdDSASigner(hexToBytes(privateKey))
-    }).toThrowError(/^bad_key: Invalid private key format.*/)
+    }).not.toThrow()
   })
 
   it('refuses wrong key size', async () => {

src/signers/EdDSASigner.ts

@@ -5,21 +5,27 @@ import { bytesToBase64url, stringToBytes } from '../util.js'
 /**
  *  Creates a configured signer function for signing data using the EdDSA (Ed25519) algorithm.
  *
- *  The signing function itself takes the data as a `Uint8Array` or `string` and returns a `base64Url`-encoded signature
+ *  The private key is expected to be a `Uint8Array` of 32 bytes, but for compatibility 64 bytes are also acceptable.
+ * Users of `@stablelib/ed25519` or `tweetnacl` will be able to use the 64 byte secret keys that library generates.
+ * These libraries precompute the public key and append it as the last 32 bytes of the secretKey, to speed up later
+ * signing operations.
+ *
+ *  The signing function itself takes the data as a `Uint8Array` or utf8 `string` and returns a `base64Url`-encoded
+ * signature
  *
  *  @example
  *  ```typescript
  *  const sign: Signer = EdDSASigner(process.env.PRIVATE_KEY)
  *  const signature: string = await sign(data)
  *  ```
  *
- *  @param    {String}    secretKey   a 64 byte secret key as `Uint8Array`
+ *  @param    {String}    secretKey   a 32 or 64 byte secret key as `Uint8Array`
  *  @return   {Function}              a configured signer function `(data: string | Uint8Array): Promise<string>`
  */
 export function EdDSASigner(secretKey: Uint8Array): Signer {
   const privateKeyBytes: Uint8Array = secretKey
-  if (privateKeyBytes.length !== 64) {
-    throw new Error(`bad_key: Invalid private key format. Expecting 64 bytes, but got ${privateKeyBytes.length}`)
+  if (![32, 64].includes(privateKeyBytes.length)) {
+    throw new Error(`bad_key: Invalid private key format. Expecting 32 or 64 bytes, but got ${privateKeyBytes.length}`)
   }
   return async (data: string | Uint8Array): Promise<string> => {
     const dataBytes: Uint8Array = typeof data === 'string' ? stringToBytes(data) : data