This read-create string field stores the actual secret key material, a 128-bit key rendered in up to 16 displayable characters, used for MD5-based authentication on this key-chain entry. An administrator sets it when provisioning or rotating shared secrets that two devices must both hold identically for authenticated protocol sessions to succeed. For example, during a scheduled key rotation, an admin writes a new secret string to this OID on both the local router and its peer to keep MD5 authentication working after the changeover.