An improved document authentication method in which critical content, such as signatures, is preserved at a high-resolution in the authentication data carried on the self-authenticating document. When generating authentication data, signatures are compressed without down-sampling to preserve their resolution and quality. The compressed signature data (a bit string) is embedded in an image segment on the document. For example, each bit of the bit string is stored in the low bits of one or more image pixels. A hash code is calculated from the bit string and stored in a barcode printed on the document. To authenticate a scanned-back document, the bit string is recovered from the image segment. A hash code is calculated from the recovered bit string and compared to the hash code extracted from the barcode. The signatures re-generated from the recovered bit string are compared to the signatures in the scanned document.