This OID counts, in frames, how many received frames on this ODU (Optical Data Unit) layer of this optical transport interface failed decryption. On a link where optical-layer encryption is enabled, a nonzero and growing count here means frames are arriving that this node cannot successfully decrypt, which could indicate a key mismatch between the two ends of the encrypted link or actual tampering with the signal. For example, if the encryption key on the ODU layer beneath an OTU4 wavelength carrying aggregated client traffic was recently rotated on only one end of the link, this counter would start climbing immediately as every frame fails to decrypt with the mismatched key.