Return friendly error if input is Base64 encoded

[lavaleri]

Problem:

We have some code that intends to return a friendly error if the first byte is base64 encoded:

• aws-encryption-sdk-javascript/modules/serialize/src/deserialize_header_v1.ts

  Lines 88 to 93 in 43813f4

  	/* Precondition: version and type must be the required values. */
  	needs(
  	version === MessageFormat.V1 && type === 128,
  	version === 65 && type === 89
  	? 'Malformed Header: This blob may be base64 encoded.'
  	: 'Malformed Header.'

• aws-encryption-sdk-javascript/modules/serialize/src/deserialize_header_v2.ts

  Lines 92 to 96 in 43813f4

  	needs(
  	version === MessageFormat.V2,
  	version === 65
  	? 'Malformed Header: This blob may be base64 encoded.'
  	: 'Malformed Header.'

However we can't ever get inside those statements because we first do a check at:

aws-encryption-sdk-javascript/modules/serialize/src/deserialize_factory.ts

Line 74 in 43813f4

needs(deserializer, 'Not a supported message format version.')

Solution:

Move the logic checking the expected first byte in Base64 before the branch that determines which version we're deserializing.

Out of scope:

n/a

[gauthaman92]

Could you please share the updated logic