Can't create Secret in Kubernetes: illegal base64 data at input

I got the decoded values "mega_secret_key" and "really_secret_value1" from from your encoded data. Seems they are not encoded in right way. So, encode your data in right way:

$ echo "mega_secret_key" | base64
bWVnYV9zZWNyZXRfa2V5Cg==

$ echo "really_secret_value1" | base64
cmVhbGx5X3NlY3JldF92YWx1ZTEK

Then check whether they are encoded properly:

$ echo "bWVnYV9zZWNyZXRfa2V5Cg==" | base64 -d
mega_secret_key

$ echo "cmVhbGx5X3NlY3JldF92YWx1ZTEK" | base64 -d
really_secret_value1

So they are ok. Now use them in your dummy-secret.yaml:

apiVersion: v1
kind: Secret
metadata:
  name: dummy-secret
type: Opaque
data:
  API_KEY: bWVnYV9zZWNyZXRfa2V5Cg==
  API_SECRET: cmVhbGx5X3NlY3JldF92YWx1ZTEK

And run $ kubectl create -f dummy-secret.yaml.

UPDATE on 11-02-2022:

The newer versions of Kubernetes support the optional stringData property where one can provide the value against any key without decoding.

All key-value pairs in the stringData field are internally merged into the data field. If a key appears in both the data and the stringData field, the value specified in the stringData field takes precedence.

apiVersion: v1
kind: Secret
metadata:
  name: dummy-secret
type: Opaque
stringData:
  API_KEY: mega_secret_key
  API_SECRET: really_secret_value1

UPDATE:

If you use -n flag while running $ echo "some_text", it will trim the trailing \n (newline) from the string you are printing.

$ echo "some_text"
some_text
$ echo -n "some_text"
some_text⏎

Just try it,

# first encode
$ echo -n "mega_secret_key" | base64
bWVnYV9zZWNyZXRfa2V5
$ echo -n "really_secret_value1" | base64
cmVhbGx5X3NlY3JldF92YWx1ZTE=
# then decode and check whether newline is stripped
$ echo "bWVnYV9zZWNyZXRfa2V5" | base64 -d
mega_secret_key⏎
$ echo "cmVhbGx5X3NlY3JldF92YWx1ZTE=" | base64 -d
really_secret_value1⏎

You can use these newly (without newline) decoded data in your secret instead. That also should fine.

$ cat - <<-EOF | kubectl apply -f -
apiVersion: v1
kind: Secret
metadata:
  name: dummy-secret
type: Opaque
data:
  API_KEY: bWVnYV9zZWNyZXRfa2V5
  API_SECRET: cmVhbGx5X3NlY3JldF92YWx1ZTE=
EOF
secret/dummy-secret created

At the time of update, my kubernetes version is,

Minor:"17", GitVersion:"v1.17.3",
GitCommit:"06ad960bfd03b39c8310aaf92d1e7c1 2ce618213",
GitTreeState:"clean", BuildDate:"2020-02-11T18:14:22Z",
GoVersion:"go1.13.6", Compiler:"gc", Platform:"l inux/amd64"} Server
Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.3",
GitCommit:"06ad960bfd03b39c8310aaf92d1e7c1 2ce618213",
GitTreeState:"clean", BuildDate:"2020-02-11T18:07:13Z",
GoVersion:"go1.13.6", Compiler:"gc", Platform:"l inux/amd64"} ```

This was already answered but for future reference, there is no need to encode the strings by using stringData instead of data field as shown below:

#secrets.yaml
apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
stringData:
  API_KEY: "STRING_IN_CLEAR_TEXT"
  API_SECRET: "STRING_IN_CLEAR_TEXT"

After a while I want to return back to this question and leave an answer with a reference to official kubernetes docs:

echo -n 'admin' | base64
YWRtaW4=
echo -n '1f2d1e2e67df' | base64
MWYyZDFlMmU2N2Rm

Pay extra attention to -n, because it guaranties that after decoding your secret key will not contain 'new line symbol'.