I too am facing the same issue. Any leads would be appreciated.
Thanks

Same in 7.31.1.

I created a client of my own to be able to work. So I had to use the AWS signature calculation and use a crypto library, CryptoJS. I encountered the following problem which could be the same as Postman's one. The CryptoJS library only takes a string as input and not a Buffer, so an implicit .toString() is made to utf8 which makes the signature invalid.

Same here. Can't POST multipart/form-data to Lambda function behind API Gateway.

It looks like that currently authorization of form data requests left unimplemented in the assumption that it is only used to do uploads directly to S3. See the relevant comment in the source code here: https://github.com/postmanlabs/postman-runtime/blob/451e9aa3ce563b6ae9529633151d4f934cdba9e0/lib/authorizer/aws4.js#L74-L76

But in my case, I want to post form data to API gateway, which is configured to bypass such payload to my Lambda function as is (with multipart/form-data specified as a binary media type).

Maybe it is better to implement signature in the same way as for raw body type to cover at least this use case for the time being?

commenting to track this

Following

I am also facing the same issue while calling an API with aws_auth using postman latest Version 8.6.1 (8.6.1). Trying to upload multiple files but getting this invalid signature error.

I was able to set up HTTPie console utility for multipart request signing and switched to it for the time being:

# Install HTTPie with required plugins for AWS request signing
pip install git+https://github.com/Envek/aws-requests-auth@fix/multipart-requests
pip install httpie httpie-aws-authv4

# Make multipart requests like this:
http --auth-type aws4 --auth REDACTED.execute-api.us-west-2.amazonaws.com \
     --multipart POST http://example.com/test field=value

See https://github.com/aidan-/httpie-aws-authv4/issues/10 for details.

We are also facing this issue with Postman. Any plan to solve this issue ?

Following

Any updates on this? This has been open for two nearly three years, and has been a longstanding issue even before then

It took me so much time to figure out that the issue is with postman. I was continuously debugging my API setup as the request was giving signature mismatch error.

Any plan to solve this?