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No minimum tag length was enforced. A user supplying a 1-byte tag only needed to match the first byte of the real tag, reducing the authentication check to a 1-in-256 brute-force
Prevented Truncated Tag comparison
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The CLI
verifycommand accepts truncated authentication tags of arbitrary length, including a single byte. The verification routine base64url-decodes the user-supplied--tagargument and uses the decoded length as the comparison length forCRYPTO_memcmp(). No minimum tag length is enforced. An attacker supplying a 1-byte tag only needs to match the first byte of the real tag.