fix: Shards are always base64 encoded

Provide encoding only for the secret.

packages/horcrux-crypto/src/tss.ts

@@ -1,5 +1,5 @@
 import * as tss from '@stablelib/tss'
-import { Encoding, encoders, decoders } from './codec'
+import { Encoding, encoders, decoders, b64 } from './codec'
 
 /**
  * Split a secret into a given amount of shards.
@@ -8,34 +8,31 @@ import { Encoding, encoders, decoders } from './codec'
  * @param secret The secret to split
  * @param numShards How many pieces to split into
  * @param threshold How many pieces are needed (min) to re-assemble the secret
- * @param encoding Encoding for the secret and the shards
+ * @param encoding Input encoding for the secret
  */
 export const splitSecret = (
   secret: string,
   numShards: number,
   threshold: number,
-  encoding: Encoding = 'base64'
+  encoding: Encoding = 'utf8'
 ) => {
-  const encode = encoders[encoding]
   const decode = decoders[encoding]
   const shards = tss.split(decode(secret), threshold, numShards)
-  return shards.map(shard => encode(shard))
+  return shards.map(shard => b64.encode(shard))
 }
 
 /**
  * Try to re-assemble the original secret from shards.
  * Will throw if anything goes wrong.
  *
  * @param shards Any number of shards
- * @param encoding Encoding for the shards and the output
- * @returns The secret in the same encoding as the shards
+ * @param encoding Encoding for the output
  */
 export const assembleSecret = (
   shards: string[],
-  encoding: Encoding = 'base64'
+  encoding: Encoding = 'utf8'
 ) => {
   const encode = encoders[encoding]
-  const decode = decoders[encoding]
-  const secret = tss.combine(shards.map(shard => decode(shard)))
+  const secret = tss.combine(shards.map(shard => b64.decode(shard)))
   return encode(secret)
 }