This paper analyses the cryptography used in the Open Smart Grid Protocol (OSGP). The authenticated encryption (AE) scheme deployed by OSGP is a non-standard composition of RC4 and a homebrewed MAC, the “OMA digest”. We present several practical key-recovery attacks against the OMA digest. The first and basic variant can achieve this with a mere 13 queries to an OMA digest oracle and negligible time complexity. A more sophisticated version breaks the OMA digest with only 4 queries and a time complexity of about \( 2^{25} \) simple operations. A different approach only requires one arbitrary valid plaintext-tag pair, and recovers the key in an average of 144 message verification queries, or one ciphertext-tag pair and 168 ciphertext verification queries. Since the encryption key is derived from the key used by the OMA digest, our attacks break both confidentiality and authenticity of OSGP.