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Let's say I want to do an SHA256HMAC digest of a file with the openssl command line utility:

openssl dgst -sha256 -hmac "$(cat $KEY_FILE)" -hex "$TARGET_FILE"

How can I protect this command against the $(cat $KEY_FILE) generating null bytes (or other potentially troublesome characters) if those happen to exist in $KEY_FILE?

  • You can't provide arbitrary binary input as arguments because strings can't contain null bytes (which suggests that the utility is not designed to take that). Quotes are not a problem though. Do you have an applied example of where you're seeing an issue? – Michael Homer May 17 '18 at 6:54
  • @MichaelHomer This is the only utility I've ever seen that is supposed to take arbitrary binary as a command line argument, so I don't have any other examples. – martin May 17 '18 at 7:00
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    It appears that -mac HMAC -macopt hexkey:1f0cda is supported. If you don't have any null bytes and the file doesn't end with linebreaks then your version is fine, though. – Michael Homer May 17 '18 at 7:08

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