Linux - openssl sha256 using key from file

Question

I have a 32 byte key stored in a file called test_key.key

I wish to use openssl to create hash of a different file called mytext.txt. I need to create the hash using HMAC-SHA-256 & the key in test_key.key

I thought I had it working with the following command:

openssl dgst -sha256 -hmac $test_key.key mytext.txt

However, on closer examination I noticed it was using the string "test_key.key", not the actual file contents. I have tried a few different variations of this for the key file such as:

"$test_key.key"
'$test_key.key'
/test_key.key

Sadly, they all seem to have the same problem.

Can anyone advise me on this? I wish to keep this as a one line command if possible.

@DavidC.Rankin I'm surprised that this works, especially if the key contains non printable character. Personally I prefer openssl dgst -sha256 -mac HMAC -macopt "hexkey:$(xxd -ps -c32 test_key.key)" mytext.txt that is explicit about the key format. — oliv
– oliv, Commented May 23, 2018 at 8:34
There are quite few caveats to the general use, but with a sha256 string of unsigned char you should be fine (as long as there isn't a '/* ' or something else that could be interpreted as a independent command) I wouldn't recommend it for much else. — David C. Rankin
– David C. Rankin, Commented May 23, 2018 at 8:41
@DavidC.Rankin calling openssl ... $(<test_key.key) ... exposes the key in the process info — bigbear3001
– bigbear3001, Commented Jul 12, 2022 at 9:15

DaveMac001 · Accepted Answer · 2018-05-23 08:26:16Z

5

openssl dgst -sha256 -hmac $(<test_key.key) mytext.txt

(credit to user David C. Rankin)

answered May 23, 2018 at 8:26

DaveMac001

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1 Comment

ndvo Over a year ago

It is helpful to use quotes to avoid issues with special characters (couldn't edit because edits must have 6 chars): openssl dgst -sha256 -hmac "$(<test_key.key)" mytext.txt

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