Nikon Image Authentication System Cracked
Not a lot of details:
ElcomSoft research shows that image metadata and image data are processed independently with a SHA-1 hash function. There are two 160-bit hash values produced, which are later encrypted with a secret (private) key by using an asymmetric RSA-1024 algorithm to create a digital signature. Two 1024-bit (128-byte) signatures are stored in EXIF MakerNote tag 0×0097 (Color Balance).
During validation, Nikon Image Authentication Software calculates two SHA-1 hashes from the same data, and uses the public key to verify the signature by decrypting stored values and comparing the result with newly calculated hash values.
The ultimate vulnerability is that the private (should-be-secret) cryptographic key is handled inappropriately, and can be extracted from camera. After obtaining the private key, it is possible to generate a digital signature value for any image, thus forging the Image Authentication System.
Canon's system is just as bad, by the way.
Fifteen years ago, I co-authored a paper on the problem. The idea was to use a hash chain to better deal with the possibility of a secret-key compromise.
Posted on May 3, 2011 at 7:54 AM • 25 Comments