I know this question is 4 years old, but it still came up at the top of my search results when I went looking for the same thing, and with no answer.
Here's the answer.
First use your script to create an encrypted string:
$a = 'MySecret'
$b = 'MySecretKey4Test'
$c = ConvertTo-SecureString $a -AsPlainText -Force | ConvertFrom-SecureString -Key ($b.ToCharArray() | % {[byte]$_})
Let's get our b (key) and c (encrypted string) into bash to begin with:
b="MySecretKey4Test"
c="76492d1116743f0423413b16050a5345MgB8ADkATgB4AEgAMQBZAHAAdABYAGgASgBJAGoAWABIAFMARgBoAHEANABhAHcAPQA9AHwAOABhADYANgBkAGUAZQBhAGIAMQBiADcANAA4ADMAZgA5AGUAYwA2ADEAZQBiADYAZAA5AGYAMgBmAGMAYgBmAGEANQBhADgAOAA4AGEAYgBhAGIAMgBiADAAYQA4AGYAMwBhAGYANwAyADUAYgA1AGMANgA4ADQAZQBjADgAMAA="
We need to convert the key to hexadecimal, being careful not to get a newline at the end.
read -r b < <(echo -n "$b"|xxd -p -c 9999)
We need to remove a fixed header from the beginning of the encrypted string. The fixed header is always 76492d1116743f0423413b16050a5345. Not sure why this is here, but it is.
c="${c/76492d1116743f0423413b16050a5345/}"
Next up, decode the encrypted string as base64 and pass it through iconv, to properly parse the utf16-le characters. Then split the result on | into an array (called parts here).
IFS='|' read -r -a parts < <(base64 -d <<< "$c"|iconv -f UTF16LE -co - -)
parts[0] is just a part count, which will be 2 and can be ignored.
parts[1] is the cbc initialization vector in base64 format, but openssl wants it in hexadecimal.
read -r iv < <(base64 -d <<< "${parts[1]}"|xxd -p -c 99999)
parts[2] is the actual encrypted data in hexadecimal format.
read -r crypt <<< "${parts[2]}"
Now we decrypt, using the key ($b), iv ($iv) and encrypted data ($crypt).
$crypt will need to be converted to binary first, so we use xxd.
read -r decrypted < <(openssl enc -d -A -aes-128-cbc -iv "$iv" -K "$b" < <(xxd -r -p <<< "$crypt"))
echo "$decrypted"
All together now:
b="MySecretKey4Test"
c="76492d1116743f0423413b16050a5345MgB8ADkATgB4AEgAMQBZAHAAdABYAGgASgBJAGoAWABIAFMARgBoAHEANABhAHcAPQA9AHwAOABhADYANgBkAGUAZQBhAGIAMQBiADcANAA4ADMAZgA5AGUAYwA2ADEAZQBiADYAZAA5AGYAMgBmAGMAYgBmAGEANQBhADgAOAA4AGEAYgBhAGIAMgBiADAAYQA4AGYAMwBhAGYANwAyADUAYgA1AGMANgA4ADQAZQBjADgAMAA="
read -r b < <(echo -n "$b"|xxd -p -c 9999)
c="${c/76492d1116743f0423413b16050a5345/}"
IFS='|' read -r -a parts < <(base64 -d <<< "$c"|iconv -f UTF16LE -co - -)
read -r iv < <(base64 -d <<< "${parts[1]}"|xxd -p -c 99999)
read -r crypt <<< "${parts[2]}"
read -r decrypted < <(openssl enc -d -A -aes-128-cbc -iv "$iv" -K "$b" < <(xxd -r -p <<< "$crypt"))
echo "$decrypted"
Results:
MySecret
Be aware that if you use a 32-byte (256bit) key instead of a 16-byte (128bit) key, use -aes-256-cbc in your openssl command instead of -aes-128-cbc
